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From:   ebiederm@gmail.com (Eric W. Biederman)
To:     madhuparnabhowmik10@gmail.com
Cc:     ebiederm@xmission.com, oleg@redhat.com,
        christian.brauner@ubuntu.com, guro@fb.com, tj@kernel.org,
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        linux-kernel-mentees@lists.linuxfoundation.org, frextrite@gmail.com
In-Reply-To: <20200205172437.10113-1-madhuparnabhowmik10@gmail.com>
        (madhuparnabhowmik's message of "Wed, 5 Feb 2020 22:54:37 +0530")
References: <20200205172437.10113-1-madhuparnabhowmik10@gmail.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
Date:   Wed, 05 Feb 2020 16:59:52 -0600
Message-ID: <87wo90myhj.fsf@x220.int.ebiederm.org>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [PATCH] signal.c: Fix sparse warnings
Sender: linux-kernel-owner@vger.kernel.org
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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.

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


Eric