Re: [PATCH] signal.c: Fix sparse warnings

From: Joel Fernandes
Date: Thu Feb 06 2020 - 15:25:15 EST


On Thu, Feb 06, 2020 at 04:30:51PM +0530, Madhuparna Bhowmik wrote:
> On Wed, Feb 05, 2020 at 04:59:52PM -0600, Eric W. Biederman wrote:
> > madhuparnabhowmik10@xxxxxxxxx writes:
> >
> > > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > >
> > > 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@xxxxxxxxx>
> > > ---
> > > 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.

But then there are several other ->parent accesses in the function. What
about something like the following? It removes the confusion Eric is
referring to and fixes the sparse errors you mentioned. Thoughts?

---8<-----------------------

diff --git a/kernel/signal.c b/kernel/signal.c
index bcd46f547db39..92f0b7bf70bf3 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1909,6 +1909,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
struct sighand_struct *psig;
bool autoreap = false;
u64 utime, stime;
+ struct task_struct *tsk_parent;

BUG_ON(sig == -1);

@@ -1918,6 +1919,9 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
BUG_ON(!tsk->ptrace &&
(tsk->group_leader != tsk || !thread_group_empty(tsk)));

+ tsk_parent = rcu_dereference_protected(tsk->parent,
+ lockdep_is_held(&tasklist_lock));
+
/* Wake up all pidfd waiters */
do_notify_pidfd(tsk);

@@ -1926,7 +1930,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
* This is only possible if parent == real_parent.
* Check if it has changed security domain.
*/
- if (tsk->parent_exec_id != tsk->parent->self_exec_id)
+ if (tsk->parent_exec_id != tsk_parent->self_exec_id)
sig = SIGCHLD;
}

@@ -1945,8 +1949,8 @@ 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(tsk_parent));
+ info.si_uid = from_kuid_munged(task_cred_xxx(tsk_parent, user_ns),
task_uid(tsk));
rcu_read_unlock();

@@ -1964,7 +1968,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
info.si_status = tsk->exit_code >> 8;
}

- psig = tsk->parent->sighand;
+ psig = tsk_parent->sighand;
spin_lock_irqsave(&psig->siglock, flags);
if (!tsk->ptrace && sig == SIGCHLD &&
(psig->action[SIGCHLD-1].sa.sa_handler == SIG_IGN ||
@@ -1989,8 +1993,8 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
sig = 0;
}
if (valid_signal(sig) && sig)
- __group_send_sig_info(sig, &info, tsk->parent);
- __wake_up_parent(tsk, tsk->parent);
+ __group_send_sig_info(sig, &info, tsk_parent);
+ __wake_up_parent(tsk, tsk_parent);
spin_unlock_irqrestore(&psig->siglock, flags);

return autoreap;
--
2.25.0.341.g760bfbb309-goog