linux-kernel - Re: [PATCH v2 09/18] mips: use simpler access

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Date:   Wed, 23 Feb 2022 12:05:04 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     Arnd Bergmann <arnd@...nel.org>, Christoph Hellwig <hch@....de>,
        linux-arch <linux-arch@...r.kernel.org>,
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Subject: Re: [PATCH v2 09/18] mips: use simpler access_ok()

On Mon, Feb 21, 2022 at 5:25 AM Thomas Bogendoerfer
<tsbogend@...ha.franken.de> wrote:
>
> With this patch
[ .. snip snip ..]
> I at least get my simple test cases fixed, but I'm not sure this is
> correct.

I think you really want to do that anyway, just to get things like
wild kernel pointers right (ie think get_kernel_nofault() and friends
for ftrace etc).

They shouldn't happen in any normal situation, but those kinds of
unverified pointers is why we _have_ get_kernel_nofault() in the first
place.

On x86-64, the roughly equivalent situation is that addresses that
aren't in canonical format do not take a #PF (page fault), they take a
#GP (general protection) fault.

So I think you want to do that fixup_exception() for any possible addresses.

> Is there a reason to not also #define TASK_SIZE_MAX   __UA_LIMIT like
> for the 32bit case ?

I would suggest against using a non-constant TASK_SIZE_MAX. Being
constant is literally one reason why it exists, when TASK_SIZE itself
has often been about other things (ie "32-bit process").

Having to load variables for things like get_user() is annoying, if
you could do it with a simple constant instead (where that "simple"
part is to avoid having to load big values from a constant pool -
often constants like "high bit set" can be loaded and compared against
more efficiently).

               Linus