[PATCH v1 1/2] uaccess: Add __user_write_access_begin().

[Linus Torvalds]

On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima <kuniyu at google.com> wrote:
>
> unsafe_put_user() can be used to save a stac/clac pair, but
> masked_user_access_begin() or user_access_begin() introduces
> an unnecessary address masking or access_ok().
>
> Add a low-level helper for such a use case.

I really suspect that you cannot actually measure the cost of the
extra masking, and would be much happier if you just used a regular
"user_access_begin()" (perhaps the "user_write_access_begin()"
variant).

The masking is very cheap - literally just a couple of ALU
instructions. And unless you can actually measure some real advantage
of avoiding it, let's not add another helper to this area.

We spent a fair amount of time undoing years of "__get_user()" and
"__put_user()" cases that didn't actually help, and sometimes only
made it hard to see where the actual user pointer validation was done.

               Linus