Re: [patch] mm, oom: remove task_lock protecting comm printing

Michal Hocko
2015-09-23T09:44:01Z

On Wed 23-09-15 12:30:22, Vladimir Davydov wrote:
> On Wed, Sep 23, 2015 at 06:13:54PM +0900, Sergey Senozhatsky wrote:
> > On (09/23/15 11:06), Vladimir Davydov wrote:
> > > Hi,
> > >
> > > On Tue, Sep 22, 2015 at 04:30:13PM -0700, David Rientjes wrote:
> > > > The oom killer takes task_lock() in a couple of places solely to protect
> > > > printing the task's comm.
> > > >
> > > > A process's comm, including current's comm, may change due to
> > > > /proc/pid/comm or PR_SET_NAME.
> > > >
> > > > The comm will always be NULL-terminated, so the worst race scenario would
> > > > only be during update. We can tolerate a comm being printed that is in
> > > > the middle of an update to avoid taking the lock.
> > > >
> > > > Other locations in the kernel have already dropped task_lock() when
> > > > printing comm, so this is consistent.
> > >
> > > Without the protection, can't reading task->comm race with PR_SET_NAME
> > > as described below?
> >
> > the previous name was already null terminated,
>
> Yeah, but if the old name is shorter than the new one, set_task_comm()
> overwrites the terminating null of the old name before writing the new
> terminating null, so there is a short time window during which tsk->comm
> might be not null-terminated, no?

Not really:
case PR_SET_NAME:
comm[sizeof(me->comm) - 1] = 0;
if (strncpy_from_user(comm, (char __user *)arg2,
sizeof(me->comm) - 1) < 0)
return -EFAULT;

So it first writes the terminating 0 and only then starts copying.
--
Michal Hocko
SUSE Labs