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

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

CCing Maddy and MPE
On Fri, Jan 10, 2025 at 02:29:42PM -0800, Andrii Nakryiko wrote:
> On Fri, Jan 10, 2025 at 2:49 AM Saket Kumar Bhaskar <skb99@linux.ibm.com> wrote:
> >
> > On Thu, Nov 21, 2024 at 04:00:13PM -0800, Andrii Nakryiko wrote:
> > > On Wed, Nov 20, 2024 at 6:52 AM Saket Kumar Bhaskar <skb99@linux.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@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.
> > > >
> > >
> > > 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.
> > >
> > Hi Andrii,
> >
> > Sorry about the delayed response, I have started looking at this after
> > a vacation.
> >
> > There are unlikely to be any old kernels that use arch-specific prefix
> > as syscall wrapper support was added to powerpc in v6.1 and
> > commit 94746890202cf that dropped the prefix also went into the same
> > kernel release (v6.1-rc1). So, is it worth it support both sys_bpf and
> > __powerpc_sys_bpf cases?
> >
> > But yes, there can be a kernel without syscall wrapper but having the
> > sys_bpf symbol. So, how about identifying syscall wrapper enablement
> > with __se_sys_bpf instead:
> >
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 66173ddb5a2d..ff69a30cfe9b 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -11163,11 +11163,15 @@ int probe_kern_syscall_wrapper(int token_fd)
> >         char syscall_name[64];
> >         const char *ksys_pfx;
> >
> > +#if defined(__powerpc__)
> > +       snprintf(syscall_name, sizeof(syscall_name), "__se_sys_bpf", ksys_pfx);
> > +#else
> >         ksys_pfx = arch_specific_syscall_pfx();
> >         if (!ksys_pfx)
> >                 return 0;
> >
> >         snprintf(syscall_name, sizeof(syscall_name), "__%s_sys_bpf", ksys_pfx);
> > +#endif
> >
> >         if (determine_kprobe_perf_type() >= 0) {
> >                 int pfd;
> > @@ -11176,16 +11180,28 @@ int probe_kern_syscall_wrapper(int token_fd)
> >                 if (pfd >= 0)
> >                         close(pfd);
> >
> > +#if defined(__powerpc__)
> >                 return pfd >= 0 ? 1 : 0;
> > +#else
> > +               return pfd >= 0 ? 1 : 0;
> > +#endif
> >         } else { /* legacy mode */
> >                 char probe_name[128];
> >
> >                 gen_kprobe_legacy_event_name(probe_name, sizeof(probe_name), syscall_name, 0);
> >                 if (add_kprobe_event_legacy(probe_name, false, syscall_name, 0) < 0)
> > +#if defined(__powerpc__)
> > +                       return 1;
> > +#else
> >                         return 0;
> > +#endif
> >
> >                 (void)remove_kprobe_event_legacy(probe_name, false);
> > +#if defined(__powerpc__)
> > +               return 0;
> > +#else
> >                 return 1;
> > +#endif
> >         }
> >  }
> >
> > Actually, all architectures could use this '__se_' prefix instead of
> > arch specific prefix  to identify if syscall wrapper is enabled.
> > Separate way to handle powerpc case may not be needed. Will
> > wait for your inputs to send v2.
> 
> the problem is that __se_sys_bpf is not traceable (it's a static
> function), so it seems like this won't work
> 
> 
> it's been a while, let me try to clarify my understanding of the
> issue. The problem is that powerpc is special in that when syscall
> wrapper is used, then, unlike all other architectures, they opted to
> not have arch-specific prefix for syscall wrappers, is that right? and
> that's why all the dancing you are trying to add. Am I right?
> 
Yes, you got it right. For more details, you can refer to the 
reasoning behind the change here:
https://github.com/torvalds/linux/commit/94746890202cf

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