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.
Signed-off-by: Valerie Aurora <valerie.aurora@xxxxxxxxx>
---
fs/super.c | 8 ++++++++
include/linux/fs.h | 7 +++++++
2 files changed, 15 insertions(+), 0 deletions(-)
diff --git a/fs/super.c b/fs/super.c
index 8819e3a..d02a4d6 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -126,6 +126,7 @@ 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);
kfree(s->s_subtype);
kfree(s->s_options);
@@ -577,6 +578,9 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
return -EBUSY;
}
+ 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)
@@ -963,6 +967,10 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
"negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);
+ error = -EROFS;
+ if (!(flags & MS_RDONLY) && mnt->mnt_sb->s_hard_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 92d248b..469e0ea 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1388,6 +1388,13 @@ struct super_block {
* generic_show_options()
*/
char *s_options;
+
+ /*
+ * 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;
};
extern struct timespec current_fs_time(struct super_block *sb);
--
1.7.0.4
--
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