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

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

On Mon, Mar 25, 2024, at 19:30, Mark Rutland wrote:
> On Mon, Mar 25, 2024 at 07:02:13PM +0100, Arnd Bergmann wrote:
>> On Mon, Mar 25, 2024, at 17:39, Mark Rutland wrote:
>
>> 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.
>
> Agreed, but do we even support such architectures within Linux?

Apparently not.

On 32-bit architectures, you often have TASK_SIZE==PAGE_OFFSET,
but not on 64-bit -- either the top few bits in PAGE_OFFSET are
always ones, or the user and kernel page tables are completely
separate.

>> It doesn't look like this is the case on riscv, but I'm not sure
>> about this part.
>
> It looks like riscv is in the same bucket as arm64 and x86 per:
>
>   https://www.kernel.org/doc/html/next/riscv/vm-layout.html
>
> ... which says:
>
> | The RISC-V privileged architecture document states that the 64bit addresses
> | "must have bits 63-48 all equal to bit 47, or else a page-fault exception
> | will occur.": that splits the virtual address space into 2 halves separated
> | by a very big hole, the lower half is where the userspace resides, the upper
> | half is where the RISC-V Linux Kernel resides.

Rihgt. I had even looked in that directory but somehow missed
the vm-layout.rst file.

     Arnd