linux-kernel - [PATCH] kcmp: Comment get_file_raw

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Date:   Wed,  2 Feb 2022 10:17:34 -0500
From:   Jason Andryuk <jandryuk@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Eric W . Biederman" <ebiederm@...ssion.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Jason Andryuk <jandryuk@...il.com>
Subject: [PATCH] kcmp: Comment get_file_raw_ptr() RCU usage

This usage of RCU appears wrong since the pointer is passed outside the
RCU region.  However, it is not dereferenced, so it is "okay".  Leave a
comment for the next reader.

Without a reference, these comparisons are racy, but even with their use
inside an RCU region, the result could go stale.

Signed-off-by: Jason Andryuk <jandryuk@...il.com>
---
I was looking for examples of task_lookup_fd_rcu()/files_lookup_fd_rcu()
and found this.  It differed from the example given in
Documentation/filesystems/files.rst, so I was initially confused.  A
comment seemed appropriate to avoid confusion.

 kernel/kcmp.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index 5353edfad8e1..4fb23f242e0f 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c
@@ -63,6 +63,9 @@ get_file_raw_ptr(struct task_struct *task, unsigned int idx)
 {
 	struct file *file;
 
+	/* This RCU locking is only present to silence warnings.  The pointer
+	 * value is only used for comparison and not dereferenced, so it is
+	 * acceptable. */
 	rcu_read_lock();
 	file = task_lookup_fd_rcu(task, idx);
 	rcu_read_unlock();
-- 
2.34.1