Re: [PATCH] x86: predict __access_ok() returning true

Mateusz Guzik
2025-04-01T21:11:43Z

On Tue, Apr 1, 2025 at 11:00 PM Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> > On Tue, Apr 1, 2025 at 10:43 PM Ingo Molnar <mingo@kernel.org> wrote:
> > >
> > >
> > > * Ingo Molnar <mingo@kernel.org> wrote:
> > >
> > > > It's also the right place to have the hint: that user addresses are
> > > > valid is the common case we optimize for.
> > > >
> > > > Thanks,
> > > >
> > > > Ingo
> > > >
> > > > arch/x86/include/asm/uaccess_64.h | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
> > > > index c52f0133425b..4c13883371aa 100644
> > > > --- a/arch/x86/include/asm/uaccess_64.h
> > > > +++ b/arch/x86/include/asm/uaccess_64.h
> > > > @@ -54,7 +54,7 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm,
> > > > #endif
> > > >
> > > > #define valid_user_address(x) \
> > > > - ((__force unsigned long)(x) <= runtime_const_ptr(USER_PTR_MAX))
> > > > + likely((__force unsigned long)(x) <= runtime_const_ptr(USER_PTR_MAX))
> > >
> > > Should we go this way, this is the safe macro variant:
> > >
> > > #define valid_user_address(x) \
> > > (likely((__force unsigned long)(x) <= runtime_const_ptr(USER_PTR_MAX)))
> > >
> >
> > Note the are 2 tests and the other one does not get covered by *this* likely:
> > valid_user_address(sum) && sum >= (__force unsigned long)ptr;
> >
> > as in sum >= ptr is left be.
> >
> > However, I confirmed that with your patch the issue also goes away so
> > I guess it is fine.
> >
> > I think it would be the safest to likely within valid_user_address()
> > like in your patch, and likely on the entire expression like in mine.
> >
> > That said, there will be no hard feelz if you just commit your patch
> > and drop mine.
>
> Feel free to turn it into a Co-developed-by patch:
>
> From: Mateusz Guzik <mjguzik@gmail.com>
> ...
>
> Co-developed-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
>
> Because all I did was to transform your fix into something a bit more
> maintainable. I didn't even test it.
>

This is 2 lines changed in total, both trivially.

I think it is going to look rather silly to have this "co-developed",
but I can do it if you insist.

My suggestion would be that you commit your thing and add:
Reported-and-tested-by: Mateusz Guzik <mjguzik@gmail.com>

feel free to steal the disasm before/after
--
Mateusz Guzik <mjguzik gmail.com>