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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Borislav Petkov <bp@alien8.de>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Peter Zijlstra <peterz@infradead.org>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	x86@kernel.org, llvm@lists.linux.dev,
	linux-sparse@vger.kernel.org, linux-kernel@vger.kernel.org,
	kernel test robot <lkp@intel.com>,
	Nathan Chancellor <nathan@kernel.org>
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__
Date: Tue, 18 Jan 2022 16:03:27 -0800	[thread overview]
Message-ID: <20220119000327.oapghqad4lebnsra@treble> (raw)
In-Reply-To: <YedOLva0zos3A1JE@zn.tnic>

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


  reply	other threads:[~2022-01-19  0:03 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-01-14  1:05 [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER__ Nick Desaulniers
2022-01-14 21:58 ` Nathan Chancellor
2022-01-16 13:32 ` Borislav Petkov
2022-01-18 19:22   ` Josh Poimboeuf
2022-01-18 23:01     ` Josh Poimboeuf
2022-01-18 23:33       ` Borislav Petkov
2022-01-19  0:03         ` Josh Poimboeuf [this message]
2022-01-19 10:01           ` Borislav Petkov
2022-01-24 23:26       ` Nick Desaulniers
2022-01-24 23:38         ` Nick Desaulniers
2022-01-25 18:49           ` Borislav Petkov
2022-01-25 23:31         ` Segher Boessenkool
2022-01-26  0:59           ` Nick Desaulniers
2022-01-26  2:12             ` Nick Desaulniers
2022-01-26 11:13               ` Segher Boessenkool
2022-01-31 20:45           ` Nick Desaulniers
2022-01-31 22:13             ` Segher Boessenkool

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