GIT: unionfs2-2.6.27.y: branch management: disallow setting leftmost branch to readonly

[Erez Zadok]

commit d3578c7b9c231b57aeb3a6d46593477e98cd9609
Author: Erez_Zadok <ezk at cs.sunysb.edu>
Date:   Mon Apr 30 16:42:30 2007 -0400

    branch management: disallow setting leftmost branch to readonly
    
    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).

diff --git a/fs/unionfs/super.c b/fs/unionfs/super.c
index 10520c0..3a8d74c 100644
--- a/fs/unionfs/super.c
+++ b/fs/unionfs/super.c
@@ -626,12 +626,20 @@ 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;
+	}
+
 	/*
 	 * Allocate space for actual pointers, if needed.  By the time we
 	 * finish this block of code, new_branches and new_lower_paths will