Re: [regression] ENOTTY returned for tty fds

[stsp]

24.07.2024 12:08, Greg KH пишет:
> On Wed, Jul 24, 2024 at 11:07:32AM +0300, stsp wrote:
> > 24.07.2024 09:51, Greg KH пишет:
> > > What caused this change/regression?
> > I have absolutely no idea.
> > I've found it by debugging userspace,
> > and wrote a test-case to make sure the
> > problem is not in user-space.
> So this has always worked this way?  Or has it changed?  If changed,
> when did it work before?

OK, I did some extensive digging, and
now am pretty sure its this one:

commit 1b8b20868a6d64cfe8174a21b25b74367bdf0560
Author:     Johan Hovold <johan@xxxxxxxxxx>
AuthorDate: Wed Apr 7 11:52:02 2021 +0200
Commit:     Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
CommitDate: Sat Apr 10 10:36:34 2021 +0200

    tty: fix return value for unsupported ioctls

    Drivers should return -ENOTTY ("Inappropriate I/O control operation")
    when an ioctl isn't supported, while -EINVAL is used for invalid
    arguments.

> > >     And does any real-world programs
> > > rely on this?
> > dosemu
> It does this today or wants to do this in the future?

It does so since 2003/03/15, according
to a change-log (so for 21 year now).
Adding Herbert Xu to CC as an author of
that feature.

> > >     What exactly are you trying to determine with this ioctl
> > > test?
> > Whether it is a PTS (Pseudo-Tty-Slave), or
> > a real comport with MSR signalling.
> Why is that needed?

To fake modem status lines, mustly DCD.


>    And why not do it how other programs (like stty)
> does it?

I am not sure stty is interested in faking
modem status lines. It doesn't seem to be
using TIOCM ioctls at all.

> > >     Is there a different way to determine that?
> > I am not aware of any "canonical" way
> > of determining this. Maybe you tell me. :)
> > So far the only fix I know, is to stop checking
> > errno. But you return ENOTTY for a tty-associated
> > fd (isatty(fd)==1), so I believe this is a
> > bug in a kernel.
> isatty() is a libc provided function, not a kernel call.
It seems to be using TCGETS ioctl() and looks
for it to not return an error.
I still think returning ENOTTY where TCGETS
succeeds, is more than strange.
I looked at the tty code of current linux kernel,
and it returns EOPNOTSUPP in most such cases.
At least that makes sense, but ENOTTY?