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Date: Thu, 8 Mar 2012 13:56:43 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: David Rientjes <rientjes@google.com>
Cc: Dave Jones <davej@redhat.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [patch] mm, hugetlb: add thread name and pid to SHM_HUGETLB
 mlock rlimit warning
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On Thu, 8 Mar 2012 13:37:57 -0800 (PST)
David Rientjes <rientjes@google.com> wrote:

> On Thu, 8 Mar 2012, Andrew Morton wrote:
> 
> > > --- a/fs/hugetlbfs/inode.c
> > > +++ b/fs/hugetlbfs/inode.c
> > > @@ -946,7 +946,11 @@ struct file *hugetlb_file_setup(const char *name, size_t size,
> > >  	if (creat_flags == HUGETLB_SHMFS_INODE && !can_do_hugetlb_shm()) {
> > >  		*user = current_user();
> > >  		if (user_shm_lock(size, *user)) {
> > > -			printk_once(KERN_WARNING "Using mlock ulimits for SHM_HUGETLB is deprecated\n");
> > > +			task_lock(current);
> > > +			printk_once(KERN_WARNING
> > > +				"%s (%d): Using mlock ulimits for SHM_HUGETLB is deprecated\n",
> > > +				current->comm, current->pid);
> > > +			task_unlock(current);
> > 
> > I assume the task_lock() is there to protect current->comm.
> 
> Yup.
> 
> > If so, it
> > is unneeded - we're protecting against prctl(PR_SET_NAME), and
> > PR_SET_NAME only operates on current, and we know this task isn't
> > currently running PR_SET_NAME.
> > 
> > If there's a way for another task to alter this task's ->comm then we
> > _do_ need locking.  But there isn't a way, I hope.
> > 
> 
> I wish there wasn't as well, it would prevent a lot of the currently buggy 
> reads to current->comm and allow us to avoid so many otherwise pointless 
> task_lock()s.
> 
> This protects against /proc/pid/comm, which is writable by threads in the 
> same thread group.

Oh crap.

>  We have a get_task_comm() that does the task_lock() 
> internally but requires a TASK_COMM_LEN buffer in the calling code.  It's 
> just easier for the calling code to the task_lock() itself for a tiny 
> little printk().

Well for a tiny little printk we could just omit the locking?  The
printk() won't oops and once in a million years one person will see a
garbled comm[] string?