Re: [PATCH bpf] bpf, libbpf: guard bpf inline asm from bpf_tail_call_static

[Yonghong Song <yhs@fb.com>]

On 10/21/20 1:32 PM, Daniel Borkmann wrote:
> Yaniv reported a compilation error after pulling latest libbpf:
> 
>    [...]
>    ../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 issue got triggered given Yaniv was compiling tracing programs with native
> target (e.g. x86) instead of BPF target, hence no BTF generated vmlinux.h nor
> CO-RE used, and later llc with -march=bpf was invoked to compile from LLVM IR
> to BPF object file. Given that clang was expecting x86 inline asm and not BPF
> one the error complained that these regs don't exist on the former.
> 
> Guard bpf_tail_call_static() with defined(__bpf__) where BPF inline asm is valid
> to use. BPF tracing programs on more modern kernels use BPF target anyway and
> thus the bpf_tail_call_static() function will be available for them. BPF inline
> asm is supported since clang 7 (clang <= 6 otherwise throws same above error),
> and __bpf_unreachable() since clang 8, therefore include the latter condition
> in order to prevent compilation errors for older clang versions. Given even an
> old Ubuntu 18.04 LTS has official LLVM packages all the way up to llvm-10, I did
> not bother to special case the __bpf_unreachable() inside bpf_tail_call_static()
> further.
> 
> Fixes: 0e9f6841f664 ("bpf, libbpf: Add bpf_tail_call_static helper for bpf programs")
> Reported-by: Yaniv Agman <yanivagman@gmail.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> Link: https://lore.kernel.org/bpf/CAMy7=ZUk08w5Gc2Z-EKi4JFtuUCaZYmE4yzhJjrExXpYKR4L8w@mail.gmail.com

Acked-by: Yonghong Song <yhs@fb.com>