Re: [syzbot] [kernfs?] possible deadlock in kernfs_fop_llseek

Al Viro
2024-04-05T16:23:43Z

On Fri, Apr 05, 2024 at 08:37:03AM -0700, syzbot wrote:
> Hello,
>
> syzbot tried to test the proposed patch but the build/boot failed:

WTF? The patch is

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index e9df2f87072c6..8502ef68459b9 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -636,11 +636,18 @@ static int kernfs_fop_open(struct inode *inode, struct file *file)
* each file a separate locking class. Let's differentiate on
* whether the file has mmap or not for now.
*
- * Both paths of the branch look the same. They're supposed to
+ * For similar reasons, writable and readonly files are given different
+ * lockdep key, because the writable file /sys/power/resume may call vfs
+ * lookup helpers for arbitrary paths and readonly files can be read by
+ * overlayfs from vfs helpers when sysfs is a lower layer of overalyfs.
+ *
+ * All three cases look the same. They're supposed to
* look that way and give @of->mutex different static lockdep keys.
*/
if (has_mmap)
mutex_init(&of->mutex);
+ else if (file->f_mode & FMODE_WRITE)
+ mutex_init(&of->mutex);
else
mutex_init(&of->mutex);

How could it possibly trigger boot failure? Test the parent, perhaps?