linux-kernel - Re: [PATCH 5/7] x86/speculation: Annotate intra-function calls

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Date:   Fri, 3 Apr 2020 11:16:07 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, peterz@...radead.org,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH 5/7] x86/speculation: Annotate intra-function calls

On Fri, Apr 03, 2020 at 11:05:38AM -0500, Josh Poimboeuf wrote:
> On Thu, Apr 02, 2020 at 10:22:18AM +0200, Alexandre Chartre wrote:
> >  .macro RETPOLINE_JMP reg:req
> > -	call	.Ldo_rop_\@
> > +	INTRA_FUNCTION_CALL .Ldo_rop_\@
> >  .Lspec_trap_\@:
> >  	pause
> >  	lfence
> > @@ -102,7 +116,7 @@
> >  .Ldo_retpoline_jmp_\@:
> >  	RETPOLINE_JMP \reg
> >  .Ldo_call_\@:
> > -	call	.Ldo_retpoline_jmp_\@
> > +	INTRA_FUNCTION_CALL .Ldo_retpoline_jmp_\@
> >  .endm
> 
> There's a catch: this is part of an alternative.  Which means if
> X86_FEATURE_RETPOLINE isn't set at runtime, then the retpoline won't be
> there and the ORC data will be wrong.
> 
> In fact objtool should probably be made smart enough to warn about this
> situation, when an alternative changes the stack state.
> 
> The only way I can think of to fix this is to have ORC alternatives :-/

Or they could be converted to use static branches instead of
alternatives.

-- 
Josh