linux-kernel - Re: [PATCH] debug: fix WARN_ON

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Date:   Mon, 17 Jul 2017 16:00:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Mike Galbraith <efault@....de>
Subject: Re: [PATCH] debug: fix WARN_ON_ONCE() for modules

On Sat, Jul 15, 2017 at 12:10:58AM -0500, Josh Poimboeuf wrote:
> Mike Galbraith reported a situation where a WARN_ON_ONCE() call in DRM
> code turned into an oops.  As it turns out, WARN_ON_ONCE() seems to be
> completely broken when called from a module.
> 
> The bug was introduced with the following commit:
> 
>   19d436268dde ("debug: Add _ONCE() logic to report_bug()")
> 
> That commit changed WARN_ON_ONCE() to move its 'once' logic into the bug
> trap handler.  It requires a writable bug table so that the BUGFLAG_DONE
> bit can be written to the flags to indicate the first warning has
> occurred.
> 
> The bug table was made writable for vmlinux, which relies on
> vmlinux.lds.S and vmlinux.lds.h for laying out the sections.  However,
> it wasn't made writable for modules, which rely on the ELF section
> header flags.
> 
> Reported-by: Mike Galbraith <efault@....de>
> Fixes: 19d436268dde ("debug: Add _ONCE() logic to report_bug()")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> ---

> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index 39e702d..aa6b202 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -35,7 +35,7 @@
>  #define _BUG_FLAGS(ins, flags)						\
>  do {									\
>  	asm volatile("1:\t" ins "\n"					\
> -		     ".pushsection __bug_table,\"a\"\n"			\
> +		     ".pushsection __bug_table,\"aw\"\n"		\
>  		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
>  		     "\t"  __BUG_REL(%c0) "\t# bug_entry::file\n"	\
>  		     "\t.word %c1"        "\t# bug_entry::line\n"	\
> @@ -52,7 +52,7 @@ do {									\
>  #define _BUG_FLAGS(ins, flags)						\
>  do {									\
>  	asm volatile("1:\t" ins "\n"					\
> -		     ".pushsection __bug_table,\"a\"\n"			\
> +		     ".pushsection __bug_table,\"aw\"\n"		\
>  		     "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n"	\
>  		     "\t.word %c0"        "\t# bug_entry::flags\n"	\
>  		     "\t.org 2b+%c1\n"					\


Ah shiny. Thanks Josh!