Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__

[Josh Poimboeuf]

On Wed, Jan 19, 2022 at 12:33:02AM +0100, Borislav Petkov wrote:
> On Tue, Jan 18, 2022 at 03:01:20PM -0800, Josh Poimboeuf wrote:
> > With the two WARN_ONs in media_request_object_complete(), GCC apparently
> > considers the two reachable() asm statements as duplicates, and it
> > removes the second one.
> 
> Could that be the same thing:
> 
> net/xfrm/xfrm_output.o: warning: objtool: xfrm_output_resume()+0x2e0: unreachable instruction
> 
> I see two WARN_ONs in asm output there too...

If one of the '__bug_table' asm snippets isn't immediately followed by
the .L[un]reachable asm, then yeah, it's the same issue.


For example it's supposed to look something like this:


# 472 "net/xfrm/xfrm_output.c" 1
	1:	.byte 0x0f, 0x0b
.pushsection __bug_table,"aw"
2:	.long 1b - 2b	# bug_entry::bug_addr
	.long .LC4 - 2b	# bug_entry::file	#
	.word 472	# bug_entry::line	#
	.word 2307	# bug_entry::flags	#
	.org 2b+12	#
.popsection
# 0 "" 2
# 472 "net/xfrm/xfrm_output.c" 1
	.Lreachable1666:
	.pushsection .discard.reachable
	.long .Lreachable1666 - .
	.popsection


NOT just this:


# 472 "net/xfrm/xfrm_output.c" 1
	1:	.byte 0x0f, 0x0b
.pushsection __bug_table,"aw"
2:	.long 1b - 2b	# bug_entry::bug_addr
	.long .LC4 - 2b	# bug_entry::file	#
	.word 472	# bug_entry::line	#
	.word 2307	# bug_entry::flags	#
	.org 2b+12	#
.popsection
# some other code here...


There's a bunch of those throughout the code base.  The current
annotate_[un]reachable() implementations are carefully written to avoid
that happening.

-- 
Josh