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

Michal Hocko
2015-09-23T10:41:37Z

On Wed 23-09-15 13:07:40, Vladimir Davydov wrote:
> On Wed, Sep 23, 2015 at 06:50:22PM +0900, Sergey Senozhatsky wrote:
> > On (09/23/15 11:43), Michal Hocko wrote:
> > [..]
> > > > > 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.
>
> It writes 0 to a temporary buffer, not to tsk->comm, so I don't think
> it's related. However, reading tsk->comm w/o locking must be safe
> anyway, because tsk->comm[TASK_COMM_LEN-1] is always 0 (inherited from
> init_task) and it never gets overwritten, because __set_task_comm() uses
> strlcpy().

Right you are! I am blind obviously... Thought we are copying directly
to the task->comm.

--
Michal Hocko
SUSE Labs