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bpf, docs: Clarify definitions of various instructions

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Clarify definitions of several instructions:

* BPF_NEG does not support BPF_X
* BPF_CALL does not support BPF_JMP32 or BPF_X
* BPF_EXIT does not support BPF_X
* BPF_JA does not support BPF_X (was implied but not explicitly stated)

Also fix a typo in the wide instruction figure where the field is
actually named "opcode" not "code".

Signed-off-by: Dave Thaler <dthaler1968@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240126040050.8464-1-dthaler1968@gmail.com
This commit is contained in:
Dave Thaler 2024-01-25 20:00:50 -08:00 committed by Daniel Borkmann
parent e6be8cd5d3
commit e48f0f4a9b
1 changed files with 27 additions and 24 deletions
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51
Documentation/bpf/standardization/instruction-set.rst
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@ -174,12 +174,12 @@ and imm containing the high 32 bits of the immediate value.
This is depicted in the following figure::
basic_instruction
.-----------------------------.
| |
code:8 regs:8 offset:16 imm:32 unused:32 imm:32
| |
'--------------'
pseudo instruction
.------------------------------.
| |
opcode:8 regs:8 offset:16 imm:32 unused:32 imm:32
| |
'--------------'
pseudo instruction
Thus the 64-bit immediate value is constructed as follows:
@ -320,6 +320,9 @@ bit operands, and zeroes the remaining upper 32 bits.
operands into 64 bit operands. Unlike other arithmetic instructions,
``BPF_MOVSX`` is only defined for register source operands (``BPF_X``).
The ``BPF_NEG`` instruction is only defined when the source bit is clear
(``BPF_K``).
Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)
for 32-bit operations.
@ -375,27 +378,27 @@ Jump instructions
otherwise identical operations.
The 'code' field encodes the operation as below:
======== ===== === =========================================== =========================================
code value src description notes
======== ===== === =========================================== =========================================
BPF_JA 0x0 0x0 PC += offset BPF_JMP class
BPF_JA 0x0 0x0 PC += imm BPF_JMP32 class
======== ===== === =============================== =============================================
code value src description notes
======== ===== === =============================== =============================================
BPF_JA 0x0 0x0 PC += offset BPF_JMP | BPF_K only
BPF_JA 0x0 0x0 PC += imm BPF_JMP32 | BPF_K only
BPF_JEQ 0x1 any PC += offset if dst == src
BPF_JGT 0x2 any PC += offset if dst > src unsigned
BPF_JGE 0x3 any PC += offset if dst >= src unsigned
BPF_JGT 0x2 any PC += offset if dst > src unsigned
BPF_JGE 0x3 any PC += offset if dst >= src unsigned
BPF_JSET 0x4 any PC += offset if dst & src
BPF_JNE 0x5 any PC += offset if dst != src
BPF_JSGT 0x6 any PC += offset if dst > src signed
BPF_JSGE 0x7 any PC += offset if dst >= src signed
BPF_CALL 0x8 0x0 call helper function by address see `Helper functions`_
BPF_CALL 0x8 0x1 call PC += imm see `Program-local functions`_
BPF_CALL 0x8 0x2 call helper function by BTF ID see `Helper functions`_
BPF_EXIT 0x9 0x0 return BPF_JMP only
BPF_JLT 0xa any PC += offset if dst < src unsigned
BPF_JLE 0xb any PC += offset if dst <= src unsigned
BPF_JSLT 0xc any PC += offset if dst < src signed
BPF_JSLE 0xd any PC += offset if dst <= src signed
======== ===== === =========================================== =========================================
BPF_JSGT 0x6 any PC += offset if dst > src signed
BPF_JSGE 0x7 any PC += offset if dst >= src signed
BPF_CALL 0x8 0x0 call helper function by address BPF_JMP | BPF_K only, see `Helper functions`_
BPF_CALL 0x8 0x1 call PC += imm BPF_JMP | BPF_K only, see `Program-local functions`_
BPF_CALL 0x8 0x2 call helper function by BTF ID BPF_JMP | BPF_K only, see `Helper functions`_
BPF_EXIT 0x9 0x0 return BPF_JMP | BPF_K only
BPF_JLT 0xa any PC += offset if dst < src unsigned
BPF_JLE 0xb any PC += offset if dst <= src unsigned
BPF_JSLT 0xc any PC += offset if dst < src signed
BPF_JSLE 0xd any PC += offset if dst <= src signed
======== ===== === =============================== =============================================
The BPF program needs to store the return value into register R0 before doing a
``BPF_EXIT``.