On Mon, Feb 20, 2023 at 2:52 PM Dave Thaler <dthaler1968=40googlemail.com@xxxxxxxxxxxxxx> wrote: > > From: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > Add text explaining helper functions. > Note that text about runtime functions (kfuncs) is part of a separate patch, > not this one. > > Signed-off-by: Dave Thaler <dthaler@xxxxxxxxxxxxx> > --- > V1 -> V2: addressed comments from Alexei and Stanislav > > V2 -> V3: addressed comments from David Vernet > --- > Documentation/bpf/clang-notes.rst | 6 ++++++ > Documentation/bpf/instruction-set.rst | 19 ++++++++++++++++++- > Documentation/bpf/linux-notes.rst | 8 ++++++++ > 3 files changed, 32 insertions(+), 1 deletion(-) > > diff --git a/Documentation/bpf/clang-notes.rst b/Documentation/bpf/clang-notes.rst > index 528feddf2db..2c872a1ee08 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. This is fine here. > + > Atomic operations > ================= > > diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst > index af515de5fc3..148dd2a2e39 100644 > --- a/Documentation/bpf/instruction-set.rst > +++ b/Documentation/bpf/instruction-set.rst > @@ -239,7 +239,7 @@ 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 > @@ -250,6 +250,23 @@ 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. > + > +Conceptually, each helper function is implemented with a commonly shared function > +signature defined as: > + > + u64 function(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5) > + > +In actuality, each helper function is defined as taking between 0 and 5 arguments, > +with the remaining registers being ignored. The definition of a helper function > +is responsible for specifying the type (e.g., integer, pointer, etc.) of the value returned, > +the number of arguments, and the type of each argument. Above is correct, but it aims to describe the calling convention which should be done in a separate BPF psABI doc and not in instruction-set.rst. And if we start describing calling convention we should talk about promotion rules, sign extensions, expectations for return values, for passing structs by value, etc. > Load and store instructions > =========================== > diff --git a/Documentation/bpf/linux-notes.rst b/Documentation/bpf/linux-notes.rst > index 956b0c86699..f43b9c797bc 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. This is fine here as well.
