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

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Message-ID: <YFTJro42GX6m7O5Q@hirez.programming.kicks-ass.net>
Date:   Fri, 19 Mar 2021 16:56:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
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 10:30:26AM -0500, Josh Poimboeuf wrote:
> 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.

UNDEF has size 0. That is, unless these symbols exist in the translation
unit (they do not) we have no clue.

Arguably I could parse the symbol name again and then we know the
register number and thus if we need REX etc.. but I figured we wanted to
avoid all that.