Date: Wed, 22 May 2019 17:04:05 -0400 From: Mark Johnston <markj@freebsd.org> To: Alan Somers <asomers@freebsd.org> Cc: Johannes Lundberg <johalun@freebsd.org>, Konstantin Belousov <kostikbel@gmail.com>, FreeBSD Current <freebsd-current@freebsd.org> Subject: Re: Weirdness when writing to pseudofs file Message-ID: <20190522210405.GA9444@spy> In-Reply-To: <CAOtMX2i63czMAWmnJik5=gEY=5r4YFhz084zJ8xoThR01POGhQ@mail.gmail.com> References: <6ec62e4d-9f93-ffe1-646c-3846c9308334@FreeBSD.org> <20190522175133.GC2748@kib.kiev.ua> <84e3001b-646d-b1d9-f206-577d63f79bf1@FreeBSD.org> <CAOtMX2i63czMAWmnJik5=gEY=5r4YFhz084zJ8xoThR01POGhQ@mail.gmail.com>
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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@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].
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