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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Fri, 18 Feb 2022 10:07:07 -0800
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Subject: Re: [PATCH v2 13/18] uaccess: generalize access_ok()
To:     David Laight <David.Laight@aculab.com>
Cc:     Arnd Bergmann <arnd@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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On Fri, Feb 18, 2022 at 1:30 AM David Laight <David.Laight@aculab.com> wrote:
>
> From: Andy Lutomirski
> > Sent: 17 February 2022 19:15
> ...
> > This isn't actually optimal.  On x86, TASK_SIZE_MAX is a bizarre
> > constant that has a very specific value to work around a bug^Wdesign
> > error^Wfeature of Intel CPUs.  TASK_SIZE_MAX is the maximum address at
> > which userspace is permitted to allocate memory, but there is a huge
> > gap between user and kernel addresses, and any value in the gap would
> > be adequate for the comparison.  If we wanted to optimize this, simply
> > checking the high bit (which x86 can do without any immediate
> > constants at all) would be sufficient and, for an access known to fit
> > in 32 bits, one could get even fancier and completely ignore the size
> > of the access.  (For accesses not known to fit in 32 bits, I suspect
> > some creativity could still come up with a construction that's
> > substantially faster than the one in your patch.)
> >
> > So there's plenty of room for optimization here.
> >
> > (This is not in any respect a NAK -- it's just an observation that
> > this could be even better.)
>
> For 64bit arch that use the top bit to separate user/kernel
> you can test '(addr | size) >> 62)'.
> The compiler optimises out constant sizes.
>
> This has all been mentioned a lot of times.
> You do get different fault types.
>
> OTOH an explicit check for constant size (less than something big)
> can use the cheaper test of the sign bit.
> Big constant sizes could be compile time errors.

The different fault type issue may well be a real problem.  Right now
the core x86 fault code reserves the right to grouch if we get #GP
instead of #PF.  We could change that.