linux-kernel - Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array

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Message-ID: <f33b9c4e-08c1-f88d-4873-82050a944010@citrix.com>
Date:   Wed, 20 Oct 2021 17:46:39 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <alexei.starovoitov@...il.com>, <ndesaulniers@...gle.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array

On 20/10/2021 16:57, Josh Poimboeuf wrote:
> On Wed, Oct 20, 2021 at 12:44:50PM +0200, Peter Zijlstra wrote:
>> Stick all the retpolines in a single symbol and have the individual
>> thunks as inner labels, this should guarantee thunk order and layout.
> How so?
>
> Just wondering what the purpose of the array is.  It doesn't seem to be
> referenced anywhere.

The array property is what makes:

> +	reg = (target - &__x86_indirect_thunk_rax) /
> +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);

safe in the next path.

~Andrew