Re: Weirdness when writing to pseudofs file

From: Johannes Lundberg <johalun_at_FreeBSD.org>
Date: Wed, 22 May 2019 16:33:47 -0700
On 5/22/19 4:22 PM, Alan Somers wrote:
> On Wed, May 22, 2019 at 5:18 PM Johannes Lundberg <johalun_at_freebsd.org> wrote:
>>
>> On 5/22/19 4:12 PM, Johannes Lundberg wrote:
>>> On 5/22/19 3:02 PM, Conrad Meyer wrote:
>>>> On Wed, May 22, 2019 at 1:58 PM Johannes Lundberg <johalun_at_freebsd.org> wrote:
>>>>>> It seems, a single '>' will cause it to try to create the file (even
>>>>>> though it already exists) and that fails (kern_openat).
>>>>>>
>>>>> I would guess because of
>>>>>
>>>>> https://github.com/freebsd/freebsd/blob/master/sys/fs/pseudofs/pseudofs_vnops.c#L1042
>>>>>
>>>>> struct vop_vector pfs_vnodeops = {
>>>>> ...
>>>>> .vop_create = VOP_EOPNOTSUPP,
>>>>> ...
>>>>> }
>>>> kern_openat -> vn_open(_cred) should only call VOP_CREATE if namei()
>>>> cannot find the named vnode (ni_vp == NULL).  Otherwise, it should
>>>> just invoke VOP_OPEN.  This suggests there might be a lookup bug in
>>>> pfs?  Tracing VOPs as Mark suggested seems like a good next step.
>>>>
>>>> Best,
>>>> Conrad
>>> Thanks Conrad. Yeah, that makes sense that it would open instead of
>>> recreating. Tracing a'la Mark points to
>>>
>>> vop_getwritemount
>>>
>>> failing.
>> Actually vop_setattr also shows up in dtrace.
>>
>> I'll continue digging..
> vop_setattr would get called to truncate the file's size down to 0.
> That's probably called by sh which is opening the file with O_TRUNC.
>
> -Alan

It works if I simply return 0 from pfs_setattr so that's the culprit
indeed. Hmm, wonder if there's any elegant solution to this..
Received on Wed May 22 2019 - 21:33:49 UTC

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