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From:   Abhinav Singh <singhabhinav9051571833@gmail.com>
To:     akpm@linux-foundation.org, brauner@kernel.org, surenb@google.com,
        mst@redhat.com, michael.christie@oracle.com,
        mathieu.desnoyers@efficios.com, mjguzik@gmail.com,
        npiggin@gmail.com, shakeelb@google.com, peterz@infradead.org
Cc:     linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Abhinav Singh <singhabhinav9051571833@gmail.com>
Subject: [PATCH] Fixing directly deferencing a __rcu pointer warning
Date:   Thu, 26 Oct 2023 17:57:48 +0530
Message-Id: <20231026122748.359162-1-singhabhinav9051571833@gmail.com>
In-Reply-To: <20231025165002.64ab92e6d55d204b66e055f4@linux-foundation.org>
References: <20231025165002.64ab92e6d55d204b66e055f4@linux-foundation.org>
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This patch fixes the warning about directly dereferencing a pointer
tagged with __rcu annotation.

Dereferencing the pointers tagged with __rcu directly should
always be avoided according to the docs. There is a rcu helper
functions rcu_dereference(...) to use when dereferencing a __rcu
pointer. This functions returns the non __rcu tagged pointer which
can be dereferenced just like a normal pointers.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
---
 kernel/fork.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 10917c3e1f03..802b7bbe3d92 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2369,7 +2369,7 @@ __latent_entropy struct task_struct *copy_process(
 
 	retval = -EAGAIN;
 	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
-		if (p->real_cred->user != INIT_USER &&
+		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
 		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
 			goto bad_fork_cleanup_count;
 	}
@@ -2692,7 +2692,7 @@ __latent_entropy struct task_struct *copy_process(
 			 */
 			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
 							 p->real_parent->signal->is_child_subreaper;
-			list_add_tail(&p->sibling, &p->real_parent->children);
+			list_add_tail(&p->sibling, &(rcu_dereference(p->real_parent)->children));
 			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			attach_pid(p, PIDTYPE_TGID);
 			attach_pid(p, PIDTYPE_PGID);
-- 
2.39.2