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

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Message-ID: <CAEf4Bzbek6CYbx5Atz_xwwx5J3gC1ELdVmW-kFrrR=CWNLMyBA@mail.gmail.com>
Date: Thu, 21 Nov 2024 16:00:13 -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 Wed, Nov 20, 2024 at 6:52 AM Saket Kumar Bhaskar <skb99@...ux.ibm.com> wrote:
>
> On Fri, Nov 08, 2024 at 10:43:54AM -0800, Andrii Nakryiko wrote:
> > 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
> >
> Hi Andrii,
>
> IMO since both the patches 7e92e01b7245(powerpc: Provide syscall wrapper)
> and 94746890202cf(powerpc: Don't add __powerpc_ prefix to syscall entry points)
> went into the same kernel version v6.1-rc1, there won't me much kernel
> versions that has only one of these patches.
>
> Also, to test more I tried this patch with ARCH_HAS_SYSCALL_WRAPPER disabled,
> and it the test passed in this case too.
>

Keep in mind that libbpf is supposed to work across many kernel
versions. So as long as there are powerpc (old) kernels that do use
arch-specific prefix, we need to detect them and supply prefix when
attaching ksyscall programs.

> Thanks,
> Saket
> > >
> > >         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
> > >