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

From: Borislav Petkov <bp@alien8.de>
To: Josh Poimboeuf <jpoimboe@redhat.com>
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: Wed, 19 Jan 2022 11:01:21 +0100	[thread overview]
Message-ID: <YefhcXouSQwETTvl@zn.tnic> (raw)
In-Reply-To: <20220119000327.oapghqad4lebnsra@treble>

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