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

From: Daniel Borkmann <daniel@iogearbox.net>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Yaniv Agman <yanivagman@gmail.com>, bpf <bpf@vger.kernel.org>,
	Yonghong Song <yhs@fb.com>
Subject: Re: libbpf error: unknown register name 'r0' in asm
Date: Fri, 9 Oct 2020 20:41:16 +0200	[thread overview]
Message-ID: <ece9975d-717c-a868-be51-c97aeae8e011@iogearbox.net> (raw)
In-Reply-To: <CAEf4Bza4KFJ_j7vmg-x_Zinp0PUM-zmWYHMq_y+2zWmX485sBQ@mail.gmail.com>

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.

>>>> 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
>>>> +#endif /* __clang_major__ >= 10 && __bpf__ */
>>>>
>>>>     /*
>>>>      * Helper structure used by eBPF C program
>>