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

[Yaniv Agman <yanivagman@gmail.com>]

‫בתאריך יום ו׳, 9 באוק׳ 2020 ב-22:58 מאת ‪Daniel Borkmann‬‏
<‪daniel@iogearbox.net‬‏>:‬
>
> On 10/9/20 9:33 PM, Yaniv Agman wrote:
> > ‫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.
>
> Yeah that was my thinking for those stuck for whatever reason on old LLVM.
>
> >>>>>>>> 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.
>
> Makes sense as well, I was mainly thinking if people include header files in
> their project which are shared between tracing & non-tracing, so they compile
> just fine, but I can see the point that wrt very specific guarantees, fully
> agree. In that sense we should just have it defined with the clang + __bpf__
> constraints mentioned earlier.
>
> Thanks,
> Daniel

Hi Daniel,

Is this change going to happen?
I'm still having a compilation error when using master branch

Thanks,
Yaniv