Re: libbpf error: unknown register name 'r0' in asm

[Yaniv Agman <yanivagman@gmail.com>]

‫בתאריך יום ו׳, 9 באוק׳ 2020 ב-22:08 מאת ‪Yonghong Song‬‏ <‪yhs@fb.com‬‏>:‬
>
>
>
> On 10/9/20 11:59 AM, Andrii Nakryiko wrote:
> > On Fri, Oct 9, 2020 at 11:41 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >>
> >> On 10/9/20 8:35 PM, Andrii Nakryiko wrote:
> >>> On Fri, Oct 9, 2020 at 11:21 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >>>> On 10/9/20 8:09 PM, Yaniv Agman wrote:
> >>>>> ‫בתאריך יום ו׳, 9 באוק׳ 2020 ב-20:39 מאת ‪Daniel Borkmann‬‏
> >>>>> <‪daniel@iogearbox.net‬‏>:‬
> >>>>>>
> >>>>>> On 10/9/20 6:56 PM, Yaniv Agman wrote:
> >>>>>>> ‫בתאריך יום ו׳, 9 באוק׳ 2020 ב-19:27 מאת ‪Daniel Borkmann‬‏
> >>>>>>> <‪daniel@iogearbox.net‬‏>:‬
> >>>>>>>>
> >>>>>>>> [ Cc +Yonghong ]
> >>>>>>>>
> >>>>>>>> On 10/9/20 6:05 PM, Yaniv Agman wrote:
> >>>>>>>>> Pulling the latest changes of libbpf and compiling my application with it,
> >>>>>>>>> I see the following error:
> >>>>>>>>>
> >>>>>>>>> ../libbpf/src//root/usr/include/bpf/bpf_helpers.h:99:10: error:
> >>>>>>>>> unknown register name 'r0' in asm
> >>>>>>>>>                           : "r0", "r1", "r2", "r3", "r4", "r5");
> >>>>>>>>>
> >>>>>>>>> The commit which introduced this change is:
> >>>>>>>>> 80c7838600d39891f274e2f7508b95a75e4227c1
> >>>>>>>>>
> >>>>>>>>> I'm not sure if I'm doing something wrong (missing include?), or this
> >>>>>>>>> is a genuine error
> >>>>>>>>
> >>>>>>>> Seems like your clang/llvm version might be too old.
> >>>>>>>
> >>>>>>> I'm using clang 10.0.1
> >>>>>>
> >>>>>> Ah, okay, I see. Would this diff do the trick for you?
> >>>>>
> >>>>> Yes! Now it compiles without any problems!
> >>>>
> >>>> Great, thx, I'll cook proper fix and check with clang6 as Yonghong mentioned.
> >>>
> >>> Am I the only one confused here?... Yonghong said it should be
> >>> supported as early as clang 6, Yaniv is using Clang 10 and is still
> >>> getting this error. Let's figure out what's the problem before adding
> >>> unnecessary checks.
> >>>
> >>> I think it's not the clang_major check that helped, rather __bpf__
> >>> check. So please hold off on the fix, let's get to the bottom of this
> >>> first.
> >>
> >> I don't see confusion here (maybe other than which minimal clang/llvm version
> >> libbpf should support). If we do `#if __clang_major__ >= 6 && defined(__bpf__)`
> >> for the final patch, then this means that user passed clang -target bpf and
> >> the min supported version for inline assembly was there, otherwise we fall back
> >> to bpf_tail_call. In Yaniv's case, he probably had native target with -emit-llvm
> >> and then used llc invocation.
> >
> > The "-emit-llvm" was the part that we were missing and had to figure
> > it out, before we could discuss the fix.
>
> Maybe Yaniv can confirm. I think the following properly happens.
>     - clang10 -O2 -g -S -emit-llvm t.c  // This is native compilation
> becasue some header files. Maybe some thing is guarded with x86 specific
> config's which is not available to -target bpf. This is mostly for
> tracing programs and Yanic mentions pt_regs which should be related
> to tracing.
>     - llc -march=bpf t.ll
>

Yes, like I said,  I do use --emit-llvm, and indeed have a tracing program

> So guarding the function with __bpf__ should be the one fixing this issue.
>
> guard with clang version >=6 should not hurt and may prevent
> compilation failures if people use < 6 llvm with clang -target bpf.
> I think most people should already use newer llvm, but who knows.
>
> >
> >>
> >>>>>> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> >>>>>> index 2bdb7d6dbad2..31e356831fcf 100644
> >>>>>> --- a/tools/lib/bpf/bpf_helpers.h
> >>>>>> +++ b/tools/lib/bpf/bpf_helpers.h
> >>>>>> @@ -72,6 +72,7 @@
> >>>>>>      /*
> >>>>>>       * Helper function to perform a tail call with a constant/immediate map slot.
> >>>>>>       */
> >>>>>> +#if __clang_major__ >= 10 && defined(__bpf__)
> >>>>>>      static __always_inline void
> >>>>>>      bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> >>>>>>      {
> >>>>>> @@ -98,6 +99,9 @@ bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
> >>>>>>                         :: [ctx]"r"(ctx), [map]"r"(map), [slot]"i"(slot)
> >>>>>>                         : "r0", "r1", "r2", "r3", "r4", "r5");
> >>>>>>      }
> >>>>>> +#else
> >>>>>> +# define bpf_tail_call_static  bpf_tail_call
> >
> > bpf_tail_call_static has very specific guarantees, so in cases where
> > we can't use inline assembly to satisfy those guarantees, I think we
> > should not just silently redefine bpf_tail_call_static as
> > bpf_tail_call, rather make compilation fail if someone is attempting
> > to use bpf_tail_call_static. _Static_assert could be used to provide a
> > better error message here, probably.
> >
> >>>>>> +#endif /* __clang_major__ >= 10 && __bpf__ */
> >>>>>>
> >>>>>>      /*
> >>>>>>       * Helper structure used by eBPF C program
> >>>>
> >>