linux-kernel - Re: [PATCH] x86,bpf: Avoid IBT objtool warning

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Message-ID: <CAADnVQJ1_9sBqRngG_J+84whx9j7d7qOSzMaJvhc0evDBQfE3w@mail.gmail.com>
Date:   Tue, 5 Apr 2022 09:58:28 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>
Cc:     X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86,bpf: Avoid IBT objtool warning

On Tue, Apr 5, 2022 at 12:55 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
>
> Clang can inline emit_indirect_jump() and then folds constants, which
> results in:
>
>   | vmlinux.o: warning: objtool: emit_bpf_dispatcher()+0x6a4: relocation to !ENDBR: .text.__x86.indirect_thunk+0x40
>   | vmlinux.o: warning: objtool: emit_bpf_dispatcher()+0x67d: relocation to !ENDBR: .text.__x86.indirect_thunk+0x40
>   | vmlinux.o: warning: objtool: emit_bpf_tail_call_indirect()+0x386: relocation to !ENDBR: .text.__x86.indirect_thunk+0x20
>   | vmlinux.o: warning: objtool: emit_bpf_tail_call_indirect()+0x35d: relocation to !ENDBR: .text.__x86.indirect_thunk+0x20
>
> Suppress the optimization such that it must emit a code reference to
> the __x86_indirect_thunk_array[] base.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/net/bpf_jit_comp.c |    1 +
>  1 file changed, 1 insertion(+)
>
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -412,6 +412,7 @@ static void emit_indirect_jump(u8 **ppro
>                 EMIT_LFENCE();
>                 EMIT2(0xFF, 0xE0 + reg);
>         } else if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) {
> +               OPTIMIZER_HIDE_VAR(reg);
>                 emit_jump(&prog, &__x86_indirect_thunk_array[reg], ip);
>         } else
>  #endif

Looks good. Please cc bpf@...r and all bpf maintainers in the future.
We can take it through the bpf tree if you prefer.