linux-kernel - Re: [RFC 4/4] {RFC} kmod.c: Add new call_usermodehelper

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Message-Id: <201203222048.HDB56245.JFOMVHQFSOOtLF@I-love.SAKURA.ne.jp>
Date:	Thu, 22 Mar 2012 20:48:31 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	bharrosh@...asas.com
Cc:	akpm@...ux-foundation.org, oleg@...hat.com, rjw@...k.pl,
	keyrings@...ux-nfs.org, linux-security-module@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-nfs@...r.kernel.org, Trond.Myklebust@...app.com,
	sbhamare@...asas.com, dhowells@...hat.com, eparis@...hat.com,
	srivatsa.bhat@...ux.vnet.ibm.com, kay.sievers@...y.org,
	jmorris@...ei.org, ebiederm@...ssion.com,
	gregkh@...uxfoundation.org, rusty@...tcorp.com.au, tj@...nel.org,
	rientjes@...gle.com
Subject: Re: [RFC 4/4] {RFC} kmod.c: Add new call_usermodehelper_timeout()API

Boaz Harrosh wrote:
> > @@ -452,22 +459,27 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info,
> >  
> >  	sub_info->complete = &done;
> >  	sub_info->wait = wait;
> > +	if (!sub_info->wait_timeout)
> > +		sub_info->wait_timeout = MAX_SCHEDULE_TIMEOUT;
> >  
> > +	/* Balanced in __call_usermodehelper or wait_for_helper */
> > +	kref_get(&sub_info->kref);
> >  	queue_work(khelper_wq, &sub_info->work);
> >  	if (wait == UMH_NO_WAIT)	/* task has freed sub_info */
> >  		goto unlock;
> > -	wait_for_completion(&done);
> > -	retval = sub_info->retval;
> > -
> > +	if (likely(wait_for_completion_timeout(&done, sub_info->wait_timeout)))
> > +		retval = sub_info->retval;
> > +	else
> > +		retval = -ETIMEDOUT;

This patch is incomplete because sub_info->complete refers on-stack variable.
Returning without waiting for completion will overwrite stack memory later.

> Anyway I thought that we are not
> suppose to use xhcg() since it is not portable to all ARCHs. ;-)

Don't worry. xchg() is used in many places. ;-)
http://tomoyo.sourceforge.jp/cgi-bin/lxr/ident?i=xchg

> But basically we both need support for the waiter to be returning before
> the child has completed.

I think basically we should not give up unless fatal events (e.g. SIGKILL or
TIF_MEMDIE) occur. Thus, I feel UMH_KILLABLE is sufficient.
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