Weirdness when writing to pseudofs file

[Alan Somers]

On Wed, May 22, 2019 at 11:59 AM Johannes Lundberg <johalun at freebsd.org> wrote:
>
>
> On 5/22/19 10:51 AM, Konstantin Belousov wrote:
> > On Wed, May 22, 2019 at 10:36:34AM -0700, Johannes Lundberg wrote:
> >> Hi
> >>
> >> I'm fiddling with lindebugfs, which is based on pseudofs. When writing
> >> to a file,
> >>
> >> this works: # echo  1 >> /path/to/file
> >>
> >> but this does not: # echo 1 > /path/to/file
> >>
> >> "Operation not supported." is returned before the pseudofs code is even
> >> entered.
> >>
> >> Is this expected behavior? (if so, why?)
> > Does the file exist ?
> >
> > Pseudofs does not implement VOP_CREATE(), which is reasonable.
>
> Yes, it exists and my custom write function is receiving the call for
> ">>". (which is for example used by drm driver debugfs to do certain
> things on demand by accepting write to a debugfs file)

First, you need to try ktrace to see exactly which system call is not
supported.  If the problem still isn't obvious, then you can try
dtrace to see exactly which VOP isn't suppoted.  Do it like this:

sudo ktrace /bin/sh -c "echo 1 > /path/to/file"
sudo kdump
sudo dtrace -i 'fbt:::return /arg1 == 45/ {trace(".")}' -c "/bin/sh -c
'echo 1 > /path/to/file/'"

The dtrace command will show you which function returned EOPNOTSUPP.
However, it will also show a lot of functions that coincidentally
return 45 even though it's not an errno, and even functions that
return void.

-Alan