nullfs and named pipes.
rwatson at FreeBSD.org
Mon Feb 19 14:01:19 UTC 2007
On Sun, 18 Feb 2007, Josef Karthauser wrote:
> On Fri, Feb 16, 2007 at 04:36:56PM +0200, Kostik Belousov wrote:
>>>>> cvs diff: Diffing .
>>>>> Index: null_subr.c
>>>>> RCS file: /home/ncvs/src/sys/fs/nullfs/null_subr.c,v
>>>>> retrieving revision 184.108.40.206
>>>>> diff -u -r220.127.116.11 null_subr.c
>>>>> --- null_subr.c 13 Mar 2006 03:05:17 -0000 18.104.22.168
>>>>> +++ null_subr.c 14 Feb 2007 00:02:28 -0000
>>>>> @@ -235,6 +235,8 @@
>>>>> xp->null_vnode = vp;
>>>>> xp->null_lowervp = lowervp;
>>>>> vp->v_type = lowervp->v_type;
>>>>> + if (vp->v_type == VSOCK || vp->v_type == VFIFO)
>>>>> + vp->v_un = lowervp->v_un;
>>>> I'm wondering is some reference counting needed there ?
>>> Yes, I find this a bit worrying also, but I don't know enough about how
>>> nullfs works to reason about it. What happens when a vnode in the bottom
>>> layer has its on-disk reference count drop to zero -- is the vnode in the
>>> top layer invalidated somehow?
>> Vnode reclamation from lower layer cannot do anithing for corresponding
>> nullfs vnode, but that vnode has reference from nullfs vnode. On the other
>> hand, can forced unmount proceed for lower layer ?
> Does know of any reason why I can't commit this as it is, at least for now.
> It doesn't appear that it would break anything that works currently, and in
> its current form it at least fixes named pipe functionality for the kinds of
> cases that people would want to use it.
Well, the worry would be that you would be replacing a clean error on failure
with an occasional panic, the normal symptom of a race condition.
I think I'm alright with the VFIFO case above, but I'm quite uncomfortable
with the VSOCK case. In particular, I suspect that if the socket is closed,
v_un will be reset in the lower layer, but continue to be a stale pointer in
the upper layer, leading to accessing free'd or re-allocated kernel memory
resulting in much badness. I've noticed tested this, but you might give it a
try and see what happens.
Robert N M Watson
University of Cambridge
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