Re: 2.6.36/2.6.37: broken compatibility with userspace input-utils ?

From: Dmitry Torokhov
Date: Tue Jan 25 2011 - 16:50:53 EST

On Wed, Jan 26, 2011 at 07:20:07AM +1000, Linus Torvalds wrote:
> On Wed, Jan 26, 2011 at 7:01 AM, Dmitry Torokhov
> <dmitry.torokhov@xxxxxxxxx> wrote:
> >
> > BTW, another issue is that evdev's ioctl returns -EINVAL for unknown
> > ioctls so applications would have hard time figuring out whether error
> > returned because of kernel being too old or because they are trying to
> > retrieve/establish invalid mapping if they had to go only by the error
> > code.
> So that's just another evdev interface bug.

Huh? I do not have lot of options here as far as error codes go. Invalid
request, invalid data in request - all goes to EINVAL.

> > As far as I can see EINVAL is a proper error for unknown ioctls:
> >
> > [dtor@hammer work]$ man 2 ioctl | grep EINVAL
> >       EINVAL Request or argp is not valid.
> Yeah, there's some confusion there.
> The "unknown ioctl" error code is (for traditional reasons) ENOTTY,
> but yes, the EINVAL thing admittedly has a lot of legacy use too.
> Inside the kernel, the preferred way to say "I don't recognize that
> ioctl number" is actually ENOIOCTLCMD. That's exactly so that various
> nested ioctl handlers can then tell the difference between "I didn't
> recognize that ioctl" and "I understand what you asked me to do, but
> your arguments were crap".
> vfs_ioctl() will then turn ENOIOCTLCMD to EINVAL to return to user space.

OK, so I can change evdev to employ ENOIOCTLCMD where needed, bit that
will not change older kernels where such distinction is needed (as never
kernels do support newer ioctl). And even if I could go back it would
not help since userspace still sees EINVAL only.

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