Re: [PATCH v2 13/18] uaccess: generalize access_ok()

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Feb 16, 2022 at 5:19 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> From: Arnd Bergmann <arnd@arndb.de>
>
> There are many different ways that access_ok() is defined across
> architectures, but in the end, they all just compare against the
> user_addr_max() value or they accept anything.
>
> Provide one definition that works for most architectures, checking
> against TASK_SIZE_MAX for user processes or skipping the check inside
> of uaccess_kernel() sections.
>
> For architectures without CONFIG_SET_FS(), this should be the fastest
> check, as it comes down to a single comparison of a pointer against a
> compile-time constant, while the architecture specific versions tend to
> do something more complex for historic reasons or get something wrong.

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.)