linux-kernel - Re: [PATCH 4/7] objtool: Add support for return trampoline call

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Date:   Mon, 6 Apr 2020 09:16:20 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Julien Thierry <jthierry@...hat.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [PATCH 4/7] objtool: Add support for return trampoline call

On Sat, Apr 04, 2020 at 05:51:26PM +0200, Peter Zijlstra wrote:
> On Sat, Apr 04, 2020 at 09:22:32AM -0500, Josh Poimboeuf wrote:
> > On Sat, Apr 04, 2020 at 03:32:18PM +0200, Peter Zijlstra wrote:
> > > On Fri, Apr 03, 2020 at 05:46:20PM +0200, Peter Zijlstra wrote:
> > > > On Fri, Apr 03, 2020 at 10:17:57AM -0500, Josh Poimboeuf wrote:
> > > > > Peter, I think my previous idea for UNWIND_HINT_ADJUST stack_add=8 would
> > > > > work here?
> > > > 
> > > > Yes, it would.
> > > 
> > > Sorry, I have reconsidered. While it will shut up objtool, it will not
> > > 'work'. That is, the ORC data generated will not correctly unwind.
> > > 
> > > I'll try and write a longer email tonight.
> > 
> > Right, that's what I've been trying to say.  The ORC data will be
> > non-deterministic unless we unroll the loop.  Or did you mean something
> > else?
> 
> The below should result in deterministic code.
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 07e95dcb40ad..109ee65f4a11 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -59,8 +59,8 @@
>  	jmp	775b;				\
>  774:						\
>  	dec	reg;				\
> -	jnz	771b;				\
> -	add	$(BITS_PER_LONG/8) * nr, sp;
> +	add	$(BITS_PER_LONG/8) * $2, sp;	\
> +	jnz	771b;
>  
>  #ifdef __ASSEMBLY__

That should work, though we should make sure it doesn't break the RSB
clearing -- I'm pretty sure it wouldn't, but you never know...

-- 
Josh