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

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Message-ID: <20211020193447.GR174703@worktop.programming.kicks-ass.net>
Date:   Wed, 20 Oct 2021 21:34:47 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array

On Wed, Oct 20, 2021 at 05:46:39PM +0100, Andrew Cooper wrote:
> 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.

Arguably I should probably write that like:

	(target - &__x86_indirect_thunk_array) / __x86_indirect_thunk_size;

or something, and also generate and expose that latter symbol from .S.