On Wed, Nov 20, 2024 at 6:52 AM Saket Kumar Bhaskar <skb99@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > > ---
> > > 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
> > >
