[PATCH 16/21] Unionfs: Disallow setting leftmost branch to readonly

["Josef 'Jeff' Sipek" <jsipek@cs.sunysb.edu>]

From: Erez Zadok <ezk@cs.sunysb.edu>

Unionfs requires the leftmost branch to be writeable for copyup to work
properly and simply.  If, through branch-management commands (add, delete,
or mode change), the leftmost branch will becomes readonly, then return an
error (and tell the user to use "remount,ro" if they want a readonly union).

[jsipek: fixed up to apply cleanly]
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>
Signed-off-by: Josef 'Jeff' Sipek <jsipek@cs.sunysb.edu>
---
 fs/unionfs/super.c |   16 ++++++++++++----
 1 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/unionfs/super.c b/fs/unionfs/super.c
index 3dee863..446faf8 100644
--- a/fs/unionfs/super.c
+++ b/fs/unionfs/super.c
@@ -629,12 +629,20 @@ static int unionfs_remount_fs(struct super_block *sb, int *flags,
 out_no_change:
 
 	/******************************************************************
-	 * WE'RE ALMOST DONE: see if we need to allocate a small-sized new
-	 * vector, copy the vectors to their correct place, release the
-	 * refcnt of the older ones, and return.
-	 * Also handle invalidating any pages that will have to be re-read.
+	 * WE'RE ALMOST DONE: check if leftmost branch might be read-only,
+	 * see if we need to allocate a small-sized new vector, copy the
+	 * vectors to their correct place, release the refcnt of the older
+	 * ones, and return.  Also handle invalidating any pages that will
+	 * have to be re-read.
 	 *******************************************************************/
 
+	if (!(tmp_data[0].branchperms & MAY_WRITE)) {
+		printk("unionfs: leftmost branch cannot be read-only "
+		       "(use \"remount,ro\" to create a read-only union)\n");
+		err = -EINVAL;
+		goto out_release;
+	}
+
 	/* (re)allocate space for new pointers to hidden dentry */
 	size = new_branches * sizeof(struct unionfs_data);
 	new_data = krealloc(tmp_data, size, GFP_KERNEL);
-- 
1.5.2.rc1.165.gaf9b