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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 5/5] debugfs: Add debugfs_create_const_str()
Date:   Tue, 16 May 2023 17:07:53 +0100
Message-ID: <20230516160753.32317-6-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 wrapper for debugfs_create_str() that takes a const char **.

It's never nice to have to cast a const pointer to a non-const to be
able to pass it to an API. It always looks suspicious and it is relying
on "knowing" that it's safe. A function that explicitly takes a const
pointer is creating a contract that a const pointer is safe.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
---
 include/linux/debugfs.h | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/include/linux/debugfs.h b/include/linux/debugfs.h
index ea2d919fd9c7..2723690aedd1 100644
--- a/include/linux/debugfs.h
+++ b/include/linux/debugfs.h
@@ -401,4 +401,31 @@ static inline void debugfs_create_xul(const char *name, umode_t mode,
 		debugfs_create_x64(name, mode, parent, (u64 *)value);
 }
 
+/**
+ * debugfs_create_const_str - create a debugfs file that is used to read a string value
+ * @name: a pointer to a string containing the name of the file to create.
+ * @mode: the permission that the file should have
+ * @parent: a pointer to the parent dentry for this file.  This should be a
+ *          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 from.
+ *         The const 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.
+ *
+ * The const 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.
+ */
+static inline void debugfs_create_const_str(const char *name, umode_t mode,
+					    struct dentry *parent,
+					    const char **value)
+{
+	debugfs_create_str(name, mode & ~0222, parent, (char **)value);
+}
+
 #endif
-- 
2.30.2