Bug#502816: [PATCH] snapshot: Implement compat_ioctl

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Thursday, December 29, 2011, Ben Hutchings wrote:
> On Tue, 2011-12-27 at 21:33 +0100, Ben Hutchings wrote:
> > This allows uswsusp built for i386 to run on an x86_64 kernel (tested
> > with Debian package version 1.0+20110509-2).
> 
> Now I wonder whether all this code was necessary.
> 
> [...]
> > +#ifdef CONFIG_COMPAT
> > +
> > +struct compat_resume_swap_area {
> > +	compat_loff_t offset;
> > +	u32 dev;
> > +} __packed;
> [...]
> 
> Is there actually any case where this doesn't have compatible layout
> with struct resume_swap_area?  It may have lower alignment, but
> misalignment is dealt with automatically when copying in/out of
> userland.

Quite frankly, I'm not 100% sure.

> If the structure size was variable then the ioctl number would be as
> well, and commit cf007e3526a785a95a738d5a8fba44f1f4fe33e0 is supposed to
> have removed the last ioctls that had that problem.
> 
> So maybe the compat function can be as simple as:
> 
> static long
> snapshot_compat_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
> {
> 	BUILD_BUG_ON(sizeof(loff_t) != sizeof(compat_loff_t));
> 	BUILD_BUG_ON(sizeof(struct resume_swap_area) !=
> 		     sizeof(struct compat_resume_swap_area));
> 

Well, I'd prefer to fail the ioctl in the above situations instead of crashing
the kernel.

> 	switch (cmd) {
> 	case SNAPSHOT_SET_SWAP_AREA:
> 	case SNAPSHOT_GET_IMAGE_SIZE:
> 	case SNAPSHOT_CREATE_IMAGE:
> 	case SNAPSHOT_AVAIL_SWAP_SIZE:
> 	case SNAPSHOT_ALLOC_SWAP_PAGE:
> 		return snapshot_ioctl(file, cmd,
> 				      (unsigned long) compat_ptr(arg));
> 
> 	default:
> 		return snapshot_ioctl(file, cmd, arg);
> 	}
> }

Does it acutally work?

Rafael