diff options
| author | Dave Thaler <dthaler@microsoft.com> | 2023-03-08 23:53:03 +0300 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2023-03-11 00:02:00 +0300 |
| commit | c1f9e14e3b676eb88fe1c9488c0b5f4fc9108a1c (patch) | |
| tree | c895db124ca1ed63b2683c3136c3a83f2fd7de0a /Documentation/bpf | |
| parent | 74843b57ec70af7b67b7e6153374834ee18d139f (diff) | |
| download | linux-c1f9e14e3b676eb88fe1c9488c0b5f4fc9108a1c.tar.xz | |
bpf, docs: Explain helper functions
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>
Diffstat (limited to 'Documentation/bpf')
| -rw-r--r-- | Documentation/bpf/clang-notes.rst | 6 | ||||
| -rw-r--r-- | Documentation/bpf/instruction-set.rst | 9 | ||||
| -rw-r--r-- | Documentation/bpf/linux-notes.rst | 8 |
3 files changed, 22 insertions, 1 deletions
diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst index 528feddf2db9..2c872a1ee08e 100644 --- a/Documentation/bpf/clang-notes.rst +++ b/Documentation/bpf/clang-notes.rst @@ -20,6 +20,12 @@ Arithmetic instructions 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. +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 ================= diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index db8789e6969e..5e43e14abe80 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -253,7 +253,7 @@ BPF_JSET 0x40 PC += off if dst & src BPF_JNE 0x50 PC += off if dst != src BPF_JSGT 0x60 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_JLT 0xa0 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 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 =========================== diff --git a/Documentation/bpf/linux-notes.rst b/Documentation/bpf/linux-notes.rst index 956b0c86699d..f43b9c797bcb 100644 --- a/Documentation/bpf/linux-notes.rst +++ b/Documentation/bpf/linux-notes.rst @@ -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. +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 ===================================== |