netdev - RE: [PATCH] vsock: ratelimit unknown ioctl error message

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Date:   Mon, 26 Oct 2020 09:46:17 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Stefano Garzarella' <sgarzare@...hat.com>
CC:     Colin King <colin.king@...onical.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] vsock: ratelimit unknown ioctl error message

From: Stefano Garzarella
> Sent: 26 October 2020 09:39
> 
> On Mon, Oct 26, 2020 at 09:13:23AM +0000, David Laight wrote:
> >From: Stefano Garzarella
> >> Sent: 26 October 2020 08:43
> >...
> >> >Isn't the canonical error for unknown ioctl codes -ENOTTY?
> >> >
> >>
> >> Oh, thanks for pointing that out!
> >>
> >> I had not paid attention to the error returned, but looking at it I
> >> noticed that perhaps the most appropriate would be -ENOIOCTLCMD.
> >> In the ioctl syscall we return -ENOTTY, if the callback returns
> >> -ENOIOCTLCMD.
> >>
> >> What do you think?
> >
> >It is 729 v 443 in favour of ENOTTY (based on grep).
> 
> Under net/ it is 6 vs 83 in favour of ENOIOCTLCMD.
> 
> >
> >No idea where ENOIOCTLCMD comes from, but ENOTTY probably
> >goes back to the early 1970s.
> 
> Me too.
> 
> >
> >The fact that the ioctl wrapper converts the value is a good
> >hint that userspace expects ENOTTY.
> 
> Agree on that, but since we are not interfacing directly with userspace,
> I think it is better to return the more specific error (ENOIOCTLCMD).

I bet Linux thought it could use a different error code then
found that 'unknown ioctl' was spelt ENOTTY.

Back in the old days error values were probably almost unique.
strerror(EAGIAN) was "No more processes" for a long time!

	David

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