From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-65.mta0.migadu.com [91.218.175.65]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 23274349CFF for ; Thu, 3 Sep 2026 05:37:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.65 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788413850; cv=none; b=MFSaYJV4NmVzufJNZV3PQfxvWp8pP6N+O1nBCS6Y86QSk6GRzWP4BVZXaIAGK7wqWOa80VpxescJqy4wVvJYwJNdO9q56dNOOkRg6KAnwmdzIWwXSB4JfZZJEOFOLghiPdmAt/G7csfgysOo7fmOeFdHYtvGfzDFdj+9JnBjgmE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788413850; c=relaxed/simple; bh=GUra9Sz1ZsJBvBMzdUk5ZKkwM32xrfNY9UlZVLF9CBw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=dsNmSJMKoCWygOvvh6oNbU2Ep91P4QLfd27rByhwKVF7eUB0IOMcnMQxRV7TsSNpoebXWdqaLwZX/OYRL+DAfs9oQZ5xODEPd46cSUGYpS7SlpafPqO8ePSAk80rkYkThwHp8QAdxyUxQV/w1n6F86Y2UujTAfS1tA0eVpDcR8Y= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=U+r/bmwK; arc=none smtp.client-ip=91.218.175.65 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="U+r/bmwK" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=GUra9Sz1ZsJBvBMzdUk5ZKkwM32xrfNY9UlZVLF9CBw=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1788413847; v=1; x=1789018647; b=U+r/bmwKHwc/2GSw7taX7xlq3WSkSI+ubK/Wigo/T0FnELu45/VTHFyl1R8Yr7Id+JzIIoUM 3n7Uh6ZtdnaZU67OFM3qJhReq6IACfmrxbO0skZpFFHbuVwC+Uhu/5u9zTNU32azQ82/uLf9JnT 0Szm+LxFR3lGsCUKg2XXCiPg= X-Envelope-To: linux-kernel@vger.kernel.org Received: by smtp.migadu.com with ESMTPS id 34ba9696c4835a6b; Thu, 03 Sep 2026 05:37:27 +0000 X-Mizu-Trace-ID: 34ba9696c4835a6b X-Migadu-Flow: FLOW_OUT Date: Wed, 2 Sep 2026 22:37:21 -0700 From: Shakeel Butt To: Greg Kroah-Hartman Cc: Tejun Heo , Christian Brauner , Meta kernel team , linux-kselftest@vger.kernel.org, driver-core@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] kernfs: take kernfs_rename_lock for same-parent renames too Message-ID: <20260903053226.GA15581@shakeel.butt@linux.dev> References: <20260903040253.670020-1-shakeel.butt@linux.dev> <2026090330-cytoplasm-resilient-900b@gregkh> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <2026090330-cytoplasm-resilient-900b@gregkh> On Thu, Sep 03, 2026 at 06:31:58AM +0200, Greg Kroah-Hartman wrote: > On Wed, Sep 02, 2026 at 09:02:51PM -0700, Shakeel Butt wrote: > > kernfs_rename_ns() only takes kernfs_rename_lock when the rename moves > > the node to a new parent. A rename that keeps the same parent, like > > renaming a network interface, changes kernfs_node::name with only > > kernfs_rwsem held. So the lock protects ->__parent but not ->name, and > > a reader that wants a stable name has to take kernfs_rwsem, the same > > lock every path lookup needs. > > > > That also makes for a small but real bug. kernfs_path_from_node() > > takes kernfs_rename_lock for reading, and kernfs_path_from_node_locked() > > then reads the name of each ancestor. It reads each one once, so a > > single same-parent rename only moves the answer from the old path to the > > new one, but two of them landing inside one walk build a path that never > > existed: > > > > CPU0 CPU1 > > kernfs_path_from_node() on /a/b/c > > reads the name of a, gets "a" > > renames a to a2 > > renames b to b2 > > reads the name of b, gets "b2" > > returns "/a/b2/c" > > > > This hits roots without KERNFS_ROOT_INVARIANT_PARENT: sysfs, where the > > bad path can reach sysfs_warn_dup() and pr_cont_kernfs_path(), and > > resctrl, which renames a mon group inside its mon_groups directory. > > cgroup sets the flag, so it skips the lock and reads names under RCU > > alone; that case needs something else and is not addressed here. > > > > So take the lock in both cases, and let kernfs_rcu_name() accept it the > > way kernfs_parent() already does for ->__parent. Same-parent renames > > are rare, the lock is per filesystem, and the locked section is at most > > three stores. It also gives a future rename sequence counter one place > > to sit that covers every rename. > > > > Fixes: 741c10b096bc ("kernfs: Use RCU to access kernfs_node::name.") > > Signed-off-by: Shakeel Butt > > --- > > fs/kernfs/dir.c | 28 +++++++++++++++------------- > > fs/kernfs/kernfs-internal.h | 9 ++++++++- > > 2 files changed, 23 insertions(+), 14 deletions(-) > > How was this found and tested? Did you forget an Assisted-by: tag? I am working on a series to improve kernfs_rwsem and going through review-prompt with AI to review my series and these were existing issues AI found. I have created reproducers with AI for these and tested that these patches those.