[PATCH v5.15-v5.10 2/4] fs: new helper vfs_empty_path()

[Miao Wang via B4 Relay <devnull+shankerwangmiao.gmail.com@kernel.org>]

From: Christian Brauner <brauner@kernel.org>

commit 1bc6d44 upstream.

Make it possible to quickly check whether AT_EMPTY_PATH is valid.
Note, after some discussion we decided to also allow NULL to be passed
instead of requiring the empty string.

Signed-off-by: Christian Brauner <brauner@kernel.org>

Cc: <stable@vger.kernel.org> # 5.10.x-5.15.x
Signed-off-by: Miao Wang <shankerwangmiao@gmail.com>
Tested-by: Xi Ruoyao <xry111@xry111.site>
---
 include/linux/fs.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 27da89d0ed5a..6fe2ae89bba6 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3736,4 +3736,21 @@ static inline int inode_drain_writes(struct inode *inode)
 	return filemap_write_and_wait(inode->i_mapping);
 }
 
+static inline bool vfs_empty_path(int dfd, const char __user *path)
+{
+	char c;
+
+	if (dfd < 0)
+		return false;
+
+	/* We now allow NULL to be used for empty path. */
+	if (!path)
+		return true;
+
+	if (unlikely(get_user(c, path)))
+		return false;
+
+	return !c;
+}
+
 #endif /* _LINUX_FS_H */

-- 
2.43.0