Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()

["Arnd Bergmann" <arnd@arndb.de>]

On Tue, Mar 19, 2024, at 17:51, Alexandre Ghiti wrote:
> On 18/03/2024 22:29, Samuel Holland wrote:
>> On 2024-03-18 3:50 PM, Alexandre Ghiti wrote:
>>> On Wed, Mar 13, 2024 at 7:00 PM Samuel Holland
>> It looks like the call to fixup_exception() [added
>> in 416721ff05fd ("riscv, mm: Perform BPF exhandler fixup on page fault")] is
>> only intended to catch null pointer dereferences. So making the change wouldn't
>> have any functional impact, but it would still be a valid optimization.
>>
>>> Or I was wondering if it would not be better to do like x86 and use an
>>> alternative, it would be more correct (even though I believe your
>>> solution works)
>>> https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/page_64.h#L82.
>> What would be the benefit of using an alternative? Any access to an address
>> between TASK_SIZE and TASK_SIZE_MAX is guaranteed to generate a page fault, so
>> the only benefit I see is returning -EFAULT slightly faster at the cost of
>> applying a few hundred alternatives at boot. But it's possible I'm missing
>> something.
>
>
> The use of alternatives allows to return right away if the buffer is 
> beyond the usable user address space, and it's not just "slightly 
> faster" for some cases (a very large buffer with only a few bytes being 
> beyond the limit or someone could fault-in all the user pages and fail 
> very late...etc). access_ok() is here to guarantee that such situations 
> don't happen, so actually it makes more sense to use an alternative to 
> avoid that.

The access_ok() function really wants a compile-time constant
value for TASK_SIZE_MAX so it can do constant folding for
repeated calls inside of one function, so for configurations
with a boot-time selected TASK_SIZE_64 it's already not ideal,
with or without alternatives.

If I read the current code correctly, riscv doesn't even
have a way to build with a compile-time selected
VA_BITS/PGDIR_SIZE, which is probably a better place to
start optimizing, since this rarely needs to be selected
dynamically.

      Arnd