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Date: Fri, 7 Feb 2020 20:34:29 +0530
From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
"Eric W. Biederman" <ebiederm@...il.com>, ebiederm@...ssion.com,
oleg@...hat.com, christian.brauner@...ntu.com, guro@...com,
tj@...nel.org, linux-kernel@...r.kernel.org, paulmck@...nel.org,
linux-kernel-mentees@...ts.linuxfoundation.org, frextrite@...il.com
Subject: Re: [PATCH] signal.c: Fix sparse warnings
On Thu, Feb 06, 2020 at 03:25:11PM -0500, Joel Fernandes wrote:
> 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@...il.com writes:
> > >
> > > > From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.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@...il.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.
>
> 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?
>
Yes, I agree this should remove the confusion.
Thank you,
Madhuparna
> ---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
>
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