linux-kernel - Re: [PATCH v2 14/14] objtool,x86: Rewrite retpoline thunk calls

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Message-ID: <20210319153026.cfqwyy36feqyunyd@treble>
Date:   Fri, 19 Mar 2021 10:30:26 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, jgross@...e.com, mbenes@...e.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 14/14] objtool,x86: Rewrite retpoline thunk calls

On Fri, Mar 19, 2021 at 09:06:44AM +0100, Peter Zijlstra wrote:
> > Also doesn't the alternative code already insert nops?
> 
> Problem is that the {call,jmp} *%\reg thing is not fixed length. They're
> 2 or 3 bytes depending on which register is picked.

Why do they need to be fixed length?  Objtool can use sym->len as the
alternative replacement length.  Then alternatives can add nops as
needed.

-- 
Josh