linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Date:   Wed, 13 Oct 2021 13:52:59 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

On Wed, Oct 13, 2021 at 02:22:21PM +0200, Peter Zijlstra wrote:
>  	/*
> +	 * Rewrite the retpolines, must be done before alternatives since
> +	 * those can rewrite the retpoline thunks.
> +	 */

Why exactly is that a problem?  This code doesn't read the thunks.

BTW, CALL_NOSPEC results in a retpoline site in .altinstr_replacement:

Relocation section [40] '.rela.retpoline_sites' for section [39] '.retpoline_sites' at offset 0x8d28 contains 1 entry:
  Offset              Type            Value               Addend Name
  000000000000000000  X86_64_PC32     000000000000000000     +10 .altinstr_replacement

Which I assume we don't want.

-- 
Josh