From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Hansen Subject: [PATCH 04/23] r/o bind mounts: stub functions Date: Wed, 11 Jul 2007 17:17:15 -0700 Message-ID: <20070712001715.B07613F8@kernel> References: <20070712001710.654CD9ED@kernel> Cc: linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, hch@infradead.org, Dave Hansen To: linux-kernel@vger.kernel.org Return-path: Received: from e3.ny.us.ibm.com ([32.97.182.143]:47416 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936357AbXGLARU (ORCPT ); Wed, 11 Jul 2007 20:17:20 -0400 In-Reply-To: <20070712001710.654CD9ED@kernel> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org This patch adds two function mnt_want_write() and mnt_drop_write(). These are used like a lock pair around and fs operations that might cause a write to the filesystem. Before these can become useful, we must first cover each place in the VFS where writes are performed with a want/drop pair. When that is complete, we can actually introduce code that will safely check the counts before allowing r/w<->r/o transitions to occur. Signed-off-by: Dave Hansen --- lxc-dave/fs/namespace.c | 46 +++++++++++++++++++++++++++++++++++++++++ lxc-dave/include/linux/mount.h | 3 ++ 2 files changed, 49 insertions(+) diff -puN fs/namespace.c~add-vfsmount-writer-count fs/namespace.c --- lxc/fs/namespace.c~add-vfsmount-writer-count 2007-07-10 12:46:04.000000000 -0700 +++ lxc-dave/fs/namespace.c 2007-07-10 12:46:04.000000000 -0700 @@ -76,6 +76,52 @@ struct vfsmount *alloc_vfsmnt(const char return mnt; } +/* + * Most r/o checks on a fs are for operations that take + * discrete amounts of time, like a write() or unlink(). + * We must keep track of when those operations start + * (for permission checks) and when they end, so that + * we can determine when writes are able to occur to + * a filesystem. + */ +/* + * This tells the low-level filesystem that a write is + * about to be performed to it, and makes sure that + * writes are allowed before returning success. When + * the write operation is finished, mnt_drop_write() + * must be called. This is effectively a refcount. + */ +int mnt_want_write(struct vfsmount *mnt) +{ + if (__mnt_is_readonly(mnt)) + return -EROFS; + return 0; +} +EXPORT_SYMBOL_GPL(mnt_want_write); + +/* + * Tells the low-level filesystem that we are done + * performing a write to it. Must be matched with + * mnt_want_write() call above. + */ +void mnt_drop_write(struct vfsmount *mnt) +{ +} +EXPORT_SYMBOL_GPL(mnt_drop_write); + +/* + * This shouldn't be used directly ouside of the VFS. + * It does not guarantee that the filesystem will stay + * r/w, just that it is right *now*e + * mnt_want/drop_write() will _keep_ the filesystem + * r/w. + */ +int __mnt_is_readonly(struct vfsmount *mnt) +{ + return (mnt->mnt_sb->s_flags & MS_RDONLY); +} +EXPORT_SYMBOL_GPL(__mnt_is_readonly); + int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb) { mnt->mnt_sb = sb; diff -puN include/linux/mount.h~add-vfsmount-writer-count include/linux/mount.h --- lxc/include/linux/mount.h~add-vfsmount-writer-count 2007-07-10 12:46:04.000000000 -0700 +++ lxc-dave/include/linux/mount.h 2007-07-10 12:46:04.000000000 -0700 @@ -70,9 +70,12 @@ static inline struct vfsmount *mntget(st return mnt; } +extern int mnt_want_write(struct vfsmount *mnt); +extern void mnt_drop_write(struct vfsmount *mnt); extern void mntput_no_expire(struct vfsmount *mnt); extern void mnt_pin(struct vfsmount *mnt); extern void mnt_unpin(struct vfsmount *mnt); +extern int __mnt_is_readonly(struct vfsmount *mnt); static inline void mntput(struct vfsmount *mnt) { _