linux-kernel - Re: [RFC][4/11][MANUX] Kernel compatibility : ioctl(2)

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Date:	Tue, 15 Apr 2014 18:00:21 +0200
From:	Emmanuel Colbus <ecolbus@...ux.info>
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
CC:	linux-kernel@...r.kernel.org
Subject: Re: [RFC][4/11][MANUX] Kernel compatibility : ioctl(2)

Le 15/04/2014 17:00, One Thousand Gnomes a écrit :
> On Tue, 15 Apr 2014 15:42:54 +0200
> Emmanuel Colbus <ecolbus@...ux.info> wrote:
> 
>> Continuing with syscalls, I would like to indicate you a modification
>> I've done with regards to ioctl's. The thing is, I have had the need for
>> ioctl's that return *file descriptors*, instead of standard return codes.
> 
> You probably only think you have ;-)
> 
> The return from an ioctl on 32bit is going to be an unsigned 32bit value,
> as is a Linux file handle. So if you do
> 
> 	fd = ioctl(foo);
> 
> then not only have you got an interface that isn't compliant with
> POSIX/SuS you also have no error reporting capability.

Wait, on 32 bits, no process can have 2^32 file handles, or even 2^31.
Thus, since the error code is a negative value, userspace will simply do :

fd = _sys_ioctl(foo);
if (fd < 0)
	errno = -fd;

Which the libc is going to do for them anyways if they call ioctl()
directly.

> 
> The expectation of ioctl is
> 
> 	err = ioctl(fd, FDIOWIBBLE, &result);
> 
> now if result is a pointer to where to store one or more file handles you
> are sorted.

Also feasible.

> 
> If you are going to use SuS/POSIX naming I'd really suggest sticking to
> the expected behaviour in the standards.

I think this is an unproblematic extension, especially since these
ioctls are only supposed to be called by low-level software shipped with
the OS.

> 
> Alan
> 

Emmanuel
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