While we can check if a file system is currently read-only, we can't
guarantee that it will stay read-only. The file system can be
remounted read-write at any time; it's also conceivable that a file
system can be mounted a second time and converted to read-write if the
underlying fs allows it. This is a problem for union mounts, which
require the underlying file system be read-only. Add a read-only
users count and don't allow remounts to change the file system to
read-write or read-write mounts if there are any read-only users.
Signed-off-by: Valerie Aurora <vaurora@xxxxxxxxxx>
---
fs/super.c | 14 ++++++++++++++
include/linux/fs.h | 5 +++++
2 files changed, 19 insertions(+), 0 deletions(-)
diff --git a/fs/super.c b/fs/super.c
index 2761d3e..c8140ac 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -553,6 +553,15 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
}
remount_rw = !(flags & MS_RDONLY) && (sb->s_flags & MS_RDONLY);
+ /* If we are remounting read/write, make sure that none of the
+ users require read-only for correct operation (such as
+ union mounts). */
+ if (remount_rw && sb->s_readonly_users) {
+ printk(KERN_INFO "%s: In use by %d read-only user(s)\n",
+ sb->s_id, sb->s_readonly_users);
+ return -EROFS;
+ }
+
if (sb->s_op->remount_fs) {
retval = sb->s_op->remount_fs(sb, &flags, data);
if (retval)
@@ -889,6 +898,11 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
if (error)
goto out_sb;
+ error = -EROFS;
+ if (!(flags & MS_RDONLY) &&
+ (mnt->mnt_sb->s_readonly_users))
+ goto out_sb;
+
mnt->mnt_mountpoint = mnt->mnt_root;
mnt->mnt_parent = mnt;
up_write(&mnt->mnt_sb->s_umount);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 73e9b64..5fb7343 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1379,6 +1379,11 @@ struct super_block {
* generic_show_options()
*/
char *s_options;
+
+ /*
+ * Users who require read-only access - e.g., union mounts
+ */
+ int s_readonly_users;
};
extern struct timespec current_fs_time(struct super_block *sb);
--
1.6.3.3
--
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