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

[Alexandre Chartre]

On 4/3/20 6:16 PM, Josh Poimboeuf wrote:
> 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 :-/

So that means that any alternative that does a stack manipulation isn't
currently supported?

alex.

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