RE: [PATCH 04/14] x86: use more conventional access_ok() definition

[David Laight <David.Laight@xxxxxxxxxx>]

From: Linus Torvalds
> Sent: 14 February 2022 20:24
> >
> > x86-64 has always(*) used TASK_SIZE_MAX for access_ok(), and the
> > get_user() assembler implementation does the same.
> 
> Side note: we could just check the sign bit instead, and avoid big
> constants that way.

The cheap test for most 64bit is (addr | size) >> 62 != 0.

I did some tests last week and the compilers correctly optimise
out constant size.

Doesn't sparc64 still need a wrap test?
Or is that assumed because there is always an unmapped page
and transfer are 'adequately' done on increasing addresses?

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)