Re: Weirdness when writing to pseudofs file

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