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userns: Remove implicit MNT_NODEV fragility.

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Replace the implict setting of MNT_NODEV on mounts that happen with
just user namespace permissions with an implicit setting of SB_I_NODEV
in s_iflags.  The visibility of the implicit MNT_NODEV has caused
problems in the past.

With this change the fragile case where an implicit MNT_NODEV needs to
be preserved in do_remount is removed.  Using SB_I_NODEV is much less
fragile as s_iflags are set during the original mount and never
changed.

In do_new_mount with the implicit setting of MNT_NODEV gone, the only
code that can affect mnt_flags is fs_fully_visible so simplify the if
statement and reduce the indentation of the code to make that clear.

Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
This commit is contained in:
Eric W. Biederman
2016-05-18 13:50:06 -05:00
parent a1935c1738
commit 67690f937c
2 changed files with 4 additions and 18 deletions
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fs/super.c
+3
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@@ -206,6 +206,9 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
init_waitqueue_head(&s->s_writers.wait_unfrozen);
s->s_bdi = &noop_backing_dev_info;
s->s_flags = flags;
if ((s->s_user_ns != &init_user_ns) &&
!(type->fs_flags & FS_USERNS_DEV_MOUNT))
s->s_iflags |= SB_I_NODEV;
INIT_HLIST_NODE(&s->s_instances);
INIT_HLIST_BL_HEAD(&s->s_anon);
mutex_init(&s->s_sync_lock);