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

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Date:   Wed, 6 Apr 2022 10:01:39 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     bpf <bpf@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86,bpf: Avoid IBT objtool warning

On Wed, Apr 6, 2022 at 3:46 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Tue, Apr 05, 2022 at 09:58:28AM -0700, Alexei Starovoitov wrote:
> > 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.
>
> Oh right, I'll go add an alias for that.
>
> > We can take it through the bpf tree if you prefer.
>
> I'll take it through the x86/urgent tree if you don't mind.

Sure. Then pls add:
Acked-by: Alexei Starovoitov <ast@...nel.org>