summaryrefslogtreecommitdiff
path: root/Documentation/networking/filter.txt
blob: 319e5e041f3808b3f0b77fcc4f6c511b4918144b (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
Linux Socket Filtering aka Berkeley Packet Filter (BPF)
=======================================================

Introduction
------------

Linux Socket Filtering (LSF) is derived from the Berkeley Packet Filter.
Though there are some distinct differences between the BSD and Linux
Kernel filtering, but when we speak of BPF or LSF in Linux context, we
mean the very same mechanism of filtering in the Linux kernel.

BPF allows a user-space program to attach a filter onto any socket and
allow or disallow certain types of data to come through the socket. LSF
follows exactly the same filter code structure as BSD's BPF, so referring
to the BSD bpf.4 manpage is very helpful in creating filters.

On Linux, BPF is much simpler than on BSD. One does not have to worry
about devices or anything like that. You simply create your filter code,
send it to the kernel via the SO_ATTACH_FILTER option and if your filter
code passes the kernel check on it, you then immediately begin filtering
data on that socket.

You can also detach filters from your socket via the SO_DETACH_FILTER
option. This will probably not be used much since when you close a socket
that has a filter on it the filter is automagically removed. The other
less common case may be adding a different filter on the same socket where
you had another filter that is still running: the kernel takes care of
removing the old one and placing your new one in its place, assuming your
filter has passed the checks, otherwise if it fails the old filter will
remain on that socket.

SO_LOCK_FILTER option allows to lock the filter attached to a socket. Once
set, a filter cannot be removed or changed. This allows one process to
setup a socket, attach a filter, lock it then drop privileges and be
assured that the filter will be kept until the socket is closed.

The biggest user of this construct might be libpcap. Issuing a high-level
filter command like `tcpdump -i em1 port 22` passes through the libpcap
internal compiler that generates a structure that can eventually be loaded
via SO_ATTACH_FILTER to the kernel. `tcpdump -i em1 port 22 -ddd`
displays what is being placed into this structure.

Although we were only speaking about sockets here, BPF in Linux is used
in many more places. There's xt_bpf for netfilter, cls_bpf in the kernel
qdisc layer, SECCOMP-BPF (SECure COMPuting [1]), and lots of other places
such as team driver, PTP code, etc where BPF is being used.

 [1] Documentation/userspace-api/seccomp_filter.rst

Original BPF paper:

Steven McCanne and Van Jacobson. 1993. The BSD packet filter: a new
architecture for user-level packet capture. In Proceedings of the
USENIX Winter 1993 Conference Proceedings on USENIX Winter 1993
Conference Proceedings (USENIX'93). USENIX Association, Berkeley,
CA, USA, 2-2. [http://www.tcpdump.org/papers/bpf-usenix93.pdf]

Structure
---------

User space applications include <linux/filter.h> which contains the
following relevant structures:

struct sock_filter {	/* Filter block */
	__u16	code;   /* Actual filter code */
	__u8	jt;	/* Jump true */
	__u8	jf;	/* Jump false */
	__u32	k;      /* Generic multiuse field */
};

Such a structure is assembled as an array of 4-tuples, that contains
a code, jt, jf and k value. jt and jf are jump offsets and k a generic
value to be used for a provided code.

struct sock_fprog {			/* Required for SO_ATTACH_FILTER. */
	unsigned short		   len;	/* Number of filter blocks */
	struct sock_filter __user *filter;
};

For socket filtering, a pointer to this structure (as shown in
follow-up example) is being passed to the kernel through setsockopt(2).

Example
-------

#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <linux/if_ether.h>
/* ... */

/* From the example above: tcpdump -i em1 port 22 -dd */
struct sock_filter code[] = {
	{ 0x28,  0,  0, 0x0000000c },
	{ 0x15,  0,  8, 0x000086dd },
	{ 0x30,  0,  0, 0x00000014 },
	{ 0x15,  2,  0, 0x00000084 },
	{ 0x15,  1,  0, 0x00000006 },
	{ 0x15,  0, 17, 0x00000011 },
	{ 0x28,  0,  0, 0x00000036 },
	{ 0x15, 14,  0, 0x00000016 },
	{ 0x28,  0,  0, 0x00000038 },
	{ 0x15, 12, 13, 0x00000016 },
	{ 0x15,  0, 12, 0x00000800 },
	{ 0x30,  0,  0, 0x00000017 },
	{ 0x15,  2,  0, 0x00000084 },
	{ 0x15,  1,  0, 0x00000006 },
	{ 0x15,  0,  8, 0x00000011 },
	{ 0x28,  0,  0, 0x00000014 },
	{ 0x45,  6,  0, 0x00001fff },
	{ 0xb1,  0,  0, 0x0000000e },
	{ 0x48,  0,  0, 0x0000000e },
	{ 0x15,  2,  0, 0x00000016 },
	{ 0x48,  0,  0, 0x00000010 },
	{ 0x15,  0,  1, 0x00000016 },
	{ 0x06,  0,  0, 0x0000ffff },
	{ 0x06,  0,  0, 0x00000000 },
};

struct sock_fprog bpf = {
	.len = ARRAY_SIZE(code),
	.filter = code,
};

sock = socket(PF_PACKET, SOCK_RAW, htons(ETH_P_ALL));
if (sock < 0)
	/* ... bail out ... */

ret = setsockopt(sock, SOL_SOCKET, SO_ATTACH_FILTER, &bpf, sizeof(bpf));
if (ret < 0)
	/* ... bail out ... */

/* ... */
close(sock);

The above example code attaches a socket filter for a PF_PACKET socket
in order to let all IPv4/IPv6 packets with port 22 pass. The rest will
be dropped for this socket.

The setsockopt(2) call to SO_DETACH_FILTER doesn't need any arguments
and SO_LOCK_FILTER for preventing the filter to be detached, takes an
integer value with 0 or 1.

Note that socket filters are not restricted to PF_PACKET sockets only,
but can also be used on other socket families.

Summary of system calls:

 * setsockopt(sockfd, SOL_SOCKET, SO_ATTACH_FILTER, &val, sizeof(val));
 * setsockopt(sockfd, SOL_SOCKET, SO_DETACH_FILTER, &val, sizeof(val));
 * setsockopt(sockfd, SOL_SOCKET, SO_LOCK_FILTER,   &val, sizeof(val));

Normally, most use cases for socket filtering on packet sockets will be
covered by libpcap in high-level syntax, so as an application developer
you should stick to that. libpcap wraps its own layer around all that.

Unless i) using/linking to libpcap is not an option, ii) the required BPF
filters use Linux extensions that are not supported by libpcap's compiler,
iii) a filter might be more complex and not cleanly implementable with
libpcap's compiler, or iv) particular filter codes should be optimized
differently than libpcap's internal compiler does; then in such cases
writing such a filter "by hand" can be of an alternative. For example,
xt_bpf and cls_bpf users might have requirements that could result in
more complex filter code, or one that cannot be expressed with libpcap
(e.g. different return codes for various code paths). Moreover, BPF JIT
implementors may wish to manually write test cases and thus need low-level
access to BPF code as well.

BPF engine and instruction set
------------------------------

Under tools/bpf/ there's a small helper tool called bpf_asm which can
be used to write low-level filters for example scenarios mentioned in the
previous section. Asm-like syntax mentioned here has been implemented in
bpf_asm and will be used for further explanations (instead of dealing with
less readable opcodes directly, principles are the same). The syntax is
closely modelled after Steven McCanne's and Van Jacobson's BPF paper.

The BPF architecture consists of the following basic elements:

  Element          Description

  A                32 bit wide accumulator
  X                32 bit wide X register
  M[]              16 x 32 bit wide misc registers aka "scratch memory
                   store", addressable from 0 to 15

A program, that is translated by bpf_asm into "opcodes" is an array that
consists of the following elements (as already mentioned):

  op:16, jt:8, jf:8, k:32

The element op is a 16 bit wide opcode that has a particular instruction
encoded. jt and jf are two 8 bit wide jump targets, one for condition
"jump if true", the other one "jump if false". Eventually, element k
contains a miscellaneous argument that can be interpreted in different
ways depending on the given instruction in op.

The instruction set consists of load, store, branch, alu, miscellaneous
and return instructions that are also represented in bpf_asm syntax. This
table lists all bpf_asm instructions available resp. what their underlying
opcodes as defined in linux/filter.h stand for:

  Instruction      Addressing mode      Description

  ld               1, 2, 3, 4, 12       Load word into A
  ldi              4                    Load word into A
  ldh              1, 2                 Load half-word into A
  ldb              1, 2                 Load byte into A
  ldx              3, 4, 5, 12          Load word into X
  ldxi             4                    Load word into X
  ldxb             5                    Load byte into X

  st               3                    Store A into M[]
  stx              3                    Store X into M[]

  jmp              6                    Jump to label
  ja               6                    Jump to label
  jeq              7, 8, 9, 10          Jump on A == <x>
  jneq             9, 10                Jump on A != <x>
  jne              9, 10                Jump on A != <x>
  jlt              9, 10                Jump on A <  <x>
  jle              9, 10                Jump on A <= <x>
  jgt              7, 8, 9, 10          Jump on A >  <x>
  jge              7, 8, 9, 10          Jump on A >= <x>
  jset             7, 8, 9, 10          Jump on A &  <x>

  add              0, 4                 A + <x>
  sub              0, 4                 A - <x>
  mul              0, 4                 A * <x>
  div              0, 4                 A / <x>
  mod              0, 4                 A % <x>
  neg                                   !A
  and              0, 4                 A & <x>
  or               0, 4                 A | <x>
  xor              0, 4                 A ^ <x>
  lsh              0, 4                 A << <x>
  rsh              0, 4                 A >> <x>

  tax                                   Copy A into X
  txa                                   Copy X into A

  ret              4, 11                Return

The next table shows addressing formats from the 2nd column:

  Addressing mode  Syntax               Description

   0               x/%x                 Register X
   1               [k]                  BHW at byte offset k in the packet
   2               [x + k]              BHW at the offset X + k in the packet
   3               M[k]                 Word at offset k in M[]
   4               #k                   Literal value stored in k
   5               4*([k]&0xf)          Lower nibble * 4 at byte offset k in the packet
   6               L                    Jump label L
   7               #k,Lt,Lf             Jump to Lt if true, otherwise jump to Lf
   8               x/%x,Lt,Lf           Jump to Lt if true, otherwise jump to Lf
   9               #k,Lt                Jump to Lt if predicate is true
  10               x/%x,Lt              Jump to Lt if predicate is true
  11               a/%a                 Accumulator A
  12               extension            BPF extension

The Linux kernel also has a couple of BPF extensions that are used along
with the class of load instructions by "overloading" the k argument with
a negative offset + a particular extension offset. The result of such BPF
extensions are loaded into A.

Possible BPF extensions are shown in the following table:

  Extension                             Description

  len                                   skb->len
  proto                                 skb->protocol
  type                                  skb->pkt_type
  poff                                  Payload start offset
  ifidx                                 skb->dev->ifindex
  nla                                   Netlink attribute of type X with offset A
  nlan                                  Nested Netlink attribute of type X with offset A
  mark                                  skb->mark
  queue                                 skb->queue_mapping
  hatype                                skb->dev->type
  rxhash                                skb->hash
  cpu                                   raw_smp_processor_id()
  vlan_tci                              skb_vlan_tag_get(skb)
  vlan_avail                            skb_vlan_tag_present(skb)
  vlan_tpid                             skb->vlan_proto
  rand                                  prandom_u32()

These extensions can also be prefixed with '#'.
Examples for low-level BPF:

** ARP packets:

  ldh [12]
  jne #0x806, drop
  ret #-1
  drop: ret #0

** IPv4 TCP packets:

  ldh [12]
  jne #0x800, drop
  ldb [23]
  jneq #6, drop
  ret #-1
  drop: ret #0

** (Accelerated) VLAN w/ id 10:

  ld vlan_tci
  jneq #10, drop
  ret #-1
  drop: ret #0

** icmp random packet sampling, 1 in 4
  ldh [12]
  jne #0x800, drop
  ldb [23]
  jneq #1, drop
  # get a random uint32 number
  ld rand
  mod #4
  jneq #1, drop
  ret #-1
  drop: ret #0

** SECCOMP filter example:

  ld [4]                  /* offsetof(struct seccomp_data, arch) */
  jne #0xc000003e, bad    /* AUDIT_ARCH_X86_64 */
  ld [0]                  /* offsetof(struct seccomp_data, nr) */
  jeq #15, good           /* __NR_rt_sigreturn */
  jeq #231, good          /* __NR_exit_group */
  jeq #60, good           /* __NR_exit */
  jeq #0, good            /* __NR_read */
  jeq #1, good            /* __NR_write */
  jeq #5, good            /* __NR_fstat */
  jeq #9, good            /* __NR_mmap */
  jeq #14, good           /* __NR_rt_sigprocmask */
  jeq #13, good           /* __NR_rt_sigaction */
  jeq #35, good           /* __NR_nanosleep */
  bad: ret #0             /* SECCOMP_RET_KILL_THREAD */
  good: ret #0x7fff0000   /* SECCOMP_RET_ALLOW */

The above example code can be placed into a file (here called "foo"), and
then be passed to the bpf_asm tool for generating opcodes, output that xt_bpf
and cls_bpf understands and can directly be loaded with. Example with above
ARP code:

$ ./bpf_asm foo
4,40 0 0 12,21 0 1 2054,6 0 0 4294967295,6 0 0 0,

In copy and paste C-like output:

$ ./bpf_asm -c foo
{ 0x28,  0,  0, 0x0000000c },
{ 0x15,  0,  1, 0x00000806 },
{ 0x06,  0,  0, 0xffffffff },
{ 0x06,  0,  0, 0000000000 },

In particular, as usage with xt_bpf or cls_bpf can result in more complex BPF
filters that might not be obvious at first, it's good to test filters before
attaching to a live system. For that purpose, there's a small tool called
bpf_dbg under tools/bpf/ in the kernel source directory. This debugger allows
for testing BPF filters against given pcap files, single stepping through the
BPF code on the pcap's packets and to do BPF machine register dumps.

Starting bpf_dbg is trivial and just requires issuing:

# ./bpf_dbg

In case input and output do not equal stdin/stdout, bpf_dbg takes an
alternative stdin source as a first argument, and an alternative stdout
sink as a second one, e.g. `./bpf_dbg test_in.txt test_out.txt`.

Other than that, a particular libreadline configuration can be set via
file "~/.bpf_dbg_init" and the command history is stored in the file
"~/.bpf_dbg_history".

Interaction in bpf_dbg happens through a shell that also has auto-completion
support (follow-up example commands starting with '>' denote bpf_dbg shell).
The usual workflow would be to ...

> load bpf 6,40 0 0 12,21 0 3 2048,48 0 0 23,21 0 1 1,6 0 0 65535,6 0 0 0
  Loads a BPF filter from standard output of bpf_asm, or transformed via
  e.g. `tcpdump -iem1 -ddd port 22 | tr '\n' ','`. Note that for JIT
  debugging (next section), this command creates a temporary socket and
  loads the BPF code into the kernel. Thus, this will also be useful for
  JIT developers.

> load pcap foo.pcap
  Loads standard tcpdump pcap file.

> run [<n>]
bpf passes:1 fails:9
  Runs through all packets from a pcap to account how many passes and fails
  the filter will generate. A limit of packets to traverse can be given.

> disassemble
l0:	ldh [12]
l1:	jeq #0x800, l2, l5
l2:	ldb [23]
l3:	jeq #0x1, l4, l5
l4:	ret #0xffff
l5:	ret #0
  Prints out BPF code disassembly.

> dump
/* { op, jt, jf, k }, */
{ 0x28,  0,  0, 0x0000000c },
{ 0x15,  0,  3, 0x00000800 },
{ 0x30,  0,  0, 0x00000017 },
{ 0x15,  0,  1, 0x00000001 },
{ 0x06,  0,  0, 0x0000ffff },
{ 0x06,  0,  0, 0000000000 },
  Prints out C-style BPF code dump.

> breakpoint 0
breakpoint at: l0:	ldh [12]
> breakpoint 1
breakpoint at: l1:	jeq #0x800, l2, l5
  ...
  Sets breakpoints at particular BPF instructions. Issuing a `run` command
  will walk through the pcap file continuing from the current packet and
  break when a breakpoint is being hit (another `run` will continue from
  the currently active breakpoint executing next instructions):

  > run
  -- register dump --
  pc:       [0]                       <-- program counter
  code:     [40] jt[0] jf[0] k[12]    <-- plain BPF code of current instruction
  curr:     l0:	ldh [12]              <-- disassembly of current instruction
  A:        [00000000][0]             <-- content of A (hex, decimal)
  X:        [00000000][0]             <-- content of X (hex, decimal)
  M[0,15]:  [00000000][0]             <-- folded content of M (hex, decimal)
  -- packet dump --                   <-- Current packet from pcap (hex)
  len: 42
    0: 00 19 cb 55 55 a4 00 14 a4 43 78 69 08 06 00 01
   16: 08 00 06 04 00 01 00 14 a4 43 78 69 0a 3b 01 26
   32: 00 00 00 00 00 00 0a 3b 01 01
  (breakpoint)
  >

> breakpoint
breakpoints: 0 1
  Prints currently set breakpoints.

> step [-<n>, +<n>]
  Performs single stepping through the BPF program from the current pc
  offset. Thus, on each step invocation, above register dump is issued.
  This can go forwards and backwards in time, a plain `step` will break
  on the next BPF instruction, thus +1. (No `run` needs to be issued here.)

> select <n>
  Selects a given packet from the pcap file to continue from. Thus, on
  the next `run` or `step`, the BPF program is being evaluated against
  the user pre-selected packet. Numbering starts just as in Wireshark
  with index 1.

> quit
#
  Exits bpf_dbg.

JIT compiler
------------

The Linux kernel has a built-in BPF JIT compiler for x86_64, SPARC,
PowerPC, ARM, ARM64, MIPS, RISC-V and s390 and can be enabled through
CONFIG_BPF_JIT. The JIT compiler is transparently invoked for each
attached filter from user space or for internal kernel users if it has
been previously enabled by root:

  echo 1 > /proc/sys/net/core/bpf_jit_enable

For JIT developers, doing audits etc, each compile run can output the generated
opcode image into the kernel log via:

  echo 2 > /proc/sys/net/core/bpf_jit_enable

Example output from dmesg:

[ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
[ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
[ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
[ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
[ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
[ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3

When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is permanently set to 1 and
setting any other value than that will return in failure. This is even the case for
setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log
is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the
generally recommended approach instead.

In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
generating disassembly out of the kernel log's hexdump:

# ./bpf_jit_disasm
70 bytes emitted from JIT compiler (pass:3, flen:6)
ffffffffa0069c8f + <x>:
   0:	push   %rbp
   1:	mov    %rsp,%rbp
   4:	sub    $0x60,%rsp
   8:	mov    %rbx,-0x8(%rbp)
   c:	mov    0x68(%rdi),%r9d
  10:	sub    0x6c(%rdi),%r9d
  14:	mov    0xd8(%rdi),%r8
  1b:	mov    $0xc,%esi
  20:	callq  0xffffffffe0ff9442
  25:	cmp    $0x800,%eax
  2a:	jne    0x0000000000000042
  2c:	mov    $0x17,%esi
  31:	callq  0xffffffffe0ff945e
  36:	cmp    $0x1,%eax
  39:	jne    0x0000000000000042
  3b:	mov    $0xffff,%eax
  40:	jmp    0x0000000000000044
  42:	xor    %eax,%eax
  44:	leaveq
  45:	retq

Issuing option `-o` will "annotate" opcodes to resulting assembler
instructions, which can be very useful for JIT developers:

# ./bpf_jit_disasm -o
70 bytes emitted from JIT compiler (pass:3, flen:6)
ffffffffa0069c8f + <x>:
   0:	push   %rbp
	55
   1:	mov    %rsp,%rbp
	48 89 e5
   4:	sub    $0x60,%rsp
	48 83 ec 60
   8:	mov    %rbx,-0x8(%rbp)
	48 89 5d f8
   c:	mov    0x68(%rdi),%r9d
	44 8b 4f 68
  10:	sub    0x6c(%rdi),%r9d
	44 2b 4f 6c
  14:	mov    0xd8(%rdi),%r8
	4c 8b 87 d8 00 00 00
  1b:	mov    $0xc,%esi
	be 0c 00 00 00
  20:	callq  0xffffffffe0ff9442
	e8 1d 94 ff e0
  25:	cmp    $0x800,%eax
	3d 00 08 00 00
  2a:	jne    0x0000000000000042
	75 16
  2c:	mov    $0x17,%esi
	be 17 00 00 00
  31:	callq  0xffffffffe0ff945e
	e8 28 94 ff e0
  36:	cmp    $0x1,%eax
	83 f8 01
  39:	jne    0x0000000000000042
	75 07
  3b:	mov    $0xffff,%eax
	b8 ff ff 00 00
  40:	jmp    0x0000000000000044
	eb 02
  42:	xor    %eax,%eax
	31 c0
  44:	leaveq
	c9
  45:	retq
	c3

For BPF JIT developers, bpf_jit_disasm, bpf_asm and bpf_dbg provides a useful
toolchain for developing and testing the kernel's JIT compiler.

BPF kernel internals
--------------------
Internally, for the kernel interpreter, a different instruction set
format with similar underlying principles from BPF described in previous
paragraphs is being used. However, the instruction set format is modelled
closer to the underlying architecture to mimic native instruction sets, so
that a better performance can be achieved (more details later). This new
ISA is called 'eBPF' or 'internal BPF' interchangeably. (Note: eBPF which
originates from [e]xtended BPF is not the same as BPF extensions! While
eBPF is an ISA, BPF extensions date back to classic BPF's 'overloading'
of BPF_LD | BPF_{B,H,W} | BPF_ABS instruction.)

It is designed to be JITed with one to one mapping, which can also open up
the possibility for GCC/LLVM compilers to generate optimized eBPF code through
an eBPF backend that performs almost as fast as natively compiled code.

The new instruction set was originally designed with the possible goal in
mind to write programs in "restricted C" and compile into eBPF with a optional
GCC/LLVM backend, so that it can just-in-time map to modern 64-bit CPUs with
minimal performance overhead over two steps, that is, C -> eBPF -> native code.

Currently, the new format is being used for running user BPF programs, which
includes seccomp BPF, classic socket filters, cls_bpf traffic classifier,
team driver's classifier for its load-balancing mode, netfilter's xt_bpf
extension, PTP dissector/classifier, and much more. They are all internally
converted by the kernel into the new instruction set representation and run
in the eBPF interpreter. For in-kernel handlers, this all works transparently
by using bpf_prog_create() for setting up the filter, resp.
bpf_prog_destroy() for destroying it. The macro
BPF_PROG_RUN(filter, ctx) transparently invokes eBPF interpreter or JITed
code to run the filter. 'filter' is a pointer to struct bpf_prog that we
got from bpf_prog_create(), and 'ctx' the given context (e.g.
skb pointer). All constraints and restrictions from bpf_check_classic() apply
before a conversion to the new layout is being done behind the scenes!

Currently, the classic BPF format is being used for JITing on most
32-bit architectures, whereas x86-64, aarch64, s390x, powerpc64,
sparc64, arm32, riscv (RV64G) perform JIT compilation from eBPF
instruction set.

Some core changes of the new internal format:

- Number of registers increase from 2 to 10:

  The old format had two registers A and X, and a hidden frame pointer. The
  new layout extends this to be 10 internal registers and a read-only frame
  pointer. Since 64-bit CPUs are passing arguments to functions via registers
  the number of args from eBPF program to in-kernel function is restricted
  to 5 and one register is used to accept return value from an in-kernel
  function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
  sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
  registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.

  Therefore, eBPF calling convention is defined as:

    * R0	- return value from in-kernel function, and exit value for eBPF program
    * R1 - R5	- arguments from eBPF program to in-kernel function
    * R6 - R9	- callee saved registers that in-kernel function will preserve
    * R10	- read-only frame pointer to access stack

  Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64,
  etc, and eBPF calling convention maps directly to ABIs used by the kernel on
  64-bit architectures.

  On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
  and may let more complex programs to be interpreted.

  R0 - R5 are scratch registers and eBPF program needs spill/fill them if
  necessary across calls. Note that there is only one eBPF program (== one
  eBPF main routine) and it cannot call other eBPF functions, it can only
  call predefined in-kernel functions, though.

- Register width increases from 32-bit to 64-bit:

  Still, the semantics of the original 32-bit ALU operations are preserved
  via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower
  subregisters that zero-extend into 64-bit if they are being written to.
  That behavior maps directly to x86_64 and arm64 subregister definition, but
  makes other JITs more difficult.

  32-bit architectures run 64-bit internal BPF programs via interpreter.
  Their JITs may convert BPF programs that only use 32-bit subregisters into
  native instruction set and let the rest being interpreted.

  Operation is 64-bit, because on 64-bit architectures, pointers are also
  64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
  so 32-bit eBPF registers would otherwise require to define register-pair
  ABI, thus, there won't be able to use a direct eBPF register to HW register
  mapping and JIT would need to do combine/split/move operations for every
  register in and out of the function, which is complex, bug prone and slow.
  Another reason is the use of atomic 64-bit counters.

- Conditional jt/jf targets replaced with jt/fall-through:

  While the original design has constructs such as "if (cond) jump_true;
  else jump_false;", they are being replaced into alternative constructs like
  "if (cond) jump_true; /* else fall-through */".

- Introduces bpf_call insn and register passing convention for zero overhead
  calls from/to other kernel functions:

  Before an in-kernel function call, the internal BPF program needs to
  place function arguments into R1 to R5 registers to satisfy calling
  convention, then the interpreter will take them from registers and pass
  to in-kernel function. If R1 - R5 registers are mapped to CPU registers
  that are used for argument passing on given architecture, the JIT compiler
  doesn't need to emit extra moves. Function arguments will be in the correct
  registers and BPF_CALL instruction will be JITed as single 'call' HW
  instruction. This calling convention was picked to cover common call
  situations without performance penalty.

  After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
  a return value of the function. Since R6 - R9 are callee saved, their state
  is preserved across the call.

  For example, consider three C functions:

  u64 f1() { return (*_f2)(1); }
  u64 f2(u64 a) { return f3(a + 1, a); }
  u64 f3(u64 a, u64 b) { return a - b; }

  GCC can compile f1, f3 into x86_64:

  f1:
    movl $1, %edi
    movq _f2(%rip), %rax
    jmp  *%rax
  f3:
    movq %rdi, %rax
    subq %rsi, %rax
    ret

  Function f2 in eBPF may look like:

  f2:
    bpf_mov R2, R1
    bpf_add R1, 1
    bpf_call f3
    bpf_exit

  If f2 is JITed and the pointer stored to '_f2'. The calls f1 -> f2 -> f3 and
  returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to
  be used to call into f2.

  For practical reasons all eBPF programs have only one argument 'ctx' which is
  already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs
  can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
  are currently not supported, but these restrictions can be lifted if necessary
  in the future.

  On 64-bit architectures all register map to HW registers one to one. For
  example, x86_64 JIT compiler can map them as ...

    R0 - rax
    R1 - rdi
    R2 - rsi
    R3 - rdx
    R4 - rcx
    R5 - r8
    R6 - rbx
    R7 - r13
    R8 - r14
    R9 - r15
    R10 - rbp

  ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
  and rbx, r12 - r15 are callee saved.

  Then the following internal BPF pseudo-program:

    bpf_mov R6, R1 /* save ctx */
    bpf_mov R2, 2
    bpf_mov R3, 3
    bpf_mov R4, 4
    bpf_mov R5, 5
    bpf_call foo
    bpf_mov R7, R0 /* save foo() return value */
    bpf_mov R1, R6 /* restore ctx for next call */
    bpf_mov R2, 6
    bpf_mov R3, 7
    bpf_mov R4, 8
    bpf_mov R5, 9
    bpf_call bar
    bpf_add R0, R7
    bpf_exit

  After JIT to x86_64 may look like:

    push %rbp
    mov %rsp,%rbp
    sub $0x228,%rsp
    mov %rbx,-0x228(%rbp)
    mov %r13,-0x220(%rbp)
    mov %rdi,%rbx
    mov $0x2,%esi
    mov $0x3,%edx
    mov $0x4,%ecx
    mov $0x5,%r8d
    callq foo
    mov %rax,%r13
    mov %rbx,%rdi
    mov $0x2,%esi
    mov $0x3,%edx
    mov $0x4,%ecx
    mov $0x5,%r8d
    callq bar
    add %r13,%rax
    mov -0x228(%rbp),%rbx
    mov -0x220(%rbp),%r13
    leaveq
    retq

  Which is in this example equivalent in C to:

    u64 bpf_filter(u64 ctx)
    {
        return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
    }

  In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
  arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
  registers and place their return value into '%rax' which is R0 in eBPF.
  Prologue and epilogue are emitted by JIT and are implicit in the
  interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve
  them across the calls as defined by calling convention.

  For example the following program is invalid:

    bpf_mov R1, 1
    bpf_call foo
    bpf_mov R0, R1
    bpf_exit

  After the call the registers R1-R5 contain junk values and cannot be read.
  An in-kernel eBPF verifier is used to validate internal BPF programs.

Also in the new design, eBPF is limited to 4096 insns, which means that any
program will terminate quickly and will only call a fixed number of kernel
functions. Original BPF and the new format are two operand instructions,
which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT.

The input context pointer for invoking the interpreter function is generic,
its content is defined by a specific use case. For seccomp register R1 points
to seccomp_data, for converted BPF filters R1 points to a skb.

A program, that is translated internally consists of the following elements:

  op:16, jt:8, jf:8, k:32    ==>    op:8, dst_reg:4, src_reg:4, off:16, imm:32

So far 87 internal BPF instructions were implemented. 8-bit 'op' opcode field
has room for new instructions. Some of them may use 16/24/32 byte encoding. New
instructions must be multiple of 8 bytes to preserve backward compatibility.

Internal BPF is a general purpose RISC instruction set. Not every register and
every instruction are used during translation from original BPF to new format.
For example, socket filters are not using 'exclusive add' instruction, but
tracing filters may do to maintain counters of events, for example. Register R9
is not used by socket filters either, but more complex filters may be running
out of registers and would have to resort to spill/fill to stack.

Internal BPF can be used as a generic assembler for last step performance
optimizations, socket filters and seccomp are using it as assembler. Tracing
filters may use it as assembler to generate code from kernel. In kernel usage
may not be bounded by security considerations, since generated internal BPF code
may be optimizing internal code path and not being exposed to the user space.
Safety of internal BPF can come from a verifier (TBD). In such use cases as
described, it may be used as safe instruction set.

Just like the original BPF, the new format runs within a controlled environment,
is deterministic and the kernel can easily prove that. The safety of the program
can be determined in two steps: first step does depth-first-search to disallow
loops and other CFG validation; second step starts from the first insn and
descends all possible paths. It simulates execution of every insn and observes
the state change of registers and stack.

eBPF opcode encoding
--------------------

eBPF is reusing most of the opcode encoding from classic to simplify conversion
of classic BPF to eBPF. For arithmetic and jump instructions the 8-bit 'code'
field is divided into three parts:

  +----------------+--------+--------------------+
  |   4 bits       |  1 bit |   3 bits           |
  | operation code | source | instruction class  |
  +----------------+--------+--------------------+
  (MSB)                                      (LSB)

Three LSB bits store instruction class which is one of:

  Classic BPF classes:    eBPF classes:

  BPF_LD    0x00          BPF_LD    0x00
  BPF_LDX   0x01          BPF_LDX   0x01
  BPF_ST    0x02          BPF_ST    0x02
  BPF_STX   0x03          BPF_STX   0x03
  BPF_ALU   0x04          BPF_ALU   0x04
  BPF_JMP   0x05          BPF_JMP   0x05
  BPF_RET   0x06          BPF_JMP32 0x06
  BPF_MISC  0x07          BPF_ALU64 0x07

When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ...

  BPF_K     0x00
  BPF_X     0x08

 * in classic BPF, this means:

  BPF_SRC(code) == BPF_X - use register X as source operand
  BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand

 * in eBPF, this means:

  BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand
  BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand

... and four MSB bits store operation code.

If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of:

  BPF_ADD   0x00
  BPF_SUB   0x10
  BPF_MUL   0x20
  BPF_DIV   0x30
  BPF_OR    0x40
  BPF_AND   0x50
  BPF_LSH   0x60
  BPF_RSH   0x70
  BPF_NEG   0x80
  BPF_MOD   0x90
  BPF_XOR   0xa0
  BPF_MOV   0xb0  /* eBPF only: mov reg to reg */
  BPF_ARSH  0xc0  /* eBPF only: sign extending shift right */
  BPF_END   0xd0  /* eBPF only: endianness conversion */

If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 [ in eBPF ], BPF_OP(code) is one of:

  BPF_JA    0x00  /* BPF_JMP only */
  BPF_JEQ   0x10
  BPF_JGT   0x20
  BPF_JGE   0x30
  BPF_JSET  0x40
  BPF_JNE   0x50  /* eBPF only: jump != */
  BPF_JSGT  0x60  /* eBPF only: signed '>' */
  BPF_JSGE  0x70  /* eBPF only: signed '>=' */
  BPF_CALL  0x80  /* eBPF BPF_JMP only: function call */
  BPF_EXIT  0x90  /* eBPF BPF_JMP only: function return */
  BPF_JLT   0xa0  /* eBPF only: unsigned '<' */
  BPF_JLE   0xb0  /* eBPF only: unsigned '<=' */
  BPF_JSLT  0xc0  /* eBPF only: signed '<' */
  BPF_JSLE  0xd0  /* eBPF only: signed '<=' */

So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF
and eBPF. There are only two registers in classic BPF, so it means A += X.
In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly,
BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous
src_reg = (u32) src_reg ^ (u32) imm32 in eBPF.

Classic BPF is using BPF_MISC class to represent A = X and X = A moves.
eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no
BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean
exactly the same operations as BPF_ALU, but with 64-bit wide operands
instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.:
dst_reg = dst_reg + src_reg

Classic BPF wastes the whole BPF_RET class to represent a single 'ret'
operation. Classic BPF_RET | BPF_K means copy imm32 into return register
and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT
in eBPF means function exit only. The eBPF program needs to store return
value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is used as
BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
operands for the comparisons instead.

For load and store instructions the 8-bit 'code' field is divided as:

  +--------+--------+-------------------+
  | 3 bits | 2 bits |   3 bits          |
  |  mode  |  size  | instruction class |
  +--------+--------+-------------------+
  (MSB)                             (LSB)

Size modifier is one of ...

  BPF_W   0x00    /* word */
  BPF_H   0x08    /* half word */
  BPF_B   0x10    /* byte */
  BPF_DW  0x18    /* eBPF only, double word */

... which encodes size of load/store operation:

 B  - 1 byte
 H  - 2 byte
 W  - 4 byte
 DW - 8 byte (eBPF only)

Mode modifier is one of:

  BPF_IMM  0x00  /* used for 32-bit mov in classic BPF and 64-bit in eBPF */
  BPF_ABS  0x20
  BPF_IND  0x40
  BPF_MEM  0x60
  BPF_LEN  0x80  /* classic BPF only, reserved in eBPF */
  BPF_MSH  0xa0  /* classic BPF only, reserved in eBPF */
  BPF_XADD 0xc0  /* eBPF only, exclusive add */

eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and
(BPF_IND | <size> | BPF_LD) which are used to access packet data.

They had to be carried over from classic to have strong performance of
socket filters running in eBPF interpreter. These instructions can only
be used when interpreter context is a pointer to 'struct sk_buff' and
have seven implicit operands. Register R6 is an implicit input that must
contain pointer to sk_buff. Register R0 is an implicit output which contains
the data fetched from the packet. Registers R1-R5 are scratch registers
and must not be used to store the data across BPF_ABS | BPF_LD or
BPF_IND | BPF_LD instructions.

These instructions have implicit program exit condition as well. When
eBPF program is trying to access the data beyond the packet boundary,
the interpreter will abort the execution of the program. JIT compilers
therefore must preserve this property. src_reg and imm32 fields are
explicit inputs to these instructions.

For example:

  BPF_IND | BPF_W | BPF_LD means:

    R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
    and R1 - R5 were scratched.

Unlike classic BPF instruction set, eBPF has generic load/store operations:

BPF_MEM | <size> | BPF_STX:  *(size *) (dst_reg + off) = src_reg
BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32
BPF_MEM | <size> | BPF_LDX:  dst_reg = *(size *) (src_reg + off)
BPF_XADD | BPF_W  | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
BPF_XADD | BPF_DW | BPF_STX: lock xadd *(u64 *)(dst_reg + off16) += src_reg

Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. Note that 1 and
2 byte atomic increments are not supported.

eBPF has one 16-byte instruction: BPF_LD | BPF_DW | BPF_IMM which consists
of two consecutive 'struct bpf_insn' 8-byte blocks and interpreted as single
instruction that loads 64-bit immediate value into a dst_reg.
Classic BPF has similar instruction: BPF_LD | BPF_W | BPF_IMM which loads
32-bit immediate value into a register.

eBPF verifier
-------------
The safety of the eBPF program is determined in two steps.

First step does DAG check to disallow loops and other CFG validation.
In particular it will detect programs that have unreachable instructions.
(though classic BPF checker allows them)

Second step starts from the first insn and descends all possible paths.
It simulates execution of every insn and observes the state change of
registers and stack.

At the start of the program the register R1 contains a pointer to context
and has type PTR_TO_CTX.
If verifier sees an insn that does R2=R1, then R2 has now type
PTR_TO_CTX as well and can be used on the right hand side of expression.
If R1=PTR_TO_CTX and insn is R2=R1+R1, then R2=SCALAR_VALUE,
since addition of two valid pointers makes invalid pointer.
(In 'secure' mode verifier will reject any type of pointer arithmetic to make
sure that kernel addresses don't leak to unprivileged users)

If register was never written to, it's not readable:
  bpf_mov R0 = R2
  bpf_exit
will be rejected, since R2 is unreadable at the start of the program.

After kernel function call, R1-R5 are reset to unreadable and
R0 has a return type of the function.

Since R6-R9 are callee saved, their state is preserved across the call.
  bpf_mov R6 = 1
  bpf_call foo
  bpf_mov R0 = R6
  bpf_exit
is a correct program. If there was R1 instead of R6, it would have
been rejected.

load/store instructions are allowed only with registers of valid types, which
are PTR_TO_CTX, PTR_TO_MAP, PTR_TO_STACK. They are bounds and alignment checked.
For example:
 bpf_mov R1 = 1
 bpf_mov R2 = 2
 bpf_xadd *(u32 *)(R1 + 3) += R2
 bpf_exit
will be rejected, since R1 doesn't have a valid pointer type at the time of
execution of instruction bpf_xadd.

At the start R1 type is PTR_TO_CTX (a pointer to generic 'struct bpf_context')
A callback is used to customize verifier to restrict eBPF program access to only
certain fields within ctx structure with specified size and alignment.

For example, the following insn:
  bpf_ld R0 = *(u32 *)(R6 + 8)
intends to load a word from address R6 + 8 and store it into R0
If R6=PTR_TO_CTX, via is_valid_access() callback the verifier will know
that offset 8 of size 4 bytes can be accessed for reading, otherwise
the verifier will reject the program.
If R6=PTR_TO_STACK, then access should be aligned and be within
stack bounds, which are [-MAX_BPF_STACK, 0). In this example offset is 8,
so it will fail verification, since it's out of bounds.

The verifier will allow eBPF program to read data from stack only after
it wrote into it.
Classic BPF verifier does similar check with M[0-15] memory slots.
For example:
  bpf_ld R0 = *(u32 *)(R10 - 4)
  bpf_exit
is invalid program.
Though R10 is correct read-only register and has type PTR_TO_STACK
and R10 - 4 is within stack bounds, there were no stores into that location.

Pointer register spill/fill is tracked as well, since four (R6-R9)
callee saved registers may not be enough for some programs.

Allowed function calls are customized with bpf_verifier_ops->get_func_proto()
The eBPF verifier will check that registers match argument constraints.
After the call register R0 will be set to return type of the function.

Function calls is a main mechanism to extend functionality of eBPF programs.
Socket filters may let programs to call one set of functions, whereas tracing
filters may allow completely different set.

If a function made accessible to eBPF program, it needs to be thought through
from safety point of view. The verifier will guarantee that the function is
called with valid arguments.

seccomp vs socket filters have different security restrictions for classic BPF.
Seccomp solves this by two stage verifier: classic BPF verifier is followed
by seccomp verifier. In case of eBPF one configurable verifier is shared for
all use cases.

See details of eBPF verifier in kernel/bpf/verifier.c

Register value tracking
-----------------------
In order to determine the safety of an eBPF program, the verifier must track
the range of possible values in each register and also in each stack slot.
This is done with 'struct bpf_reg_state', defined in include/linux/
bpf_verifier.h, which unifies tracking of scalar and pointer values.  Each
register state has a type, which is either NOT_INIT (the register has not been
written to), SCALAR_VALUE (some value which is not usable as a pointer), or a
pointer type.  The types of pointers describe their base, as follows:
    PTR_TO_CTX          Pointer to bpf_context.
    CONST_PTR_TO_MAP    Pointer to struct bpf_map.  "Const" because arithmetic
                        on these pointers is forbidden.
    PTR_TO_MAP_VALUE    Pointer to the value stored in a map element.
    PTR_TO_MAP_VALUE_OR_NULL
                        Either a pointer to a map value, or NULL; map accesses
                        (see section 'eBPF maps', below) return this type,
                        which becomes a PTR_TO_MAP_VALUE when checked != NULL.
                        Arithmetic on these pointers is forbidden.
    PTR_TO_STACK        Frame pointer.
    PTR_TO_PACKET       skb->data.
    PTR_TO_PACKET_END   skb->data + headlen; arithmetic forbidden.
    PTR_TO_SOCKET       Pointer to struct bpf_sock_ops, implicitly refcounted.
    PTR_TO_SOCKET_OR_NULL
                        Either a pointer to a socket, or NULL; socket lookup
                        returns this type, which becomes a PTR_TO_SOCKET when
                        checked != NULL. PTR_TO_SOCKET is reference-counted,
                        so programs must release the reference through the
                        socket release function before the end of the program.
                        Arithmetic on these pointers is forbidden.
However, a pointer may be offset from this base (as a result of pointer
arithmetic), and this is tracked in two parts: the 'fixed offset' and 'variable
offset'.  The former is used when an exactly-known value (e.g. an immediate
operand) is added to a pointer, while the latter is used for values which are
not exactly known.  The variable offset is also used in SCALAR_VALUEs, to track
the range of possible values in the register.
The verifier's knowledge about the variable offset consists of:
* minimum and maximum values as unsigned
* minimum and maximum values as signed
* knowledge of the values of individual bits, in the form of a 'tnum': a u64
'mask' and a u64 'value'.  1s in the mask represent bits whose value is unknown;
1s in the value represent bits known to be 1.  Bits known to be 0 have 0 in both
mask and value; no bit should ever be 1 in both.  For example, if a byte is read
into a register from memory, the register's top 56 bits are known zero, while
the low 8 are unknown - which is represented as the tnum (0x0; 0xff).  If we
then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0;
0x1ff), because of potential carries.

Besides arithmetic, the register state can also be updated by conditional
branches.  For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch
it will have a umin_value (unsigned minimum value) of 9, whereas in the 'false'
branch it will have a umax_value of 8.  A signed compare (with BPF_JSGT or
BPF_JSGE) would instead update the signed minimum/maximum values.  Information
from the signed and unsigned bounds can be combined; for instance if a value is
first tested < 8 and then tested s> 4, the verifier will conclude that the value
is also > 4 and s< 8, since the bounds prevent crossing the sign boundary.

PTR_TO_PACKETs with a variable offset part have an 'id', which is common to all
pointers sharing that same variable offset.  This is important for packet range
checks: after adding a variable to a packet pointer register A, if you then copy
it to another register B and then add a constant 4 to A, both registers will
share the same 'id' but the A will have a fixed offset of +4.  Then if A is
bounds-checked and found to be less than a PTR_TO_PACKET_END, the register B is
now known to have a safe range of at least 4 bytes.  See 'Direct packet access',
below, for more on PTR_TO_PACKET ranges.

The 'id' field is also used on PTR_TO_MAP_VALUE_OR_NULL, common to all copies of
the pointer returned from a map lookup.  This means that when one copy is
checked and found to be non-NULL, all copies can become PTR_TO_MAP_VALUEs.
As well as range-checking, the tracked information is also used for enforcing
alignment of pointer accesses.  For instance, on most systems the packet pointer
is 2 bytes after a 4-byte alignment.  If a program adds 14 bytes to that to jump
over the Ethernet header, then reads IHL and addes (IHL * 4), the resulting
pointer will have a variable offset known to be 4n+2 for some n, so adding the 2
bytes (NET_IP_ALIGN) gives a 4-byte alignment and so word-sized accesses through
that pointer are safe.
The 'id' field is also used on PTR_TO_SOCKET and PTR_TO_SOCKET_OR_NULL, common
to all copies of the pointer returned from a socket lookup. This has similar
behaviour to the handling for PTR_TO_MAP_VALUE_OR_NULL->PTR_TO_MAP_VALUE, but
it also handles reference tracking for the pointer. PTR_TO_SOCKET implicitly
represents a reference to the corresponding 'struct sock'. To ensure that the
reference is not leaked, it is imperative to NULL-check the reference and in
the non-NULL case, and pass the valid reference to the socket release function.

Direct packet access
--------------------
In cls_bpf and act_bpf programs the verifier allows direct access to the packet
data via skb->data and skb->data_end pointers.
Ex:
1:  r4 = *(u32 *)(r1 +80)  /* load skb->data_end */
2:  r3 = *(u32 *)(r1 +76)  /* load skb->data */
3:  r5 = r3
4:  r5 += 14
5:  if r5 > r4 goto pc+16
R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
6:  r0 = *(u16 *)(r3 +12) /* access 12 and 13 bytes of the packet */

this 2byte load from the packet is safe to do, since the program author
did check 'if (skb->data + 14 > skb->data_end) goto err' at insn #5 which
means that in the fall-through case the register R3 (which points to skb->data)
has at least 14 directly accessible bytes. The verifier marks it
as R3=pkt(id=0,off=0,r=14).
id=0 means that no additional variables were added to the register.
off=0 means that no additional constants were added.
r=14 is the range of safe access which means that bytes [R3, R3 + 14) are ok.
Note that R5 is marked as R5=pkt(id=0,off=14,r=14). It also points
to the packet data, but constant 14 was added to the register, so
it now points to 'skb->data + 14' and accessible range is [R5, R5 + 14 - 14)
which is zero bytes.

More complex packet access may look like:
 R0=inv1 R1=ctx R3=pkt(id=0,off=0,r=14) R4=pkt_end R5=pkt(id=0,off=14,r=14) R10=fp
 6:  r0 = *(u8 *)(r3 +7) /* load 7th byte from the packet */
 7:  r4 = *(u8 *)(r3 +12)
 8:  r4 *= 14
 9:  r3 = *(u32 *)(r1 +76) /* load skb->data */
10:  r3 += r4
11:  r2 = r1
12:  r2 <<= 48
13:  r2 >>= 48
14:  r3 += r2
15:  r2 = r3
16:  r2 += 8
17:  r1 = *(u32 *)(r1 +80) /* load skb->data_end */
18:  if r2 > r1 goto pc+2
 R0=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R1=pkt_end R2=pkt(id=2,off=8,r=8) R3=pkt(id=2,off=0,r=8) R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)) R5=pkt(id=0,off=14,r=14) R10=fp
19:  r1 = *(u8 *)(r3 +4)
The state of the register R3 is R3=pkt(id=2,off=0,r=8)
id=2 means that two 'r3 += rX' instructions were seen, so r3 points to some
offset within a packet and since the program author did
'if (r3 + 8 > r1) goto err' at insn #18, the safe range is [R3, R3 + 8).
The verifier only allows 'add'/'sub' operations on packet registers. Any other
operation will set the register state to 'SCALAR_VALUE' and it won't be
available for direct packet access.
Operation 'r3 += rX' may overflow and become less than original skb->data,
therefore the verifier has to prevent that.  So when it sees 'r3 += rX'
instruction and rX is more than 16-bit value, any subsequent bounds-check of r3
against skb->data_end will not give us 'range' information, so attempts to read
through the pointer will give "invalid access to packet" error.
Ex. after insn 'r4 = *(u8 *)(r3 +12)' (insn #7 above) the state of r4 is
R4=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) which means that upper 56 bits
of the register are guaranteed to be zero, and nothing is known about the lower
8 bits. After insn 'r4 *= 14' the state becomes
R4=inv(id=0,umax_value=3570,var_off=(0x0; 0xfffe)), since multiplying an 8-bit
value by constant 14 will keep upper 52 bits as zero, also the least significant
bit will be zero as 14 is even.  Similarly 'r2 >>= 48' will make
R2=inv(id=0,umax_value=65535,var_off=(0x0; 0xffff)), since the shift is not sign
extending.  This logic is implemented in adjust_reg_min_max_vals() function,
which calls adjust_ptr_min_max_vals() for adding pointer to scalar (or vice
versa) and adjust_scalar_min_max_vals() for operations on two scalars.

The end result is that bpf program author can access packet directly
using normal C code as:
  void *data = (void *)(long)skb->data;
  void *data_end = (void *)(long)skb->data_end;
  struct eth_hdr *eth = data;
  struct iphdr *iph = data + sizeof(*eth);
  struct udphdr *udp = data + sizeof(*eth) + sizeof(*iph);

  if (data + sizeof(*eth) + sizeof(*iph) + sizeof(*udp) > data_end)
          return 0;
  if (eth->h_proto != htons(ETH_P_IP))
          return 0;
  if (iph->protocol != IPPROTO_UDP || iph->ihl != 5)
          return 0;
  if (udp->dest == 53 || udp->source == 9)
          ...;
which makes such programs easier to write comparing to LD_ABS insn
and significantly faster.

eBPF maps
---------
'maps' is a generic storage of different types for sharing data between kernel
and userspace.

The maps are accessed from user space via BPF syscall, which has commands:
- create a map with given type and attributes
  map_fd = bpf(BPF_MAP_CREATE, union bpf_attr *attr, u32 size)
  using attr->map_type, attr->key_size, attr->value_size, attr->max_entries
  returns process-local file descriptor or negative error

- lookup key in a given map
  err = bpf(BPF_MAP_LOOKUP_ELEM, union bpf_attr *attr, u32 size)
  using attr->map_fd, attr->key, attr->value
  returns zero and stores found elem into value or negative error

- create or update key/value pair in a given map
  err = bpf(BPF_MAP_UPDATE_ELEM, union bpf_attr *attr, u32 size)
  using attr->map_fd, attr->key, attr->value
  returns zero or negative error

- find and delete element by key in a given map
  err = bpf(BPF_MAP_DELETE_ELEM, union bpf_attr *attr, u32 size)
  using attr->map_fd, attr->key

- to delete map: close(fd)
  Exiting process will delete maps automatically

userspace programs use this syscall to create/access maps that eBPF programs
are concurrently updating.

maps can have different types: hash, array, bloom filter, radix-tree, etc.

The map is defined by:
  . type
  . max number of elements
  . key size in bytes
  . value size in bytes

Pruning
-------
The verifier does not actually walk all possible paths through the program.  For
each new branch to analyse, the verifier looks at all the states it's previously
been in when at this instruction.  If any of them contain the current state as a
subset, the branch is 'pruned' - that is, the fact that the previous state was
accepted implies the current state would be as well.  For instance, if in the
previous state, r1 held a packet-pointer, and in the current state, r1 holds a
packet-pointer with a range as long or longer and at least as strict an
alignment, then r1 is safe.  Similarly, if r2 was NOT_INIT before then it can't
have been used by any path from that point, so any value in r2 (including
another NOT_INIT) is safe.  The implementation is in the function regsafe().
Pruning considers not only the registers but also the stack (and any spilled
registers it may hold).  They must all be safe for the branch to be pruned.
This is implemented in states_equal().

Understanding eBPF verifier messages
------------------------------------

The following are few examples of invalid eBPF programs and verifier error
messages as seen in the log:

Program with unreachable instructions:
static struct bpf_insn prog[] = {
  BPF_EXIT_INSN(),
  BPF_EXIT_INSN(),
};
Error:
  unreachable insn 1

Program that reads uninitialized register:
  BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
  BPF_EXIT_INSN(),
Error:
  0: (bf) r0 = r2
  R2 !read_ok

Program that doesn't initialize R0 before exiting:
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_1),
  BPF_EXIT_INSN(),
Error:
  0: (bf) r2 = r1
  1: (95) exit
  R0 !read_ok

Program that accesses stack out of bounds:
  BPF_ST_MEM(BPF_DW, BPF_REG_10, 8, 0),
  BPF_EXIT_INSN(),
Error:
  0: (7a) *(u64 *)(r10 +8) = 0
  invalid stack off=8 size=8

Program that doesn't initialize stack before passing its address into function:
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_LD_MAP_FD(BPF_REG_1, 0),
  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
  BPF_EXIT_INSN(),
Error:
  0: (bf) r2 = r10
  1: (07) r2 += -8
  2: (b7) r1 = 0x0
  3: (85) call 1
  invalid indirect read from stack off -8+0 size 8

Program that uses invalid map_fd=0 while calling to map_lookup_elem() function:
  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_LD_MAP_FD(BPF_REG_1, 0),
  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
  BPF_EXIT_INSN(),
Error:
  0: (7a) *(u64 *)(r10 -8) = 0
  1: (bf) r2 = r10
  2: (07) r2 += -8
  3: (b7) r1 = 0x0
  4: (85) call 1
  fd 0 is not pointing to valid bpf_map

Program that doesn't check return value of map_lookup_elem() before accessing
map element:
  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_LD_MAP_FD(BPF_REG_1, 0),
  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
  BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
  BPF_EXIT_INSN(),
Error:
  0: (7a) *(u64 *)(r10 -8) = 0
  1: (bf) r2 = r10
  2: (07) r2 += -8
  3: (b7) r1 = 0x0
  4: (85) call 1
  5: (7a) *(u64 *)(r0 +0) = 0
  R0 invalid mem access 'map_value_or_null'

Program that correctly checks map_lookup_elem() returned value for NULL, but
accesses the memory with incorrect alignment:
  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_LD_MAP_FD(BPF_REG_1, 0),
  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
  BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 1),
  BPF_ST_MEM(BPF_DW, BPF_REG_0, 4, 0),
  BPF_EXIT_INSN(),
Error:
  0: (7a) *(u64 *)(r10 -8) = 0
  1: (bf) r2 = r10
  2: (07) r2 += -8
  3: (b7) r1 = 1
  4: (85) call 1
  5: (15) if r0 == 0x0 goto pc+1
   R0=map_ptr R10=fp
  6: (7a) *(u64 *)(r0 +4) = 0
  misaligned access off 4 size 8

Program that correctly checks map_lookup_elem() returned value for NULL and
accesses memory with correct alignment in one side of 'if' branch, but fails
to do so in the other side of 'if' branch:
  BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_LD_MAP_FD(BPF_REG_1, 0),
  BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
  BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2),
  BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 0),
  BPF_EXIT_INSN(),
  BPF_ST_MEM(BPF_DW, BPF_REG_0, 0, 1),
  BPF_EXIT_INSN(),
Error:
  0: (7a) *(u64 *)(r10 -8) = 0
  1: (bf) r2 = r10
  2: (07) r2 += -8
  3: (b7) r1 = 1
  4: (85) call 1
  5: (15) if r0 == 0x0 goto pc+2
   R0=map_ptr R10=fp
  6: (7a) *(u64 *)(r0 +0) = 0
  7: (95) exit

  from 5 to 8: R0=imm0 R10=fp
  8: (7a) *(u64 *)(r0 +0) = 1
  R0 invalid mem access 'imm'

Program that performs a socket lookup then sets the pointer to NULL without
checking it:
value:
  BPF_MOV64_IMM(BPF_REG_2, 0),
  BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_MOV64_IMM(BPF_REG_3, 4),
  BPF_MOV64_IMM(BPF_REG_4, 0),
  BPF_MOV64_IMM(BPF_REG_5, 0),
  BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
  BPF_MOV64_IMM(BPF_REG_0, 0),
  BPF_EXIT_INSN(),
Error:
  0: (b7) r2 = 0
  1: (63) *(u32 *)(r10 -8) = r2
  2: (bf) r2 = r10
  3: (07) r2 += -8
  4: (b7) r3 = 4
  5: (b7) r4 = 0
  6: (b7) r5 = 0
  7: (85) call bpf_sk_lookup_tcp#65
  8: (b7) r0 = 0
  9: (95) exit
  Unreleased reference id=1, alloc_insn=7

Program that performs a socket lookup but does not NULL-check the returned
value:
  BPF_MOV64_IMM(BPF_REG_2, 0),
  BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_2, -8),
  BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
  BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
  BPF_MOV64_IMM(BPF_REG_3, 4),
  BPF_MOV64_IMM(BPF_REG_4, 0),
  BPF_MOV64_IMM(BPF_REG_5, 0),
  BPF_EMIT_CALL(BPF_FUNC_sk_lookup_tcp),
  BPF_EXIT_INSN(),
Error:
  0: (b7) r2 = 0
  1: (63) *(u32 *)(r10 -8) = r2
  2: (bf) r2 = r10
  3: (07) r2 += -8
  4: (b7) r3 = 4
  5: (b7) r4 = 0
  6: (b7) r5 = 0
  7: (85) call bpf_sk_lookup_tcp#65
  8: (95) exit
  Unreleased reference id=1, alloc_insn=7

Testing
-------

Next to the BPF toolchain, the kernel also ships a test module that contains
various test cases for classic and internal BPF that can be executed against
the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and
enabled via Kconfig:

  CONFIG_TEST_BPF=m

After the module has been built and installed, the test suite can be executed
via insmod or modprobe against 'test_bpf' module. Results of the test cases
including timings in nsec can be found in the kernel log (dmesg).

Misc
----

Also trinity, the Linux syscall fuzzer, has built-in support for BPF and
SECCOMP-BPF kernel fuzzing.

Written by
----------

The document was written in the hope that it is found useful and in order
to give potential BPF hackers or security auditors a better overview of
the underlying architecture.

Jay Schulist <jschlst@samba.org>
Daniel Borkmann <daniel@iogearbox.net>
Alexei Starovoitov <ast@kernel.org>