Re: [PATCH] libbpf: define bpf_tail_call_static when __clang__ is not defined

[Yonghong Song]

On 9/9/22 5:26 PM, Andrii Nakryiko wrote:
> On Fri, Sep 9, 2022 at 3:46 PM James Hilliard <james.hilliard1@xxxxxxxxx> wrote:
> > The bpf_tail_call_static function is currently not defined unless
> > using clang >= 8.
> >
> > To support bpf_tail_call_static on GCC we can check if __clang__ is
> > not defined to enable bpf_tail_call_static.
> >
> > Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
> > Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > ---
> >   tools/lib/bpf/bpf_helpers.h | 10 +++++-----
> >   1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> > index 7349b16b8e2f..30fc95e7cd76 100644
> > --- a/tools/lib/bpf/bpf_helpers.h
> > +++ b/tools/lib/bpf/bpf_helpers.h
> > @@ -131,7 +131,7 @@
> >   /*
> >    * Helper function to perform a tail call with a constant/immediate map slot.
> >    */
> > -#if __clang_major__ >= 8 && defined(__bpf__)
> > +#if (!defined(__clang__) || __clang_major__ >= 8) && defined(__bpf__)
> >   static __always_inline void
> >   bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> >   {
> > @@ -139,8 +139,8 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> >                  __bpf_unreachable();
> >
> >          /*
> > -        * Provide a hard guarantee that LLVM won't optimize setting r2 (map
> > -        * pointer) and r3 (constant map index) from _different paths_ ending
> > +        * Provide a hard guarantee that the compiler won't optimize setting r2
> > +        * (map pointer) and r3 (constant map index) from _different paths_ ending
> >           * up at the _same_ call insn as otherwise we won't be able to use the
> >           * jmpq/nopl retpoline-free patching by the x86-64 JIT in the kernel
> >           * given they mismatch. See also d2e4c1e6c294 ("bpf: Constant map key
> > @@ -148,8 +148,8 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> >           *
> >           * Note on clobber list: we need to stay in-line with BPF calling
> >           * convention, so even if we don't end up using r0, r4, r5, we need
> > -        * to mark them as clobber so that LLVM doesn't end up using them
> > -        * before / after the call.
> > +        * to mark them as clobber so that the compiler doesn't end up using
> > +        * them before / after the call.
> >           */
> >          asm volatile("r1 = %[ctx]\n\t"
> >                       "r2 = %[map]\n\t"
>
> will this compile as is on GCC-BPF? I'm trying to understand what's
> the point. Once GCC supports this ASM syntax we can add similar check
> to __clang_major__, instead of allowing it for all GCC versions?
>
> We must have done __clang_major__ check for a reason, old Clangs
> probably had some problems compiling this. Maybe Daniel remembers?

Yes, clang >= 8 is needed to ensure 'slot' is propagated as 'const' 
instead of variables. clang 6 added support for inline asm but clang 8
is needed for above 'const' propagation into asm code.

> > --
> > 2.34.1