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

[Peter Zijlstra]

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__