Duplicate network filesystems

[aaptel@suse.com (Aurélien Aptel)]

Hi,

In fs/namespace.c do_add_mount() we do this check:

	/* Refuse the same filesystem on the same mount point */
	err = -EBUSY;
	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
	    path->mnt->mnt_root == path->dentry)
		goto unlock;

So that mount fails with EBUSY. But for networked filesystems (at least
cifs and nfs) you can do this:

mount //foo /mnt -o A
mount //foo /mnt -o B # different options

Since the SB are different it works, fine.

But mounting a 3rd time with options A succeeds, where from a user POV I
would have expected to fail.

So to recap:

mount //foo /mnt -o A
mount //foo /mnt -o A
# EBUSY => expected behaviour

mount //foo /mnt -o A
mount //foo /mnt -o B
# ok => expected behaviour

mount //foo /mnt -o A
mount //foo /mnt -o B
mount //foo /mnt -o A
# ok => what?

Shouldn't we check the stack of filesystems mounted at the path instead of
just the last one?

Cheers,

-- 
Aurélien Aptel / SUSE Labs Samba Team
GPG: 1839 CB5F 9F5B FB9B AA97  8C99 03C8 A49B 521B D5D3
SUSE Linux GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)