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

[Alexei Starovoitov]

On Tue, Apr 5, 2022 at 12:55 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
>  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@vger and all bpf maintainers in the future.
We can take it through the bpf tree if you prefer.