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

[Yonghong Song <yhs@fb.com>]

On 9/9/22 5:26 PM, Andrii Nakryiko wrote:
> On Fri, Sep 9, 2022 at 3:46 PM James Hilliard <james.hilliard1@gmail.com> 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@gmail.com>
>> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
>> ---
>>   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
>>