From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 6CC5734D384; Thu, 3 Sep 2026 05:43:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788414218; cv=none; b=TkV2CNb7gmZqlVAU3h7fyluxtR57HdXV4ICYBkNyEbCOjz0I4bYfKmEC+KF+z4KLs6J1cmqVp9llav6Gxwy03B09XAjSR71mnqgHL3PkwwTdzMITxZZKtesc7ea85tLMWWHjEq2MpSbJcTgBjkISSMtxJvb8oSqykpfeO5UPSGU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788414218; c=relaxed/simple; bh=56+B6ZGap2zpjnDcmP30lgJpoQQaz0l2GeKgqVqc3/k=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=FnM0qJViX1W1hJSxqzZBMdPRE6URwj99jlUHXv206SBqxbkKGwanhjVMKZLyjpHUGXr2FagW6R0fMy6X7U1ct5LcaOvDXTt2yxv9pDVjkNgo6TOrJ0OGcxFTAJstYv6MxdKpZQDATfiX2TEZUvHKhCFY3w2Egobkn3saE9xa/ko= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=uKky5BCz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="uKky5BCz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id AC91D1F00A3A; Thu, 3 Sep 2026 05:43:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1788414217; bh=Jy6YIArcCpTZAYf77Oc3uKkEC44H2h6IYfvF/LWfans=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=uKky5BCzpzXYdLKMCS06t//S/hlpcQnaHdw7OT19nrrTGbf1guYURgpkmQbEgUS4i QhkCdXRdanZ2X/Wk/lnAbpTPqlLRxTr3xTYawUaDIDlAN4f64hI+QIWxNVCrh3N0f2 bswibKmq5LSynFtlN9g7XN8a1cQK+uM4RRcHWYO0= Date: Thu, 3 Sep 2026 07:41:53 +0200 From: Greg Kroah-Hartman To: Shakeel Butt 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: <2026090351-turbofan-tux-d75e@gregkh> References: <20260903040253.670020-1-shakeel.butt@linux.dev> <2026090330-cytoplasm-resilient-900b@gregkh> <6a990797.3e7a366d.3bd849.72d1SMTPIN_ADDED_BROKEN@mx.google.com> Precedence: bulk X-Mailing-List: driver-core@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6a990797.3e7a366d.3bd849.72d1SMTPIN_ADDED_BROKEN@mx.google.com> On Wed, Sep 02, 2026 at 10:37:21PM -0700, Shakeel Butt wrote: > 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. Then please read our documentation for how to properly document this usage of a LLM tool. If you have reproducers, please add them to the kernfs tests as well as patches part of this series when you resend them. thanks, gre gk-h