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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 24 Sep 2018 23:17:01 +0200
Message-ID: <CAK8P3a1Bh9NnncbcT0X4fKWC7B0FnqYyEMUwB6MvmzaNO7VhFQ@mail.gmail.com>
Subject: Re: [PATCH v2 05/17] compat_ioctl: move more drivers to generic_compat_ioctl_ptrarg
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Darren Hart <dvhart@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
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On Mon, Sep 24, 2018 at 10:35 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> On Mon, Sep 24, 2018 at 10:18:52PM +0200, Arnd Bergmann wrote:
> > On Tue, Sep 18, 2018 at 7:59 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > > On Tue, Sep 18, 2018 at 10:51:08AM -0700, Darren Hart wrote:
> > > > On Fri, Sep 14, 2018 at 09:57:48PM +0100, Al Viro wrote:
> > > > > On Fri, Sep 14, 2018 at 01:35:06PM -0700, Darren Hart wrote:
> > We already do this inside of some subsystems, notably drivers/media/,
> > and it simplifies the implementation of the ioctl handler function
> > significantly. We obviously cannot do this in general, both because of
> > traditional drivers that have 16-bit command codes (drivers/tty and others)
> > and also because of drivers that by accident defined the commands
> > incorrectly and use the wrong type or the wrong direction in the
> > definition.
>
> That could work well, but the first idea could be done globally and
> mechanically, while this would require very careful per-driver
> investigation.
>
> Particularly if the core code has worse performance.. ie due to
> kmalloc calls or something.
>
> I think it would make more sense to start by having the core do the
> case to __user and then add another entry point to have the core do
> the copy_from_user, and so on.

Having six separate callback pointers to implement a single
system call seems a bit excessive though.

        Arnd