[PATCH] mtd: provide proper 32/64-bit compat_ioctl() support for BLKPG

[Brian Norris]

On Mon, Sep 21, 2015 at 01:26:59PM -0700, Brian Norris wrote:
> After a bit of poking around wondering why my 32-bit user-space can't
> seem to send a proper ioctl(BLKPG) to an MTD on my 64-bit kernel
> (ARM64), I noticed that struct blkpg_ioctl_arg is actually pretty
> unsuitable for use in the ioctl() ABI, due to its use of raw pointers,
> and its lack of alignment/packing restrictions (32-bit arch'es tend to
> pack the 4 fields into 4 32-bit words, whereas 64-bit arch'es would add
> padding after the third int, and make this 6 32-bit words).
> 
> Anyway, this means BLKPG deserves some special compat_ioctl handling. Do
> the conversion in a small shim for MTD.
> 
> block/compat_ioctl.c already has compat support for the block subsystem,
> but it does so by a re-marshalling data to/from user-space (see
> compat_blkpg_ioctl()). Personally, I think this approach is cleaner.
> 
> Tested only on MTD, with an ARM32 user space on an ARM64 kernel.
> 
> Signed-off-by: Brian Norris <computersforpeace at gmail.com>
> ---
> You can find the initial bug report / patch here:
> 
>   http://article.gmane.org/gmane.linux.kernel/2028927
>   https://lkml.org/lkml/2015/8/28/594
> 
> Changes since RFC:
>  * create new non-UAPI header, instead of cluttering the UAPI header with
>    compat stuff
>  * remove mention of "same bug in block/", since block/ solves this problem
>    already, just in a slightly different way

Pushed to l2-mtd.git