Re: [PATCH v2 01/17] compat_ioctl: add generic_compat_ioctl_ptrarg()

[Al Viro]

On Wed, Sep 12, 2018 at 05:01:02PM +0200, Arnd Bergmann wrote:
> Many drivers have ioctl() handlers that are completely compatible
> between 32-bit and 64-bit architectures, except for the argument
> that is passed down from user space and may have to be passed
> through compat_ptr() in order to become a valid 64-bit pointer.
> 
> Using ".compat_ptr=generic_compat_ioctl_ptrarg" in file operations
> should let us simplify a lot of those drivers to avoid #ifdef
> checks, and convert additional drivers that don't have proper
> compat handling yet.

Just keep in mind that this should *only* be used when all
ioctls implemented in a given instance do take pointers.
Because otherwise you are asking for trouble - e.g. if one of
them takes an u32 used as a bitmap, this will run into trouble
as soon as somebody uses bit 31.  With no visible warnings.

IOW, it shouldn't be used blindly and it should come with big
fat warning.