Re: [PATCH 4/7] objtool: Add support for return trampoline call
From: Alexandre Chartre
Date: Mon Apr 06 2020 - 06:59:33 EST
On 4/6/20 11:31 AM, Peter Zijlstra wrote:
On Mon, Apr 06, 2020 at 10:19:56AM +0200, Alexandre Chartre wrote:
On 4/4/20 5:51 PM, 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__
Nice. This works fine and allows to remove ANNOTATE_NOSPEC_ALTERNATIVE when
using __FILL_RETURN_BUFFER. However this is probably less performant because
we now have nr/2 add instructions instead of just 1.
Does it actually matter though? That is, can you measure the difference?
I didn't do any measurement, I am just anticipating concerns others might have
as this code is used during context and vmexit. But I agree this might not be
significant.
alex.