linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Date:   Fri, 22 Oct 2021 08:22:35 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, X86 ML <x86@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Fri, Oct 22, 2021 at 4:33 AM Peter Zijlstra <peterz@...radead.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.