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

[Saket Kumar Bhaskar <skb99@linux.ibm.com>]

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@linux.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@linux.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.

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
> >