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

[Arnd Bergmann]

On Mon, Mar 25, 2024, at 17:39, Mark Rutland wrote:

> Using a compile-time constant TASK_SIZE_MAX allows the compiler to generate
> much better code for access_ok(), and on arm64 we use a compile-time constant
> even when our page table depth can change at runtime (and when native/compat
> task sizes differ). The only abosolute boundary that needs to be maintained is
> that access_ok() fails for kernel addresses.

As I understand, this works on arm64 and x86 because the kernel
mapping starts on negative 64-bit addresses, so the highest user
address (TASK_SIZE = 0x000fffffffffffff) is still smaller than the
lowest kernel address (PAGE_OFFSET = 0xfff0000000000000).

If an architecture ignores all the top bits of a virtual address,
the largest TASK_SIZE would be higher than the smallest (positive,
unsigned) PAGE_OFFSET, so you need TASK_SIZE_MAX to be dynamic.
It doesn't look like this is the case on riscv, but I'm not sure
about this part.

      Arnd