summaryrefslogtreecommitdiff
path: root/Documentation/core-api/printk-formats.rst
blob: 4346ae17a72c356ee640406636cc0db709e8dbf8 (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
=========================================
How to get printk format specifiers right
=========================================

.. _printk-specifiers:

:Author: Randy Dunlap <rdunlap@infradead.org>
:Author: Andrew Murray <amurray@mpc-data.co.uk>


Integer types
=============

::

	If variable is of Type,		use printk format specifier:
	------------------------------------------------------------
		char			%d or %x
		unsigned char		%u or %x
		short int		%d or %x
		unsigned short int	%u or %x
		int			%d or %x
		unsigned int		%u or %x
		long			%ld or %lx
		unsigned long		%lu or %lx
		long long		%lld or %llx
		unsigned long long	%llu or %llx
		size_t			%zu or %zx
		ssize_t			%zd or %zx
		s8			%d or %x
		u8			%u or %x
		s16			%d or %x
		u16			%u or %x
		s32			%d or %x
		u32			%u or %x
		s64			%lld or %llx
		u64			%llu or %llx


If <type> is architecture-dependent for its size (e.g., cycles_t, tcflag_t) or
is dependent on a config option for its size (e.g., blk_status_t), use a format
specifier of its largest possible type and explicitly cast to it.

Example::

	printk("test: latency: %llu cycles\n", (unsigned long long)time);

Reminder: sizeof() returns type size_t.

The kernel's printf does not support %n. Floating point formats (%e, %f,
%g, %a) are also not recognized, for obvious reasons. Use of any
unsupported specifier or length qualifier results in a WARN and early
return from vsnprintf().

Pointer types
=============

A raw pointer value may be printed with %p which will hash the address
before printing. The kernel also supports extended specifiers for printing
pointers of different types.

Some of the extended specifiers print the data on the given address instead
of printing the address itself. In this case, the following error messages
might be printed instead of the unreachable information::

	(null)	 data on plain NULL address
	(efault) data on invalid address
	(einval) invalid data on a valid address

Plain Pointers
--------------

::

	%p	abcdef12 or 00000000abcdef12

Pointers printed without a specifier extension (i.e unadorned %p) are
hashed to prevent leaking information about the kernel memory layout. This
has the added benefit of providing a unique identifier. On 64-bit machines
the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it
gathers enough entropy.

When possible, use specialised modifiers such as %pS or %pB (described below)
to avoid the need of providing an unhashed address that has to be interpreted
post-hoc. If not possible, and the aim of printing the address is to provide
more information for debugging, use %p and boot the kernel with the
``no_hash_pointers`` parameter during debugging, which will print all %p
addresses unmodified. If you *really* always want the unmodified address, see
%px below.

If (and only if) you are printing addresses as a content of a virtual file in
e.g. procfs or sysfs (using e.g. seq_printf(), not printk()) read by a
userspace process, use the %pK modifier described below instead of %p or %px.

Error Pointers
--------------

::

	%pe	-ENOSPC

For printing error pointers (i.e. a pointer for which IS_ERR() is true)
as a symbolic error name. Error values for which no symbolic name is
known are printed in decimal, while a non-ERR_PTR passed as the
argument to %pe gets treated as ordinary %p.

Symbols/Function Pointers
-------------------------

::

	%pS	versatile_init+0x0/0x110
	%ps	versatile_init
	%pSR	versatile_init+0x9/0x110
		(with __builtin_extract_return_addr() translation)
	%pB	prev_fn_of_versatile_init+0x88/0x88


The ``S`` and ``s`` specifiers are used for printing a pointer in symbolic
format. They result in the symbol name with (S) or without (s)
offsets. If KALLSYMS are disabled then the symbol address is printed instead.

The ``B`` specifier results in the symbol name with offsets and should be
used when printing stack backtraces. The specifier takes into
consideration the effect of compiler optimisations which may occur
when tail-calls are used and marked with the noreturn GCC attribute.

Probed Pointers from BPF / tracing
----------------------------------

::

	%pks	kernel string
	%pus	user string

The ``k`` and ``u`` specifiers are used for printing prior probed memory from
either kernel memory (k) or user memory (u). The subsequent ``s`` specifier
results in printing a string. For direct use in regular vsnprintf() the (k)
and (u) annotation is ignored, however, when used out of BPF's bpf_trace_printk(),
for example, it reads the memory it is pointing to without faulting.

Kernel Pointers
---------------

::

	%pK	01234567 or 0123456789abcdef

For printing kernel pointers which should be hidden from unprivileged
users. The behaviour of %pK depends on the kptr_restrict sysctl - see
Documentation/admin-guide/sysctl/kernel.rst for more details.

This modifier is *only* intended when producing content of a file read by
userspace from e.g. procfs or sysfs, not for dmesg. Please refer to the
section about %p above for discussion about how to manage hashing pointers
in printk().

Unmodified Addresses
--------------------

::

	%px	01234567 or 0123456789abcdef

For printing pointers when you *really* want to print the address. Please
consider whether or not you are leaking sensitive information about the
kernel memory layout before printing pointers with %px. %px is functionally
equivalent to %lx (or %lu). %px is preferred because it is more uniquely
grep'able. If in the future we need to modify the way the kernel handles
printing pointers we will be better equipped to find the call sites.

Before using %px, consider if using %p is sufficient together with enabling the
``no_hash_pointers`` kernel parameter during debugging sessions (see the %p
description above). One valid scenario for %px might be printing information
immediately before a panic, which prevents any sensitive information to be
exploited anyway, and with %px there would be no need to reproduce the panic
with no_hash_pointers.

Pointer Differences
-------------------

::

	%td	2560
	%tx	a00

For printing the pointer differences, use the %t modifier for ptrdiff_t.

Example::

	printk("test: difference between pointers: %td\n", ptr2 - ptr1);

Struct Resources
----------------

::

	%pr	[mem 0x60000000-0x6fffffff flags 0x2200] or
		[mem 0x0000000060000000-0x000000006fffffff flags 0x2200]
	%pR	[mem 0x60000000-0x6fffffff pref] or
		[mem 0x0000000060000000-0x000000006fffffff pref]

For printing struct resources. The ``R`` and ``r`` specifiers result in a
printed resource with (R) or without (r) a decoded flags member.

Passed by reference.

Physical address types phys_addr_t
----------------------------------

::

	%pa[p]	0x01234567 or 0x0123456789abcdef

For printing a phys_addr_t type (and its derivatives, such as
resource_size_t) which can vary based on build options, regardless of the
width of the CPU data path.

Passed by reference.

DMA address types dma_addr_t
----------------------------

::

	%pad	0x01234567 or 0x0123456789abcdef

For printing a dma_addr_t type which can vary based on build options,
regardless of the width of the CPU data path.

Passed by reference.

Raw buffer as an escaped string
-------------------------------

::

	%*pE[achnops]

For printing raw buffer as an escaped string. For the following buffer::

		1b 62 20 5c 43 07 22 90 0d 5d

A few examples show how the conversion would be done (excluding surrounding
quotes)::

		%*pE		"\eb \C\a"\220\r]"
		%*pEhp		"\x1bb \C\x07"\x90\x0d]"
		%*pEa		"\e\142\040\\\103\a\042\220\r\135"

The conversion rules are applied according to an optional combination
of flags (see :c:func:`string_escape_mem` kernel documentation for the
details):

	- a - ESCAPE_ANY
	- c - ESCAPE_SPECIAL
	- h - ESCAPE_HEX
	- n - ESCAPE_NULL
	- o - ESCAPE_OCTAL
	- p - ESCAPE_NP
	- s - ESCAPE_SPACE

By default ESCAPE_ANY_NP is used.

ESCAPE_ANY_NP is the sane choice for many cases, in particularly for
printing SSIDs.

If field width is omitted then 1 byte only will be escaped.

Raw buffer as a hex string
--------------------------

::

	%*ph	00 01 02  ...  3f
	%*phC	00:01:02: ... :3f
	%*phD	00-01-02- ... -3f
	%*phN	000102 ... 3f

For printing small buffers (up to 64 bytes long) as a hex string with a
certain separator. For larger buffers consider using
:c:func:`print_hex_dump`.

MAC/FDDI addresses
------------------

::

	%pM	00:01:02:03:04:05
	%pMR	05:04:03:02:01:00
	%pMF	00-01-02-03-04-05
	%pm	000102030405
	%pmR	050403020100

For printing 6-byte MAC/FDDI addresses in hex notation. The ``M`` and ``m``
specifiers result in a printed address with (M) or without (m) byte
separators. The default byte separator is the colon (:).

Where FDDI addresses are concerned the ``F`` specifier can be used after
the ``M`` specifier to use dash (-) separators instead of the default
separator.

For Bluetooth addresses the ``R`` specifier shall be used after the ``M``
specifier to use reversed byte order suitable for visual interpretation
of Bluetooth addresses which are in the little endian order.

Passed by reference.

IPv4 addresses
--------------

::

	%pI4	1.2.3.4
	%pi4	001.002.003.004
	%p[Ii]4[hnbl]

For printing IPv4 dot-separated decimal addresses. The ``I4`` and ``i4``
specifiers result in a printed address with (i4) or without (I4) leading
zeros.

The additional ``h``, ``n``, ``b``, and ``l`` specifiers are used to specify
host, network, big or little endian order addresses respectively. Where
no specifier is provided the default network/big endian order is used.

Passed by reference.

IPv6 addresses
--------------

::

	%pI6	0001:0002:0003:0004:0005:0006:0007:0008
	%pi6	00010002000300040005000600070008
	%pI6c	1:2:3:4:5:6:7:8

For printing IPv6 network-order 16-bit hex addresses. The ``I6`` and ``i6``
specifiers result in a printed address with (I6) or without (i6)
colon-separators. Leading zeros are always used.

The additional ``c`` specifier can be used with the ``I`` specifier to
print a compressed IPv6 address as described by
https://tools.ietf.org/html/rfc5952

Passed by reference.

IPv4/IPv6 addresses (generic, with port, flowinfo, scope)
---------------------------------------------------------

::

	%pIS	1.2.3.4		or 0001:0002:0003:0004:0005:0006:0007:0008
	%piS	001.002.003.004	or 00010002000300040005000600070008
	%pISc	1.2.3.4		or 1:2:3:4:5:6:7:8
	%pISpc	1.2.3.4:12345	or [1:2:3:4:5:6:7:8]:12345
	%p[Ii]S[pfschnbl]

For printing an IP address without the need to distinguish whether it's of
type AF_INET or AF_INET6. A pointer to a valid struct sockaddr,
specified through ``IS`` or ``iS``, can be passed to this format specifier.

The additional ``p``, ``f``, and ``s`` specifiers are used to specify port
(IPv4, IPv6), flowinfo (IPv6) and scope (IPv6). Ports have a ``:`` prefix,
flowinfo a ``/`` and scope a ``%``, each followed by the actual value.

In case of an IPv6 address the compressed IPv6 address as described by
https://tools.ietf.org/html/rfc5952 is being used if the additional
specifier ``c`` is given. The IPv6 address is surrounded by ``[``, ``]`` in
case of additional specifiers ``p``, ``f`` or ``s`` as suggested by
https://tools.ietf.org/html/draft-ietf-6man-text-addr-representation-07

In case of IPv4 addresses, the additional ``h``, ``n``, ``b``, and ``l``
specifiers can be used as well and are ignored in case of an IPv6
address.

Passed by reference.

Further examples::

	%pISfc		1.2.3.4		or [1:2:3:4:5:6:7:8]/123456789
	%pISsc		1.2.3.4		or [1:2:3:4:5:6:7:8]%1234567890
	%pISpfc		1.2.3.4:12345	or [1:2:3:4:5:6:7:8]:12345/123456789

UUID/GUID addresses
-------------------

::

	%pUb	00010203-0405-0607-0809-0a0b0c0d0e0f
	%pUB	00010203-0405-0607-0809-0A0B0C0D0E0F
	%pUl	03020100-0504-0706-0809-0a0b0c0e0e0f
	%pUL	03020100-0504-0706-0809-0A0B0C0E0E0F

For printing 16-byte UUID/GUIDs addresses. The additional ``l``, ``L``,
``b`` and ``B`` specifiers are used to specify a little endian order in
lower (l) or upper case (L) hex notation - and big endian order in lower (b)
or upper case (B) hex notation.

Where no additional specifiers are used the default big endian
order with lower case hex notation will be printed.

Passed by reference.

dentry names
------------

::

	%pd{,2,3,4}
	%pD{,2,3,4}

For printing dentry name; if we race with :c:func:`d_move`, the name might
be a mix of old and new ones, but it won't oops.  %pd dentry is a safer
equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
last components.  %pD does the same thing for struct file.

Passed by reference.

block_device names
------------------

::

	%pg	sda, sda1 or loop0p1

For printing name of block_device pointers.

struct va_format
----------------

::

	%pV

For printing struct va_format structures. These contain a format string
and va_list as follows::

	struct va_format {
		const char *fmt;
		va_list *va;
	};

Implements a "recursive vsnprintf".

Do not use this feature without some mechanism to verify the
correctness of the format string and va_list arguments.

Passed by reference.

Device tree nodes
-----------------

::

	%pOF[fnpPcCF]


For printing device tree node structures. Default behaviour is
equivalent to %pOFf.

	- f - device node full_name
	- n - device node name
	- p - device node phandle
	- P - device node path spec (name + @unit)
	- F - device node flags
	- c - major compatible string
	- C - full compatible string

The separator when using multiple arguments is ':'

Examples::

	%pOF	/foo/bar@0			- Node full name
	%pOFf	/foo/bar@0			- Same as above
	%pOFfp	/foo/bar@0:10			- Node full name + phandle
	%pOFfcF	/foo/bar@0:foo,device:--P-	- Node full name +
	                                          major compatible string +
						  node flags
							D - dynamic
							d - detached
							P - Populated
							B - Populated bus

Passed by reference.

Fwnode handles
--------------

::

	%pfw[fP]

For printing information on fwnode handles. The default is to print the full
node name, including the path. The modifiers are functionally equivalent to
%pOF above.

	- f - full name of the node, including the path
	- P - the name of the node including an address (if there is one)

Examples (ACPI)::

	%pfwf	\_SB.PCI0.CIO2.port@1.endpoint@0	- Full node name
	%pfwP	endpoint@0				- Node name

Examples (OF)::

	%pfwf	/ocp@68000000/i2c@48072000/camera@10/port/endpoint - Full name
	%pfwP	endpoint				- Node name

Time and date
-------------

::

	%pt[RT]			YYYY-mm-ddTHH:MM:SS
	%pt[RT]s		YYYY-mm-dd HH:MM:SS
	%pt[RT]d		YYYY-mm-dd
	%pt[RT]t		HH:MM:SS
	%pt[RT][dt][r][s]

For printing date and time as represented by::

	R  struct rtc_time structure
	T  time64_t type

in human readable format.

By default year will be incremented by 1900 and month by 1.
Use %pt[RT]r (raw) to suppress this behaviour.

The %pt[RT]s (space) will override ISO 8601 separator by using ' ' (space)
instead of 'T' (Capital T) between date and time. It won't have any effect
when date or time is omitted.

Passed by reference.

struct clk
----------

::

	%pC	pll1
	%pCn	pll1

For printing struct clk structures. %pC and %pCn print the name of the clock
(Common Clock Framework) or a unique 32-bit ID (legacy clock framework).

Passed by reference.

bitmap and its derivatives such as cpumask and nodemask
-------------------------------------------------------

::

	%*pb	0779
	%*pbl	0,3-6,8-10

For printing bitmap and its derivatives such as cpumask and nodemask,
%*pb outputs the bitmap with field width as the number of bits and %*pbl
output the bitmap as range list with field width as the number of bits.

The field width is passed by value, the bitmap is passed by reference.
Helper macros cpumask_pr_args() and nodemask_pr_args() are available to ease
printing cpumask and nodemask.

Flags bitfields such as page flags, gfp_flags
---------------------------------------------

::

	%pGp	referenced|uptodate|lru|active|private|node=0|zone=2|lastcpupid=0x1fffff
	%pGg	GFP_USER|GFP_DMA32|GFP_NOWARN
	%pGv	read|exec|mayread|maywrite|mayexec|denywrite

For printing flags bitfields as a collection of symbolic constants that
would construct the value. The type of flags is given by the third
character. Currently supported are [p]age flags, [v]ma_flags (both
expect ``unsigned long *``) and [g]fp_flags (expects ``gfp_t *``). The flag
names and print order depends on the particular	type.

Note that this format should not be used directly in the
:c:func:`TP_printk()` part of a tracepoint. Instead, use the show_*_flags()
functions from <trace/events/mmflags.h>.

Passed by reference.

Network device features
-----------------------

::

	%pNF	0x000000000000c000

For printing netdev_features_t.

Passed by reference.

V4L2 and DRM FourCC code (pixel format)
---------------------------------------

::

	%p4cc

Print a FourCC code used by V4L2 or DRM, including format endianness and
its numerical value as hexadecimal.

Passed by reference.

Examples::

	%p4cc	BG12 little-endian (0x32314742)
	%p4cc	Y10  little-endian (0x20303159)
	%p4cc	NV12 big-endian (0xb231564e)

Thanks
======

If you add other %p extensions, please extend <lib/test_printf.c> with
one or more test cases, if at all feasible.

Thank you for your cooperation and attention.