Re: [RFC][PATCH] Add prctl to set sibling thread names

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 21 Oct 2009 17:52:24 -0700 john stultz <johnstul@us.ibm.com> wrote:

> On Wed, 2009-10-21 at 17:48 -0700, Arjan van de Ven wrote:
> > On Wed, 21 Oct 2009 16:21:37 -0700
> > john stultz <johnstul@us.ibm.com> wrote:
> > 
> > > 
> > > Taking a very raw attempt at this, I scratched out the following
> > > simple implementation. I'd appreciate any review or suggestions for
> > > improvements. I'm not at all certain the passing of the thread pid_t
> > > through the unsigned long is valid, for instance, or if
> > > same_thread_group() is the right check to make sure we only change
> > > siblings and not tid from other processes. So any advice on better
> > > approaches would be great.
> > > 
> > > +				return -EINVAL;
> > > +
> > > +			set_task_comm(tsk, comm);
> > 
> > 
> > you're pretty much the first now who touches ->comm from
> > not-the-thread-itself.... are you sure that is safe?
> 
> No, I'm not sure at all :)
> 
> Thanks for pointing this out. I'll see whats needed in set_task_comm().
> 

set_task_comm() is OK.  The problem will be the unwritten rule that
processes can read *their own* ->comm without task_lock(), because nobody
ever alters ->comm apart from tack which owns it.

You've changed that, so all the open-coded accesses to current->comm are
now racy.

Also, you appear to be running set_task_comm() against a task_struct
without holding a reference on that task.  Will a well-timed exit() cause a
modify-after-free?