linux-kernel - Re: [PATCH 2/3] libbpf: Remove powerpc prefix from syscall function names

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Message-ID: <CAEf4BzZ9Bz8a_hY-jDkqaYg6Phi9bjvoxbBeVZqcgjYXg4a-mA@mail.gmail.com>
Date: Fri, 8 Nov 2024 10:43:54 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
Cc: bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-kernel@...r.kernel.org, ast@...nel.org, hbathini@...ux.ibm.com, 
	andrii@...nel.org, daniel@...earbox.net, martin.lau@...ux.dev, 
	eddyz87@...il.com, song@...nel.org, yonghong.song@...ux.dev, 
	john.fastabend@...il.com, kpsingh@...nel.org, sdf@...ichev.me, 
	haoluo@...gle.com, jolsa@...nel.org, shuah@...nel.org, mykolal@...com
Subject: Re: [PATCH 2/3] libbpf: Remove powerpc prefix from syscall function names

On Sun, Nov 3, 2024 at 9:00 PM Saket Kumar Bhaskar <skb99@...ux.ibm.com> wrote:
>
> Since commit 94746890202cf ("powerpc: Don't add __powerpc_ prefix to
> syscall entry points") drops _powerpc prefix to syscall entry points,
> even though powerpc now supports syscall wrapper, so /proc/kallsyms
> have symbols for syscall entry without powerpc prefix(sys_*).
>
> For this reason, arch specific prefix for syscall functions in powerpc
> is dropped.
>
> Signed-off-by: Saket Kumar Bhaskar <skb99@...ux.ibm.com>
> ---
>  tools/lib/bpf/libbpf.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 219facd0e66e..3a370fa37d8a 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -11110,9 +11110,7 @@ static const char *arch_specific_syscall_pfx(void)
>  #elif defined(__riscv)
>         return "riscv";
>  #elif defined(__powerpc__)
> -       return "powerpc";
> -#elif defined(__powerpc64__)
> -       return "powerpc64";
> +       return "";
>  #else
>         return NULL;
>  #endif
> @@ -11127,7 +11125,11 @@ int probe_kern_syscall_wrapper(int token_fd)
>         if (!ksys_pfx)
>                 return 0;
>
> +#if defined(__powerpc__)
> +       snprintf(syscall_name, sizeof(syscall_name), "sys_bpf");
> +#else
>         snprintf(syscall_name, sizeof(syscall_name), "__%s_sys_bpf", ksys_pfx);
> +#endif

The problem is that on older versions of kernel it will have this
prefix, while on newer ones it won't. So to not break anything on old
kernels, we'd need to do feature detection and pick whether to use
prefix or not, right?

So it seems like this change needs a bit more work.

pw-bot: cr

>
>         if (determine_kprobe_perf_type() >= 0) {
>                 int pfd;
> @@ -11272,8 +11274,12 @@ struct bpf_link *bpf_program__attach_ksyscall(const struct bpf_program *prog,
>                  * compiler does not know that we have an explicit conditional
>                  * as well.
>                  */
> +#if defined(__powerpc__)
> +               snprintf(func_name, sizeof(func_name), "sys_%s", syscall_name);
> +#else
>                 snprintf(func_name, sizeof(func_name), "__%s_sys_%s",
>                          arch_specific_syscall_pfx() ? : "", syscall_name);
> +#endif
>         } else {
>                 snprintf(func_name, sizeof(func_name), "__se_sys_%s", syscall_name);
>         }
> --
> 2.43.5
>