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

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Date:   Wed, 6 Apr 2022 12:46:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
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 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.