Weirdness when writing to pseudofs file

[Mark Johnston]

On Wed, May 22, 2019 at 12:03:00PM -0600, Alan Somers wrote:
> 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.

You can also trace VOPs with vfs::vop_*:return.  The return value is in
args[2].