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

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

[James Hilliard]

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"
-- 
2.34.1

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

[Andrii Nakryiko]

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?

> --
> 2.34.1
>

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@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
>>

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

[James Hilliard]

On Fri, Sep 9, 2022 at 6:26 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> 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?

So, it's a bit different since the ASM syntax issue is more of an
issue with GAS,
we would need a check along these lines I think for a proper migration path:
https://lore.kernel.org/bpf/20220912063514.2824432-1-james.hilliard1@gmail.com/

We would set something like once GAS support for the llvm syntax is added:
.ifdef .gasversion. < 24000

I don't think we can do a compiler version check here like with llvm since GCC
release cycles are out of sync with GAS(llvm assembler is kept in sync with the
compiler AFAIU).

>
> We must have done __clang_major__ check for a reason, old Clangs
> probably had some problems compiling this. Maybe Daniel remembers?
>
> > --
> > 2.34.1
> >