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authorDave Hansen <haveblue@us.ibm.com>2008-02-15 14:37:30 -0800
committerAl Viro <viro@zeniv.linux.org.uk>2008-04-19 00:25:32 -0400
commit8366025eb80dfa0d8d94b286d53027081c280ef1 (patch)
tree052c9244cd9d763ad8467bb95b25d903d58b7f11 /fs/namespace.c
parenta70e65df8812c52252fa07a2eb92a46451a4427f (diff)
[PATCH] r/o bind mounts: stub functions
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. Acked-by: Serge Hallyn <serue@us.ibm.com> Acked-by: Al Viro <viro@ZenIV.linux.org.uk> Signed-off-by: Christoph Hellwig <hch@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Dave Hansen <haveblue@us.ibm.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/namespace.c')
-rw-r--r--fs/namespace.c54
1 files changed, 54 insertions, 0 deletions
diff --git a/fs/namespace.c b/fs/namespace.c
index 94f026ec990..066b393578c 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -80,6 +80,60 @@ struct vfsmount *alloc_vfsmnt(const char *name)
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.
+ */
+/**
+ * mnt_want_write - get write access to a mount
+ * @mnt: the mount on which to take a write
+ *
+ * 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);
+
+/**
+ * mnt_drop_write - give up write access to a mount
+ * @mnt: the mount on which to give up write access
+ *
+ * Tells the low-level filesystem that we are done
+ * performing writes to it. Must be matched with
+ * mnt_want_write() call above.
+ */
+void mnt_drop_write(struct vfsmount *mnt)
+{
+}
+EXPORT_SYMBOL_GPL(mnt_drop_write);
+
+/*
+ * __mnt_is_readonly: check whether a mount is read-only
+ * @mnt: the mount to check for its write status
+ *
+ * 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*. This can not and
+ * should not be used in place of IS_RDONLY(inode).
+ */
+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;