[Linux-kernel-mentees] [PATCH] signal.c: Fix sparse warnings

[Madhuparna Bhowmik]

On Wed, Feb 05, 2020 at 04:59:52PM -0600, Eric W. Biederman wrote:
> madhuparnabhowmik10 at gmail.com writes:
> 
> > From: Madhuparna Bhowmik <madhuparnabhowmik10 at gmail.com>
> >
> > This patch fixes the following two sparse warnings caused due to
> > accessing RCU protected pointer tsk->parent without rcu primitives.
> >
> > kernel/signal.c:1948:65: warning: incorrect type in argument 1 (different address spaces)
> > kernel/signal.c:1948:65:    expected struct task_struct *tsk
> > kernel/signal.c:1948:65:    got struct task_struct [noderef] <asn:4> *parent
> > kernel/signal.c:1949:40: warning: incorrect type in argument 1 (different address spaces)
> > kernel/signal.c:1949:40:    expected void const volatile *p
> > kernel/signal.c:1949:40:    got struct cred const [noderef] <asn:4> *[noderef] <asn:4> *
> > kernel/signal.c:1949:40: warning: incorrect type in argument 1 (different address spaces)
> > kernel/signal.c:1949:40:    expected void const volatile *p
> > kernel/signal.c:1949:40:    got struct cred const [noderef] <asn:4> *[noderef] <asn:4> *
> >
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10 at gmail.com>
> > ---
> >  kernel/signal.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/signal.c b/kernel/signal.c
> > index 9ad8dea93dbb..8227058ea8c4 100644
> > --- a/kernel/signal.c
> > +++ b/kernel/signal.c
> > @@ -1945,8 +1945,8 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
> >  	 * correct to rely on this
> >  	 */
> >  	rcu_read_lock();
> > -	info.si_pid = task_pid_nr_ns(tsk, task_active_pid_ns(tsk->parent));
> > -	info.si_uid = from_kuid_munged(task_cred_xxx(tsk->parent, user_ns),
> > +	info.si_pid = task_pid_nr_ns(tsk, task_active_pid_ns(rcu_dereference(tsk->parent)));
> > +	info.si_uid = from_kuid_munged(task_cred_xxx(rcu_dereference(tsk->parent), user_ns),
> >  				       task_uid(tsk));
> >  	rcu_read_unlock();
> 
> 
> Still wrong because that access fundamentally depends upon the
> task_list_lock no the rcu_read_lock.  Things need to be consistent for
> longer than the rcu_read_lock is held.
>
Okay, then how about something like rcu_dereference_protected(tsk->parent, lockdep_is_held(&tasklist_lock))?
Let me know if this looks fine to you.

Thank you,
Madhuparna

> This patch makes sparse happy and confuses programmers who are trying to
> read the code.
> 
> 
> Eric