Re: [PATCH 6/6 v5] nfsd: Allows user un-mounting filesystem where nfsd exports base on

[Al Viro <viro@ZenIV.linux.org.uk>]

On Wed, Jun 17, 2015 at 03:53:12PM +0800, Kinglong Mee wrote:
>  static void expkey_put(struct kref *ref)
>  {
>  	struct svc_expkey *key = container_of(ref, struct svc_expkey, h.ref);
>  
>  	if (test_bit(CACHE_VALID, &key->h.flags) &&
> -	    !test_bit(CACHE_NEGATIVE, &key->h.flags))
> -		path_put(&key->ek_path);
> -	auth_domain_put(key->ek_client);
> -	kfree(key);
> +	    !test_bit(CACHE_NEGATIVE, &key->h.flags)) {
> +		rcu_read_lock();
> +		complete(&key->ek_done);
> +		pin_kill(&key->ek_pin);
> +	} else
> +		expkey_destroy(key);
>  }

> +static void expkey_pin_kill(struct fs_pin *pin)
> +{
> +	struct svc_expkey *key = container_of(pin, struct svc_expkey, ek_pin);
> +
> +	if (!completion_done(&key->ek_done)) {
> +		schedule_work(&key->ek_work);
> +		wait_for_completion(&key->ek_done);
> +	}
> +	path_put_unpin(&key->ek_path, &key->ek_pin);
> +	expkey_destroy(key);
> +}

So basically you want umount(2) to hang until all references are gone.
How long can they stick around and, more to the point, what happens
if some sucker does path_get(&key->ek_path) while umount(2) had been
waiting?  You'll drop the reference to svc_export you'd copied ->ek_path
from and get through the whole dance, letting umount(2) go.

Now what?  Sure, with your previous patch vfsmount will survive - no oopsen
there.  However, you have had umount run to completion, with filesystem
still not shut down.  This is badly broken.

You can't do "I can grab reference at any time until the call of
expkey_pin_kill()" - it's not going to work.  You _must_ grab it
carefully, being ready to cope with "umount has already decided it's
not busy, so it's a goner and we must fail" kind of situations.

You need to
	grab mount_lock with read_seqlock_excl()
	check that vfsmount isn't marked MNT_SYNC_UMOUNT or MNT_DOOMED
and bump refcount in such case; give up on attempt otherwise.
	drop mount_lock
for those attempts to grab the first reference.  And be ready to cope with
failures.
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