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bpf, docs: Explain helper functions

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Add brief text about existence of helper functions, with details to go in
separate psABI text.

Note that text about runtime functions (kfuncs) is part of a separate patch,
not this one.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20230308205303.1308-1-dthaler1968@googlemail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Dave Thaler 2023-03-08 20:53:03 +00:00 committed by Alexei Starovoitov
parent 74843b57ec
commit c1f9e14e3b
3 changed files with 22 additions and 1 deletions
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6
Documentation/bpf/clang-notes.rst
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@ -20,6 +20,12 @@ Arithmetic instructions
For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with For CPU versions prior to 3, Clang v7.0 and later can enable ``BPF_ALU`` support with
``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included. ``-Xclang -target-feature -Xclang +alu32``. In CPU version 3, support is automatically included.
Jump instructions
=================
If ``-O0`` is used, Clang will generate the ``BPF_CALL | BPF_X | BPF_JMP`` (0x8d)
instruction, which is not supported by the Linux kernel verifier.
Atomic operations Atomic operations
================= =================

9
Documentation/bpf/instruction-set.rst
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@ -253,7 +253,7 @@ BPF_JSET 0x40 PC += off if dst & src
BPF_JNE 0x50 PC += off if dst != src BPF_JNE 0x50 PC += off if dst != src
BPF_JSGT 0x60 PC += off if dst > src signed BPF_JSGT 0x60 PC += off if dst > src signed
BPF_JSGE 0x70 PC += off if dst >= src signed BPF_JSGE 0x70 PC += off if dst >= src signed
BPF_CALL 0x80 function call BPF_CALL 0x80 function call see `Helper functions`_
BPF_EXIT 0x90 function / program return BPF_JMP only BPF_EXIT 0x90 function / program return BPF_JMP only
BPF_JLT 0xa0 PC += off if dst < src unsigned BPF_JLT 0xa0 PC += off if dst < src unsigned
BPF_JLE 0xb0 PC += off if dst <= src unsigned BPF_JLE 0xb0 PC += off if dst <= src unsigned
@ -264,6 +264,13 @@ BPF_JSLE 0xd0 PC += off if dst <= src signed
The eBPF program needs to store the return value into register R0 before doing a The eBPF program needs to store the return value into register R0 before doing a
BPF_EXIT. BPF_EXIT.
Helper functions
~~~~~~~~~~~~~~~~
Helper functions are a concept whereby BPF programs can call into a
set of function calls exposed by the runtime. Each helper
function is identified by an integer used in a ``BPF_CALL`` instruction.
The available helper functions may differ for each program type.
Load and store instructions Load and store instructions
=========================== ===========================

8
Documentation/bpf/linux-notes.rst
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@ -12,6 +12,14 @@ Byte swap instructions
``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively. ``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively.
Jump instructions
=================
``BPF_CALL | BPF_X | BPF_JMP`` (0x8d), where the helper function
integer would be read from a specified register, is not currently supported
by the verifier. Any programs with this instruction will fail to load
until such support is added.
Legacy BPF Packet access instructions Legacy BPF Packet access instructions
===================================== =====================================