[PATCH 1/5] fs: Do not allow get_file() to resurrect 0 f_count

[Kees Cook]

If f_count reaches 0, calling get_file() should be a failure. Adjust to
use atomic_long_inc_not_zero() and return NULL on failure. In the future
get_file() can be annotated with __must_check, though that is not
currently possible.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
---
 include/linux/fs.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 00fc429b0af0..210bbbfe9b83 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1038,7 +1038,8 @@ struct file_handle {
 
 static inline struct file *get_file(struct file *f)
 {
-	atomic_long_inc(&f->f_count);
+	if (unlikely(!atomic_long_inc_not_zero(&f->f_count)))
+		return NULL;
 	return f;
 }
 
-- 
2.34.1