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

[David Laight <david.laight.linux@gmail.com>]

On Wed, 22 Oct 2025 19:37:27 -1000
Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Wed, 22 Oct 2025 at 14:05, Kuniyuki Iwashima <kuniyu@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).

Or wait for scoped_user_write_access() to get committed and then use that.

	David

> 
> 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
>