linux-kernel - Re: [GIT PULL] trace: module: Maintain a valid user count

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Date:	Thu, 8 May 2014 09:17:58 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Romain Izard <romain.izard.pro@...il.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [GIT PULL] trace: module: Maintain a valid user count

Linus,

You can hold off on pulling this if you want. I just received a patch
that fixes a "use after free" bug that was introduced in this merge
window. I've added that patch after this commit, and I'm starting my
tests now. I should have another pull request later today if all goes
well.

-- Steve


On Thu, 8 May 2014 07:13:27 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> Linus,
> 
> This is a long standing bug that causes bogus data to show up
> in the refcnt field of the module_refcnt tracepoint. It was
> introduced by a merge conflict resolution back in 2.6.35-rc days.
> 
> The result should be refcnt = incs - decs, but instead it did
> refcnt = incs + decs.
> 
> Please pull the latest trace-fixes-v3.15-rc4 tree, which can be found at:
> 
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git
> trace-fixes-v3.15-rc4
> 
> Tag SHA1: df839af2f97c5761d24b7a1c80f5ac8c4fa63dfe
> Head SHA1: 098507ae3ec2331476fb52e85d4040c1cc6d0ef4
> 
> 
> Romain Izard (1):
>       trace: module: Maintain a valid user count
> 
> ----
>  include/trace/events/module.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> ---------------------------
> commit 098507ae3ec2331476fb52e85d4040c1cc6d0ef4
> Author: Romain Izard <romain.izard.pro@...il.com>
> Date:   Tue Mar 4 10:09:39 2014 +0100
> 
>     trace: module: Maintain a valid user count
>     
>     The replacement of the 'count' variable by two variables 'incs' and
>     'decs' to resolve some race conditions during module unloading was done
>     in parallel with some cleanup in the trace subsystem, and was integrated
>     as a merge.
>     
>     Unfortunately, the formula for this replacement was wrong in the tracing
>     code, and the refcount in the traces was not usable as a result.
>     
>     Use 'count = incs - decs' to compute the user count.
>     
>     Link: http://lkml.kernel.org/p/1393924179-9147-1-git-send-email-romain.izard.pro@gmail.com
>     
>     Acked-by: Ingo Molnar <mingo@...nel.org>
>     Cc: Rusty Russell <rusty@...tcorp.com.au>
>     Cc: Frederic Weisbecker <fweisbec@...il.com>
>     Cc: stable@...r.kernel.org # 2.6.35
>     Fixes: c1ab9cab7509 "merge conflict resolution"
>     Signed-off-by: Romain Izard <romain.izard.pro@...il.com>
>     Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> 
> diff --git a/include/trace/events/module.h b/include/trace/events/module.h
> index 11fd51b..daa60c7 100644
> --- a/include/trace/events/module.h
> +++ b/include/trace/events/module.h
> @@ -80,7 +80,7 @@ DECLARE_EVENT_CLASS(module_refcnt,
>  
>  	TP_fast_assign(
>  		__entry->ip	= ip;
> -		__entry->refcnt	= __this_cpu_read(mod->refptr->incs) + __this_cpu_read(mod->refptr->decs);
> +		__entry->refcnt	= __this_cpu_read(mod->refptr->incs) - __this_cpu_read(mod->refptr->decs);
>  		__assign_str(name, mod->name);
>  	),
>  

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