Re: [PATCH 0/4] riscv: uaccess: optimizations

[Will Deacon]

On Sun, Jun 30, 2024 at 09:59:36AM -0700, Linus Torvalds wrote:
> On Tue, 25 Jun 2024 at 11:12, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > But yes, it would be lovely if we did things as "implement the
> > low-level accessor functions and we'll wrap them for the generic case"
> > rather than have every architecture have to do the wrapping..
> 
> Btw, to do that _well_, we need to expand on the user access functions
> a bit more.

[...]

> Will/Catalin - would that
> 
>         src = masked_user_access_begin(src);
> 
> work on arm64? The code does do something like that with
> __uaccess_mask_ptr() already, but at least currently it doesn't do the
> "avoid conditional entirely", the masking is just in _addition_ to the
> access_ok().

I think we'd need to go back to our old __uaccess_mask_ptr()
implementation, where kernel addresses end up being forced to NULL. In
other words, revert 2305b809be93 ("arm64: uaccess: simplify
uaccess_mask_ptr()"). If we then want to drop the access_ok() entirely,
we'd probably want to use an address that lives between the two TTBRs
(i.e. in the "guard region" you mentioned above), just in case somebody
has fscked around with /proc/sys/vm/mmap_min_addr.

Will