[PATCH] r/o bind mounts: stub functions

Dave Hansen
2008-02-16T01:32:37Z

Linus,

We've been causing Ted a lot of pain having to keep different copies of
his patches for mainline and for -mm since -mm has these functions and
mainline doesn't. I'm pretty darn sure at that these are the right API
and we just need another run in -mm to make sure that the reset of the
series in in good shape.

Could we bump this one in a bit ahead of the rest to ease Ted's pain? I
think the odds of this breaking anything by itself are pretty low.

---

This patch adds two functions: 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.

Note that these will replace many of the uses of IS_RDONLY(inode)
currently in the kernel.

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>
---

linux-2.6.git-dave/fs/namespace.c | 54 +++++++++++++++++++++++++++++++
linux-2.6.git-dave/include/linux/mount.h | 3 +
2 files changed, 57 insertions(+)

diff -puN fs/namespace.c~r-o-bind-mounts-stub-functions fs/namespace.c
--- linux-2.6.git/fs/namespace.c~r-o-bind-mounts-stub-functions 2008-02-15 13:25:45.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c 2008-02-15 13:25:45.000000000 -0800
@@ -80,6 +80,60 @@ 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.
+ */
+/**
+ * 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;
diff -puN include/linux/mount.h~r-o-bind-mounts-stub-functions include/linux/mount.h
--- linux-2.6.git/include/linux/mount.h~r-o-bind-mounts-stub-functions 2008-02-15 13:25:45.000000000 -0800
+++ linux-2.6.git-dave/include/linux/mount.h 2008-02-15 13:25:45.000000000 -0800
@@ -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)
{
_