[PATCH v8 4/6] kernfs: Don't re-lock kernfs_root::kernfs_rwsem in kernfs_fop_readdir().

Sebastian Andrzej Siewior
2025-02-13T14:50:30Z

The readdir operation iterates over all entries and invokes dir_emit()
for every entry passing kernfs_node::name as argument.
Since the name argument can change, and become invalid, the
kernfs_root::kernfs_rwsem lock should not be dropped to prevent renames
during the operation.

The lock drop around dir_emit() has been initially introduced in commit
1e5289c97bba2 ("sysfs: Cache the last sysfs_dirent to improve readdir scalability v2")

to avoid holding a global lock during a page fault. The lock drop is
wrong since the support of renames and not a big burden since the lock
is no longer global.

Don't re-acquire kernfs_root::kernfs_rwsem while copying the name to the
userpace buffer.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
fs/kernfs/dir.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 5f0f8b95f44c0..43fbada678381 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1869,10 +1869,10 @@ static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
file->private_data = pos;
kernfs_get(pos);

- up_read(&root->kernfs_rwsem);
- if (!dir_emit(ctx, name, len, ino, type))
+ if (!dir_emit(ctx, name, len, ino, type)) {
+ up_read(&root->kernfs_rwsem);
return 0;
- down_read(&root->kernfs_rwsem);
+ }
}
up_read(&root->kernfs_rwsem);
file->private_data = NULL;
--
2.47.2