netdev - Re: [PATCH bpf] bpf, libbpf: guard bpf inline asm from bpf_tail_call

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Date:   Wed, 21 Oct 2020 16:18:58 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Yaniv Agman <yanivagman@...il.com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf] bpf, libbpf: guard bpf inline asm from bpf_tail_call_static

On Wed, Oct 21, 2020 at 1:33 PM Daniel Borkmann <daniel@...earbox.net> 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@...il.com>
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Link: https://lore.kernel.org/bpf/CAMy7=ZUk08w5Gc2Z-EKi4JFtuUCaZYmE4yzhJjrExXpYKR4L8w@mail.gmail.com
> ---

LGTM!

Acked-by: Andrii Nakryiko <andrii@...nel.org>

>  tools/lib/bpf/bpf_helpers.h | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> index 2bdb7d6dbad2..72b251110c4d 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__ >= 8 && defined(__bpf__)
>  static __always_inline void
>  bpf_tail_call_static(void *ctx, const void *map, const __u32 slot)
>  {
> @@ -98,6 +99,7 @@ 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");
>  }
> +#endif
>
>  /*
>   * Helper structure used by eBPF C program
> --
> 2.17.1
>