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From:   Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Date:   Fri, 7 Feb 2020 20:34:29 +0530
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
        "Eric W. Biederman" <ebiederm@gmail.com>, ebiederm@xmission.com,
        oleg@redhat.com, christian.brauner@ubuntu.com, guro@fb.com,
        tj@kernel.org, linux-kernel@vger.kernel.org, paulmck@kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org, frextrite@gmail.com
Subject: Re: [PATCH] signal.c: Fix sparse warnings
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 <20200206110051.GA4531@madhuparna-HP-Notebook>
 <20200206202511.GC36876@google.com>
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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@gmail.com writes:
> > > 
> > > > From: Madhuparna Bhowmik <madhuparnabhowmik10@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@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.
> 
> 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
>