linux-kernel - Re: [PATCH] objtool: prefer memory clobber & %= to volatile & __COUNTER_

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Message-ID: <YefhcXouSQwETTvl@zn.tnic>
Date:   Wed, 19 Jan 2022 11:01:21 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        x86@...nel.org, llvm@...ts.linux.dev, linux-sparse@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel test robot <lkp@...el.com>,
        Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH] objtool: prefer memory clobber & %= to volatile &
 __COUNTER__

On Tue, Jan 18, 2022 at 04:03:27PM -0800, Josh Poimboeuf wrote:
> 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.

Found one.

AFAICT, that's the WARN_ON_ONCE(1) catch-all in the default: label of
the switch-case in nf_hook(). That thing is followed by other gunk and
no *reachable label near it.

Damn - that was too good to be true. Gotta love those compilers. :-P

# ./include/linux/netfilter.h:252: 		WARN_ON_ONCE(1);
#APP
# 252 "./include/linux/netfilter.h" 1
	1:	.byte 0x0f, 0x0b
.pushsection __bug_table,"aw"
2:	.long 1b - 2b	# bug_entry::bug_addr
	.long .LC5 - 2b	# bug_entry::file	#
	.word 252	# bug_entry::line	#
	.word 2307	# bug_entry::flags	#
	.org 2b+12	#
.popsection
# 0 "" 2
#NO_APP
	.p2align 4,,3
	jmp	.L344	#
.L265:
# ./include/linux/netfilter.h:229: 		hook_head = rcu_dereference(net->nf.hooks_ipv4[hook]);

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette