netdev - Re: [PATCH v2 06/10] mm/kmemleak: Replace strncpy() with __get_task

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Date: Thu, 13 Jun 2024 20:10:17 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Catalin Marinas <catalin.marinas@....com>
Cc: torvalds@...ux-foundation.org, ebiederm@...ssion.com, 
	alexei.starovoitov@...il.com, rostedt@...dmis.org, linux-mm@...ck.org, 
	linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	audit@...r.kernel.org, linux-security-module@...r.kernel.org, 
	selinux@...r.kernel.org, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	dri-devel@...ts.freedesktop.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 06/10] mm/kmemleak: Replace strncpy() with __get_task_comm()

On Thu, Jun 13, 2024 at 4:37 PM Catalin Marinas <catalin.marinas@....com> 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 <laoar.shao@...il.com>
> > Cc: Catalin Marinas <catalin.marinas@....com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > ---
> >  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/20240613023044.45873-2-laoar.shao@gmail.com/

-- 
Regards
Yafang