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From:   Richard Fitzgerald <rf@opensource.cirrus.com>
To:     <gregkh@linuxfoundation.org>, <rafael@kernel.org>
CC:     <linux-kernel@vger.kernel.org>, <patches@opensource.cirrus.com>,
        Richard Fitzgerald <rf@opensource.cirrus.com>
Subject: [PATCH 3/5] debugfs: Update debugfs_create_str() kerneldoc to warn about pointer race
Date:   Tue, 16 May 2023 17:07:51 +0100
Message-ID: <20230516160753.32317-4-rf@opensource.cirrus.com>
In-Reply-To: <20230516160753.32317-1-rf@opensource.cirrus.com>
References: <20230516160753.32317-1-rf@opensource.cirrus.com>
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Add a warning to the debugfs_create_str() kerneldoc that the char * pointer
value must not change after the function returns, because of a race with
debugfs_read_file_str() accessing the pointer.

The only safe case is a change from NULL to non-NULL because in that case
debugfs_read_file_str() will see either the NULL or the valid pointer.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
---
 fs/debugfs/file.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index 0c039a3d9a42..77794871f26d 100644
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -938,11 +938,18 @@ static const struct file_operations fops_str_wo = {
  *          directory dentry if set.  If this parameter is %NULL, then the
  *          file will be created in the root of the debugfs filesystem.
  * @value: a pointer to the variable that the file should read to and write
- *         from.
+ *         from. The char* pointer must not change, except from NULL to
+ *         non-NULL.
  *
  * This function creates a file in debugfs with the given name that
  * contains the value of the variable @value.  If the @mode variable is so
  * set, it can be read from, and written to.
+ *
+ * The char* pointed to by @value must not change after calling this
+ * function EXCEPT that it may change from NULL to non-NULL. This is to
+ * prevent the file read from accessing a stale pointer. A change from
+ * NULL to non-NULL is the only safe change, because the read will
+ * instantaneously see either NULL or the valid pointer.
  */
 void debugfs_create_str(const char *name, umode_t mode,
 			struct dentry *parent, char **value)
-- 
2.30.2