From: Valerie Aurora <vaurora@xxxxxxxxxx>
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 mounted
or remounted read-write at any time. This is a problem for union
mounts, which require the underlying file system be read-only for the
entire duration of the union mount.
Add a hard read-only users count to the superblock. When this count
is non-zero, don't allow any read-write mounts of this super, or any
read-write remounts of existing mounts.
Original-author: Valerie Aurora <vaurora@xxxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---
fs/super.c | 11 +++++++++++
include/linux/fs.h | 6 ++++++
2 files changed, 17 insertions(+), 0 deletions(-)
diff --git a/fs/super.c b/fs/super.c
index f343eda..732e19b 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -202,6 +202,7 @@ static inline void destroy_super(struct super_block *s)
#ifdef CONFIG_SMP
free_percpu(s->s_files);
#endif
+ BUG_ON(s->s_hard_readonly_users);
security_sb_free(s);
WARN_ON(!list_empty(&s->s_mounts));
kfree(s->s_subtype);
@@ -758,6 +759,9 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
}
}
+ if (!(flags & MS_RDONLY) && sb->s_hard_readonly_users)
+ return -EROFS;
+
if (sb->s_op->remount_fs) {
retval = sb->s_op->remount_fs(sb, &flags, data);
if (retval) {
@@ -1150,9 +1154,16 @@ mount_fs(struct file_system_type *type, int flags, const char *name, void *data)
WARN((sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
"negative value (%lld)\n", type->name, sb->s_maxbytes);
+ if (!(flags & MS_RDONLY) && sb->s_hard_readonly_users)
+ goto out_sb_is_hard_ro;
+
up_write(&sb->s_umount);
free_secdata(secdata);
return root;
+
+out_sb_is_hard_ro:
+ up_write(&sb->s_umount);
+ error = -EROFS;
out_sb:
dput(root);
deactivate_locked_super(sb);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index d851be9..b8276c0 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1496,6 +1496,12 @@ struct super_block {
/* Being remounted read-only */
int s_readonly_remount;
+
+ /* Number of mounts requiring that the underlying file system never
+ * transition to read-write. Protected by s_umount. Decremented by
+ * free_vfsmnt() if MNT_HARD_READONLY is set.
+ */
+ int s_hard_readonly_users;
};
/* superblock cache pruning functions */
--
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