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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Fri, 22 Oct 2021 08:22:35 -0700
Message-ID: <CAADnVQKJojWGaTCpUhkmU+vUxXORPacX_ByjyHWY0V03hGH7KA@mail.gmail.com>
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Josh Poimboeuf <jpoimboe@redhat.com>, X86 ML <x86@kernel.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        bpf <bpf@vger.kernel.org>, Andrii Nakryiko <andrii@kernel.org>
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On Fri, Oct 22, 2021 at 4:33 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Thu, Oct 21, 2021 at 04:42:12PM -0700, Alexei Starovoitov wrote:
>
> > Ahh. Right. It's potentially a different offset for every prog.
> > Let's put it into struct jit_context then.
>
> Something like this...

Yep. Looks nice and clean to me.

> -       poke->tailcall_bypass = image + (addr - poke_off - X86_PATCH_SIZE);
> +       poke->tailcall_bypass = ip + (prog - start);
>         poke->adj_off = X86_TAIL_CALL_OFFSET;
> -       poke->tailcall_target = image + (addr - X86_PATCH_SIZE);
> +       poke->tailcall_target = ip + ctx->tail_call_direct_label - X86_PATCH_SIZE;

This part looks correct too, but this is Daniel's magic.
He'll probably take a look next week when he comes back from PTO.
I don't recall which test exercises this tailcall poking logic.
It's only used with dynamic updates to prog_array.
insmod test_bpf.ko and test_verifier won't go down this path.