From patchwork Thu Jan 16 05:23:17 2025 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Al Viro X-Patchwork-Id: 13941241 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [62.89.141.173]) (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 A7032157E88; Thu, 16 Jan 2025 05:23:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=62.89.141.173 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737005004; cv=none; b=PAirpko2MqnnbziI4PiWtYNeJbhlhtZAyFaZcwKgp0GyqWgOodyeol/TB9/nIGNRzIwP7p5axWvZ3s0BVHby7a6AtSySkXOTJ6TUnKRSQ3fq9Y+9eEwigQlJOEf7PHdXKvOTX4MSvMlEc3v35PbPDwQsJXuYkJ3ZS2V6mHbKUBU= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737005004; c=relaxed/simple; bh=pebguC7yBaT1CfCeU8IoKa1qp9w/nenKal0WBuZHXFo=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=m4BB6yvjgXiHc4tVoj7IB1r/uOjNsrXrI4HmOvBH/MUwbCwjtP1WyXlKtBsCVJKIUxi3KRQaE+4lh8B9i7Ai6qk+za8MO7gqnIIFpKn5bl3hNEpNctUSM1+EstyE+qFuWvWQScIBc6dnLwR6gMQUrTjpsVsmJSQxLPXGIWKikUc= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zeniv.linux.org.uk; spf=none smtp.mailfrom=ftp.linux.org.uk; dkim=pass (2048-bit key) header.d=linux.org.uk header.i=@linux.org.uk header.b=wvZL/YdY; arc=none smtp.client-ip=62.89.141.173 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=ftp.linux.org.uk Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linux.org.uk header.i=@linux.org.uk header.b="wvZL/YdY" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:Content-Transfer-Encoding: MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc:To:From: Reply-To:Content-Type:Content-ID:Content-Description; bh=1cyqCeqgV8SV7Bbn/l88t8DR8FQG+BtXpkABEHx6sIY=; b=wvZL/YdYkWVnTwHOXUHFTY6W3b yk9UoduAo7QTUfwIYCigCZdTqBSz9oOdVirOO3iCV7bew+mFvhaP38vkWe5o7d/Wtv5dAsN+aYvD7 +PW89IAB2WFqIkf3JEKQEf32WVHiqfiMDnXCR1L7b8MvSAOds5908z69ET8ZdAAdiQIx9O2rEb3Vy lXO3iOVD35LSQBwGzcqXN2e3M+Zdk64wRjilH1nJuKTi7arvYAsYnGDL0cQc/Im4ihEYAI+aZepup YfD+IXfMR6P3mI40/2LYNS8/28n2ttiCmUEAz8CHOFpvfGJe/yBZhH3sDeJ2WzOk3XYTlK5gfBE1R rBb8csRg==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.98 #2 (Red Hat Linux)) id 1tYILg-000000022Ir-0n2j; Thu, 16 Jan 2025 05:23:20 +0000 From: Al Viro To: linux-fsdevel@vger.kernel.org Cc: agruenba@redhat.com, amir73il@gmail.com, brauner@kernel.org, ceph-devel@vger.kernel.org, dhowells@redhat.com, hubcap@omnibond.com, jack@suse.cz, krisman@kernel.org, linux-nfs@vger.kernel.org, miklos@szeredi.hu, torvalds@linux-foundation.org Subject: [PATCH v2 20/20] 9p: fix ->rename_sem exclusion Date: Thu, 16 Jan 2025 05:23:17 +0000 Message-ID: <20250116052317.485356-20-viro@zeniv.linux.org.uk> X-Mailer: git-send-email 2.47.1 In-Reply-To: <20250116052317.485356-1-viro@zeniv.linux.org.uk> References: <20250116052103.GF1977892@ZenIV> <20250116052317.485356-1-viro@zeniv.linux.org.uk> Precedence: bulk X-Mailing-List: ceph-devel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Sender: Al Viro 9p wants to be able to build a path from given dentry to fs root and keep it valid over a blocking operation. ->s_vfs_rename_mutex would be a natural candidate, but there are places where we need that and where we have no way to tell if ->s_vfs_rename_mutex is already held deeper in callchain. Moreover, it's only held for cross-directory renames; name changes within the same directory happen without it. Solution: * have d_move() done in ->rename() rather than in its caller * maintain a 9p-private rwsem (per-filesystem) * hold it exclusive over the relevant part of ->rename() * hold it shared over the places where we want the path. That almost works. FS_RENAME_DOES_D_MOVE is enough to put all d_move() and d_exchange() calls under filesystem's control. However, there's also __d_unalias(), which isn't covered by any of that. If ->lookup() hits a directory inode with preexisting dentry elsewhere (due to e.g. rename done on server behind our back), d_splice_alias() called by ->lookup() will move/rename that alias. Add a couple of optional methods, so that __d_unalias() would do if alias->d_op->d_unalias_trylock != NULL if (!alias->d_op->d_unalias_trylock(alias)) fail (resulting in -ESTALE from lookup) __d_move(...) if alias->d_op->d_unalias_unlock != NULL alias->d_unalias_unlock(alias) where it currently does __d_move(). 9p instances do down_write_trylock() and up_write() of ->rename_mutex. Signed-off-by: Al Viro --- Documentation/filesystems/locking.rst | 4 ++++ Documentation/filesystems/vfs.rst | 21 +++++++++++++++++++++ fs/9p/v9fs.h | 2 +- fs/9p/vfs_dentry.c | 16 ++++++++++++++++ fs/dcache.c | 5 +++++ include/linux/dcache.h | 2 ++ 6 files changed, 49 insertions(+), 1 deletion(-) diff --git a/Documentation/filesystems/locking.rst b/Documentation/filesystems/locking.rst index 146e7d8aa736..d20a32b77b60 100644 --- a/Documentation/filesystems/locking.rst +++ b/Documentation/filesystems/locking.rst @@ -31,6 +31,8 @@ prototypes:: struct vfsmount *(*d_automount)(struct path *path); int (*d_manage)(const struct path *, bool); struct dentry *(*d_real)(struct dentry *, enum d_real_type type); + bool (*d_unalias_trylock)(const struct dentry *); + void (*d_unalias_unlock)(const struct dentry *); locking rules: @@ -50,6 +52,8 @@ d_dname: no no no no d_automount: no no yes no d_manage: no no yes (ref-walk) maybe d_real no no yes no +d_unalias_trylock yes no no no +d_unalias_unlock yes no no no ================== =========== ======== ============== ======== inode_operations diff --git a/Documentation/filesystems/vfs.rst b/Documentation/filesystems/vfs.rst index 7c352ebaae98..31eea688609a 100644 --- a/Documentation/filesystems/vfs.rst +++ b/Documentation/filesystems/vfs.rst @@ -1265,6 +1265,8 @@ defined: struct vfsmount *(*d_automount)(struct path *); int (*d_manage)(const struct path *, bool); struct dentry *(*d_real)(struct dentry *, enum d_real_type type); + bool (*d_unalias_trylock)(const struct dentry *); + void (*d_unalias_unlock)(const struct dentry *); }; ``d_revalidate`` @@ -1428,6 +1430,25 @@ defined: For non-regular files, the 'dentry' argument is returned. +``d_unalias_trylock`` + if present, will be called by d_splice_alias() before moving a + preexisting attached alias. Returning false prevents __d_move(), + making d_splice_alias() fail with -ESTALE. + + Rationale: setting FS_RENAME_DOES_D_MOVE will prevent d_move() + and d_exchange() calls from the outside of filesystem methods; + however, it does not guarantee that attached dentries won't + be renamed or moved by d_splice_alias() finding a preexisting + alias for a directory inode. Normally we would not care; + however, something that wants to stabilize the entire path to + root over a blocking operation might need that. See 9p for one + (and hopefully only) example. + +``d_unalias_unlock`` + should be paired with ``d_unalias_trylock``; that one is called after + __d_move() call in __d_unalias(). + + Each dentry has a pointer to its parent dentry, as well as a hash list of child dentries. Child dentries are basically like files in a directory. diff --git a/fs/9p/v9fs.h b/fs/9p/v9fs.h index 698c43dd5dc8..f28bc763847a 100644 --- a/fs/9p/v9fs.h +++ b/fs/9p/v9fs.h @@ -202,7 +202,7 @@ static inline struct v9fs_session_info *v9fs_inode2v9ses(struct inode *inode) return inode->i_sb->s_fs_info; } -static inline struct v9fs_session_info *v9fs_dentry2v9ses(struct dentry *dentry) +static inline struct v9fs_session_info *v9fs_dentry2v9ses(const struct dentry *dentry) { return dentry->d_sb->s_fs_info; } diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c index 872c1abe3295..5061f192eafd 100644 --- a/fs/9p/vfs_dentry.c +++ b/fs/9p/vfs_dentry.c @@ -105,14 +105,30 @@ static int v9fs_lookup_revalidate(struct inode *dir, const struct qstr *name, return __v9fs_lookup_revalidate(dentry, flags); } +static bool v9fs_dentry_unalias_trylock(const struct dentry *dentry) +{ + struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry); + return down_write_trylock(&v9ses->rename_sem); +} + +static void v9fs_dentry_unalias_unlock(const struct dentry *dentry) +{ + struct v9fs_session_info *v9ses = v9fs_dentry2v9ses(dentry); + up_write(&v9ses->rename_sem); +} + const struct dentry_operations v9fs_cached_dentry_operations = { .d_revalidate = v9fs_lookup_revalidate, .d_weak_revalidate = __v9fs_lookup_revalidate, .d_delete = v9fs_cached_dentry_delete, .d_release = v9fs_dentry_release, + .d_unalias_trylock = v9fs_dentry_unalias_trylock, + .d_unalias_unlock = v9fs_dentry_unalias_unlock, }; const struct dentry_operations v9fs_dentry_operations = { .d_delete = always_delete_dentry, .d_release = v9fs_dentry_release, + .d_unalias_trylock = v9fs_dentry_unalias_trylock, + .d_unalias_unlock = v9fs_dentry_unalias_unlock, }; diff --git a/fs/dcache.c b/fs/dcache.c index 6f36d3e8c739..695406e48937 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -2961,7 +2961,12 @@ static int __d_unalias(struct dentry *dentry, struct dentry *alias) goto out_err; m2 = &alias->d_parent->d_inode->i_rwsem; out_unalias: + if (alias->d_op->d_unalias_trylock && + !alias->d_op->d_unalias_trylock(alias)) + goto out_err; __d_move(alias, dentry, false); + if (alias->d_op->d_unalias_unlock) + alias->d_op->d_unalias_unlock(alias); ret = 0; out_err: if (m2) diff --git a/include/linux/dcache.h b/include/linux/dcache.h index 4a6bdadf2f29..9a1a30857763 100644 --- a/include/linux/dcache.h +++ b/include/linux/dcache.h @@ -159,6 +159,8 @@ struct dentry_operations { struct vfsmount *(*d_automount)(struct path *); int (*d_manage)(const struct path *, bool); struct dentry *(*d_real)(struct dentry *, enum d_real_type type); + bool (*d_unalias_trylock)(const struct dentry *); + void (*d_unalias_unlock)(const struct dentry *); } ____cacheline_aligned; /*