linux-kernel - [PATCH v6 4/6] kernfs: Don't re-lock kernfs_root::kernfs_rwsem in kernfs_fop

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Message-ID: <20250130140207.1914339-5-bigeasy@linutronix.de>
Date: Thu, 30 Jan 2025 15:02:05 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Cc: Michal Koutný <mkoutny@...e.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Hillf Danton <hdanton@...a.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Marco Elver <elver@...gle.com>,
	Tejun Heo <tj@...nel.org>,
	tglx@...utronix.de,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [PATCH v6 4/6] kernfs: Don't re-lock kernfs_root::kernfs_rwsem in kernfs_fop_readdir().

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@...nel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.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 458519e416fe7..5a1fea414996e 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1868,10 +1868,10 @@ 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