[PATCH v14 1/4] asm-generic,arm64: create task variant of access_ok

[Arnd Bergmann]

On Wed, Mar 29, 2023, at 18:03, Oleg Nesterov wrote:
> On 03/29, Arnd Bergmann wrote:
>>
>> I think the idea of TASK_SIZE_MAX is that it is a compile-time constant and in fact independent of current, while TASK_SIZE
>> takes TIF_32BIT into account.
>
> Say, arch/loongarch defines TASK_SIZE which depends on 
> test_thread_flag(TIF_32BIT_ADDR)
> but it doesn't define TASK_SIZE_MAX, so __access_ok() will use TASK_SIZE.

I'd consider that a bug in loongarch, though it's
as harmless as it gets: The only downside is that
it's missing an optimization from constant-folding
the value, and since there is no CONFIG_COMPAT on
loongarch yet, it doesn't even have a different
value.

TASK_SIZE_MAX become mandatory here when I worked
on the optimized access_ok() across architectures,
and the reason it's safe to use is that access_ok()
has to only guarantee that a task cannot access
data that it can't already access, i.e. kernel
data. Passing a pointer between TASK_SIZE and
TASK_SIZE_MAX will still cause a -EFAULT error
because of the trap.

    Arnd