linux-kernel - RE: [PATCH 04/14] x86: use more conventional access

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Message-ID: <2dda07f893cb4ef9b5ea2265adccb98f@AcuMS.aculab.com>
Date:   Mon, 14 Feb 2022 22:13:34 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Arnd Bergmann <arnd@...nel.org>
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Subject: RE: [PATCH 04/14] x86: use more conventional access_ok() definition

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

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