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authorChristoph Hellwig <hch@lst.de>2022-01-31 19:36:36 +0100
committerAlexei Starovoitov <ast@kernel.org>2022-02-03 09:29:31 -0800
commit15175336270a76695412aedf68f3eab746d84b4b (patch)
tree50eb4ee3b4ae1cfcffc3e7d8e105fd078e817b96 /Documentation/bpf
parent63d8c242b9a502f1517b3e8c45d2125649dd5d7b (diff)
bpf, docs: Better document the legacy packet access instruction
Use consistent terminology and structured RST elements to better document these two oddball instructions. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20220131183638.3934982-4-hch@lst.de
Diffstat (limited to 'Documentation/bpf')
-rw-r--r--Documentation/bpf/instruction-set.rst54
1 files changed, 32 insertions, 22 deletions
diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 03da88530172..94682e23cd2f 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -213,8 +213,8 @@ The mode modifier is one of:
mode modifier value description
============= ===== ====================================
BPF_IMM 0x00 used for 64-bit mov
- BPF_ABS 0x20 legacy BPF packet access
- BPF_IND 0x40 legacy BPF packet access
+ BPF_ABS 0x20 legacy BPF packet access (absolute)
+ BPF_IND 0x40 legacy BPF packet access (indirect)
BPF_MEM 0x60 regular load and store operations
BPF_ATOMIC 0xc0 atomic operations
============= ===== ====================================
@@ -294,29 +294,39 @@ 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.
-Packet access instructions
---------------------------
+Legacy BPF Packet access instructions
+-------------------------------------
-eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and
-(BPF_IND | <size> | BPF_LD) which are used to access packet data.
+eBPF has special instructions for access to packet data that have been
+carried over from classic BPF to retain the performance of legacy socket
+filters running in the eBPF interpreter.
-They had to be carried over from classic BPF 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.
+The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and
+``BPF_IND | <size> | BPF_LD``.
-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.
+These instructions are used to access packet data and can only be used when
+the program context is a pointer to networking packet. ``BPF_ABS``
+accesses packet data at an absolute offset specified by the immediate data
+and ``BPF_IND`` access packet data at an offset that includes the value of
+a register in addition to the immediate data.
-For example, BPF_IND | BPF_W | BPF_LD means::
+These instructions have seven implicit operands:
- R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
+ * Register R6 is an implicit input that must contain pointer to a
+ struct sk_buff.
+ * Register R0 is an implicit output which contains the data fetched from
+ the packet.
+ * Registers R1-R5 are scratch registers that are clobbered after a call to
+ ``BPF_ABS | BPF_LD`` or ``BPF_IND`` | BPF_LD instructions.
+
+These instructions have an implicit program exit condition as well. When an
+eBPF program is trying to access the data beyond the packet boundary, the
+program execution will be aborted.
+
+``BPF_ABS | BPF_W | BPF_LD`` means::
-and R1 - R5 are clobbered.
+ R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + imm32))
+
+``BPF_IND | BPF_W | BPF_LD`` means::
+
+ R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))