linux-kernel - Re: [PATCH v3 1/2] tty: add compat

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Date:   Tue, 6 Jun 2017 14:46:20 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Aleksa Sarai <asarai@...e.de>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-alpha@...r.kernel.org,
        "open list:RALINK MIPS ARCHITECTURE" <linux-mips@...ux-mips.org>,
        Parisc List <linux-parisc@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-sh@...r.kernel.org, sparclinux <sparclinux@...r.kernel.org>,
        linux-xtensa@...ux-xtensa.org,
        linux-arch <linux-arch@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Valentin Rothberg <vrothberg@...e.com>
Subject: Re: [PATCH v3 1/2] tty: add compat_ioctl callbacks

On Tue, Jun 6, 2017 at 1:05 PM, Aleksa Sarai <asarai@...e.de> wrote:
>>> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
>>> index 65799575c666..2a6bd9ae3f8b 100644
>>> --- a/drivers/tty/pty.c
>>> +++ b/drivers/tty/pty.c
>>> @@ -481,6 +481,16 @@ static int pty_bsd_ioctl(struct tty_struct *tty,
>>>          return -ENOIOCTLCMD;
>>>   }
>>>
>>> +static long pty_bsd_compat_ioctl(struct tty_struct *tty,
>>> +                                unsigned int cmd, unsigned long arg)
>>> +{
>>> +       /*
>>> +        * PTY ioctls don't require any special translation between
>>> 32-bit and
>>> +        * 64-bit userspace, they are already compatible.
>>> +        */
>>> +       return pty_bsd_ioctl(tty, cmd, arg);
>>> +}
>>> +
>>
>>
>> This looks correct but unnecessary, you can simply point both
>> function pointers to the same function:
>
>
> They have different types, since they have different return types:
>
> int  (*ioctl)(struct tty_struct *tty,
>             unsigned int cmd, unsigned long arg);
> long (*compat_ioctl)(struct tty_struct *tty,
>                      unsigned int cmd, unsigned long arg);
>
> If you like, I can change (*ioctl) to return longs as well, and then change
> all of the call-sites (since unlocked_ioctl also returns long).

Ah, my mistake. In most other data structures that have a compat_ioctl
callback pointer, the prototypes are the same, and I had not realized
that tty_operations is an exception.

        Arnd