Re: [PATCH bpf-next v4 0/3] bpf, x86: allow function arguments up to 12 for TRACING

[Yonghong Song <yhs@meta.com>]

On 6/9/23 2:56 AM, menglong8.dong@gmail.com wrote:
> From: Menglong Dong <imagedong@tencent.com>
> 
> For now, the BPF program of type BPF_PROG_TYPE_TRACING can only be used
> on the kernel functions whose arguments count less than 6. This is not
> friendly at all, as too many functions have arguments count more than 6.
> According to the current kernel version, below is a statistics of the
> function arguments count:
> 
> argument count | function count
> 7              | 704
> 8              | 270
> 9              | 84
> 10             | 47
> 11             | 47
> 12             | 27
> 13             | 22
> 14             | 5
> 15             | 0
> 16             | 1
> 
> Therefore, let's enhance it by increasing the function arguments count
> allowed in arch_prepare_bpf_trampoline(), for now, only x86_64.
> 
> In the 1st patch, we make arch_prepare_bpf_trampoline() support to copy
> function arguments in stack for x86 arch. Therefore, the maximum
> arguments can be up to MAX_BPF_FUNC_ARGS for FENTRY and FEXIT.
> 
> In the 2nd patch, we clean garbage value in upper bytes of the trampoline
> when we store the arguments from regs or on-stack into stack.
> 
> And the 3rd patches are for the testcases of the 1st patch.
> 
> Changes since v3:
> - try make the stack pointer 16-byte aligned. Not sure if I'm right :)
> - introduce clean_garbage() to clean the grabage when argument count is 7
> - use different data type in bpf_testmod_fentry_test{7,12}
> - add testcase for grabage values in ctx
> 
> Changes since v2:
> - keep MAX_BPF_FUNC_ARGS still
> - clean garbage value in upper bytes in the 2nd patch
> - move bpf_fentry_test{7,12} to bpf_testmod.c and rename them to
>    bpf_testmod_fentry_test{7,12} meanwhile in the 3rd patch
> 
> Changes since v1:
> - change the maximun function arguments to 14 from 12
> - add testcases (Jiri Olsa)
> - instead EMIT4 with EMIT3_off32 for "lea" to prevent overflow
> 
> Menglong Dong (3):
>    bpf, x86: allow function arguments up to 12 for TRACING
>    bpf, x86: clean garbage value in the stack of trampoline
>    selftests/bpf: add testcase for FENTRY/FEXIT with 6+ arguments
> 
>   arch/x86/net/bpf_jit_comp.c                   | 145 ++++++++++++++++--
>   .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  19 ++-
>   .../selftests/bpf/prog_tests/fentry_fexit.c   |   4 +-
>   .../selftests/bpf/prog_tests/fentry_test.c    |   2 +
>   .../selftests/bpf/prog_tests/fexit_test.c     |   2 +
>   .../testing/selftests/bpf/progs/fentry_test.c |  33 ++++
>   .../testing/selftests/bpf/progs/fexit_test.c  |  57 +++++++
>   7 files changed, 245 insertions(+), 17 deletions(-)

Also replace rebase on top of bpf-next. Patch 3 cannot be applied.