Re: [PATCH v5 01/18] xfs: compat_ioctl: use compat_ptr()

From: Arnd Bergmann <arnd@arndb.de>
To: Dave Chinner <david@fromorbit.com>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	linux-xfs <linux-xfs@vger.kernel.org>,
	Brian Foster <bfoster@redhat.com>,
	Allison Collins <allison.henderson@oracle.com>,
	Nick Bowler <nbowler@draconx.ca>,
	Eric Sandeen <sandeen@sandeen.net>,
	Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH v5 01/18] xfs: compat_ioctl: use compat_ptr()
Date: Thu, 15 Aug 2019 08:43:24 +0200	[thread overview]
Message-ID: <CAK8P3a0CAAtxXcfYt8NwmNSmF5tWhSSihLBkOtuQ62onjst4sA@mail.gmail.com> (raw)
In-Reply-To: <20190814213753.GP6129@dread.disaster.area>

On Wed, Aug 14, 2019 at 11:39 PM Dave Chinner <david@fromorbit.com> wrote:
> >       case XFS_IOC_BULKSTAT:
> >       case XFS_IOC_INUMBERS:
> > -             return xfs_file_ioctl(filp, cmd, p);
> > +             return xfs_file_ioctl(filp, cmd, (unsigned long)arg);
>
> I don't really like having to sprinkle special casts through the
> code because of this.
>
> Perhaps do something like:
>
> static inline unsigned long compat_ptr_mask(unsigned long p)
> {
>         return (unsigned long)compat_ptr(p);
> }
>
> and then up front you can do:
>
>         void    __user *arg;
>
>         p = compat_ptr_mask(p);
>         arg = (void __user *)p;
>
>
> and then the rest of the code remains unchanged by now uses p
> correctly instead of having to change all the code to cast arg back
> to an unsigned long...
>

In part 1 of the series, I define this function as a global:

long compat_ptr_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
        if (!file->f_op->unlocked_ioctl)
                return -ENOIOCTLCMD;

        return file->f_op->unlocked_ioctl(file, cmd, (unsigned
long)compat_ptr(arg));
}

How about using that to replace the individual casts:

-       return xfs_file_ioctl(filp, cmd, (unsigned long)arg);
+      return compat_ptr_ioctl(filp, cmd, arg);

It adds another indirection, but it avoids all the casts and
uses existing mechanism.

     Arnd