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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 v3] Fixing directly deferencing a __rcu pointer warning
Date:   Sun, 29 Oct 2023 02:16:09 +0530
Message-Id: <20231028204609.426841-1-singhabhinav9051571833@gmail.com>
In-Reply-To: <93115738-6919-4855-81b7-eb9b2fc60137@gmail.com>
References: <93115738-6919-4855-81b7-eb9b2fc60137@gmail.com>
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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
function rcu_dereference(...) to use when dereferencing a __rcu
pointer inside rcu read side critical sections. This function 
returns the non __rcu tagged pointer which can be dereferenced 
just like a normal pointer.

Signed-off-by: Abhinav Singh <singhabhinav9051571833@gmail.com>
---

Link to original patch 
 https://lore.kernel.org/all/20231025222811.855336-1-singhabhinav9051571833@gmail.com/

Change from original -> v2 : 
 1. removed the null check before dereferencing the dereferenced rcu
    pointer at line 2372.
 2. added rcu_dereference(...) at line 2694
    
Changes from v2 -> v3 
 1. added rcu_dereference(...) at line 2693

 kernel/fork.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 10917c3e1f03..e78649974669 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;
 	}
@@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
 			 * tasklist_lock with adding child to the process tree
 			 * for propagate_has_child_subreaper optimization.
 			 */
-			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);
+			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
+							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
+			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