netdev - Re: [PATCH v2 05/17] compat_ioctl: move more drivers to generic_compat_ioctl

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Date:   Mon, 24 Sep 2018 22:18:52 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Darren Hart <dvhart@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        gregkh <gregkh@...uxfoundation.org>,
        David Miller <davem@...emloft.net>,
        driverdevel <devel@...verdev.osuosl.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        qat-linux@...el.com,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        Linux Media Mailing List <linux-media@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linaro-mm-sig@...ts.linaro.org, amd-gfx@...ts.freedesktop.org,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        linux-iio@...r.kernel.org, linux-rdma <linux-rdma@...r.kernel.org>,
        linux-nvdimm@...ts.01.org, linux-nvme@...ts.infradead.org,
        linux-pci <linux-pci@...r.kernel.org>,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
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        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 05/17] compat_ioctl: move more drivers to generic_compat_ioctl_ptrarg

On Tue, Sep 18, 2018 at 7:59 PM Jason Gunthorpe <jgg@...pe.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:
> > >
> > > > Acked-by: Darren Hart (VMware) <dvhart@...radead.org>
> > > >
> > > > As for a longer term solution, would it be possible to init fops in such
> > > > a way that the compat_ioctl call defaults to generic_compat_ioctl_ptrarg
> > > > so we don't have to duplicate this boilerplate for every ioctl fops
> > > > structure?
> > >
> > >     Bad idea, that...  Because several years down the road somebody will add
> > > an ioctl that takes an unsigned int for argument.  Without so much as looking
> > > at your magical mystery macro being used to initialize file_operations.
> >
> > Fair, being explicit in the declaration as it is currently may be
> > preferable then.
>
> It would be much cleaner and safer if you could arrange things to add
> something like this to struct file_operations:
>
>   long (*ptr_ioctl) (struct file *, unsigned int, void __user *);
>
> Where the core code automatically converts the unsigned long to the
> void __user * as appropriate.
>
> Then it just works right always and the compiler will help address
> Al's concern down the road.

I think if we wanted to do this with a new file operation, the best
way would be to do the copy_from_user()/copy_to_user() in the caller
as well.

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.

       Arnd