linux-kernel - Re: [PATCH bpf-next v13 2/2] selftests/bpf: Enrich subtest_basic_usdt case in selftests to cover SIB handling logic

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Message-ID: <CAEf4Bzbs3hV_Q47+d93tTX13WkrpkpOb4=U04mZCjHyZg4aVdw@mail.gmail.com>
Date: Fri, 22 Aug 2025 15:59:49 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Jiawei Zhao <phoenix500526@....com>
Cc: ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	yonghong.song@...ux.dev, bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v13 2/2] selftests/bpf: Enrich subtest_basic_usdt
 case in selftests to cover SIB handling logic

On Fri, Aug 22, 2025 at 8:16 AM Jiawei Zhao <phoenix500526@....com> wrote:
>
> When using GCC on x86-64 to compile an usdt prog with -O1 or higher
> optimization, the compiler will generate SIB addressing mode for global
> array and PC-relative addressing mode for global variable,
> e.g. "1@-96(%rbp,%rax,8)" and "-1@...1(%rip)".
>
> In this patch:
> - enrich subtest_basic_usdt test case to cover SIB addressing usdt argument spec
>   handling logic
>
> Signed-off-by: Jiawei Zhao <phoenix500526@....com>
> ---
>  tools/testing/selftests/bpf/prog_tests/usdt.c | 44 ++++++++++++++++++-
>  tools/testing/selftests/bpf/progs/test_usdt.c | 30 +++++++++++++
>  2 files changed, 72 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/usdt.c b/tools/testing/selftests/bpf/prog_tests/usdt.c
> index 9057e983cc54..c04b416aa4a8 100644
> --- a/tools/testing/selftests/bpf/prog_tests/usdt.c
> +++ b/tools/testing/selftests/bpf/prog_tests/usdt.c
> @@ -25,6 +25,10 @@ unsigned short test_usdt0_semaphore SEC(".probes");
>  unsigned short test_usdt3_semaphore SEC(".probes");
>  unsigned short test_usdt12_semaphore SEC(".probes");
>
> +#if ((defined(__x86_64__) || defined(__i386__)) && defined(__GNUC__) && !defined(__clang__))

does clang define __GNUC__ as well? otherwise why !define(__clang__) ?

> +unsigned short test_usdt_sib_semaphore SEC(".probes");
> +#endif
> +
>  static void __always_inline trigger_func(int x) {
>         long y = 42;
>
> @@ -40,12 +44,29 @@ static void __always_inline trigger_func(int x) {
>         }
>  }
>
> +#if ((defined(__x86_64__) || defined(__i386__)) && defined(__GNUC__) && !defined(__clang__))
> +static __attribute__((optimize("O1"))) void trigger_sib_spec(void)

you use assembly directly, so optimize() should be irrelevant, no?

So we can make this non-GCC specific, right?

> +{
> +       /* Base address + offset + (index * scale) */
> +       /* Force SIB addressing with inline assembly */
> +       asm volatile(
> +               "# probe point with memory access\n"
> +               STAP_PROBE_ASM(test, usdt_sib, -2@(%%rdx,%%rax,2))

is it guaranteed that nums address will end up in rdx and a in rax?...

I'd feel more comfortable if you explicitly set up rdx and rax in
assembly, then add USDT with STAP_PROBE_ASM. That should be possible
with embedded assembly, no?

> +               "# end probe point"

why these unnecessary comments embedded in the assembly?...

> +               :
> +               : "d"(nums), "a"(0)
> +               : "memory"
> +       );
> +}

[...]

> +
> +int usdt_sib_called;
> +u64 usdt_sib_cookie;
> +int usdt_sib_arg_cnt;
> +int usdt_sib_arg_ret;
> +u64 usdt_sib_arg;
> +int usdt_sib_arg_size;
> +
> +// Note: usdt_sib is only tested on x86-related architectures, so it requires
> +// manual attach since auto-attach will panic tests under other architectures

don't use c++ style comments

> +SEC("usdt")
> +int usdt_sib(struct pt_regs *ctx)
> +{
> +       long tmp;
> +
> +       if (my_pid != (bpf_get_current_pid_tgid() >> 32))
> +               return 0;
> +
> +       __sync_fetch_and_add(&usdt_sib_called, 1);
> +
> +       usdt_sib_cookie = bpf_usdt_cookie(ctx);
> +       usdt_sib_arg_cnt = bpf_usdt_arg_cnt(ctx);
> +
> +       usdt_sib_arg_ret = bpf_usdt_arg(ctx, 0, &tmp);
> +       usdt_sib_arg = (short)tmp;
> +       usdt_sib_arg_size = bpf_usdt_arg_size(ctx, 0);
> +
> +       return 0;
> +}
> +
>  char _license[] SEC("license") = "GPL";
> --
> 2.43.0
>