Subject: Re: [PATCH v2 06/10] mm/kmemleak: Replace strncpy() with __get_task_comm()

From: Catalin Marinas <[email-masked]>
Date: Fri, 14 Jun 2024 11:57:21 +0100

On Thu, Jun 13, 2024 at 08:10:17PM +0800, Yafang Shao wrote:
> On Thu, Jun 13, 2024 at 4:37 PM Catalin Marinas <[email-masked]> wrote:
> > On Thu, Jun 13, 2024 at 10:30:40AM +0800, Yafang Shao wrote:
> > > Using __get_task_comm() to read the task comm ensures that the name is
> > > always NUL-terminated, regardless of the source string. This approach also
> > > facilitates future extensions to the task comm.
> > >
> > > Signed-off-by: Yafang Shao <[email-masked]>
> > > Cc: Catalin Marinas <[email-masked]>
> > > Cc: Andrew Morton <[email-masked]>
> > > ---
> > >  mm/kmemleak.c | 8 +-------
> > >  1 file changed, 1 insertion(+), 7 deletions(-)
> > >
> > > diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> > > index d5b6fba44fc9..ef29aaab88a0 100644
> > > --- a/mm/kmemleak.c
> > > +++ b/mm/kmemleak.c
> > > @@ -663,13 +663,7 @@ static struct kmemleak_object *__alloc_object(gfp_t gfp)
> > >               strncpy(object->comm, "softirq", sizeof(object->comm));
> > >       } else {
> > >               object->pid = current->pid;
> > > -             /*
> > > -              * There is a small chance of a race with set_task_comm(),
> > > -              * however using get_task_comm() here may cause locking
> > > -              * dependency issues with current->alloc_lock. In the worst
> > > -              * case, the command line is not correct.
> > > -              */
> > > -             strncpy(object->comm, current->comm, sizeof(object->comm));
> > > +             __get_task_comm(object->comm, sizeof(object->comm), current);
> > >       }
> >
> > You deleted the comment stating why it does not use get_task_comm()
> > without explaining why it would be safe now. I don't recall the details
> > but most likely lockdep warned of some potential deadlocks with this
> > function being called with the task_lock held.
> >
> > So, you either show why this is safe or just use strscpy() directly here
> > (not sure we'd need strscpy_pad(); I think strscpy() would do, we just
> > need the NUL-termination).
> 
> The task_lock was dropped in patch #1 [0]. My apologies for not
> including you in the CC for that change. After this modification, it
> is now safe to use __get_task_comm().
> 
> [0] https://lore.kernel.org/all/[email-masked]/

Ah, great. For this patch:

Acked-by: Catalin Marinas <[email-masked]>

You may want to add a comment in the commit log that since
__get_task_comm() no longer takes a long, it's safe to call it from
kmemleak.

-- 
Catalin