Re: [PATCH v10 04/12] mm, arm64: untag user pointers passed to memory syscalls

[Andrey Konovalov]

On Sat, Feb 23, 2019 at 12:07 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 2/22/19 4:53 AM, Andrey Konovalov wrote:
> > --- a/mm/mprotect.c
> > +++ b/mm/mprotect.c
> > @@ -578,6 +578,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
> >  SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
> >               unsigned long, prot)
> >  {
> > +     start = untagged_addr(start);
> >       return do_mprotect_pkey(start, len, prot, -1);
> >  }
> >
> > @@ -586,6 +587,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
> >  SYSCALL_DEFINE4(pkey_mprotect, unsigned long, start, size_t, len,
> >               unsigned long, prot, int, pkey)
> >  {
> > +     start = untagged_addr(start);
> >       return do_mprotect_pkey(start, len, prot, pkey);
> >  }
>
> This seems to have taken the approach of going as close as possible to
> the syscall boundary and untagging the pointer there.  I guess that's
> OK, but it does lead to more churn than necessary.  For instance, why
> not just do the untagging in do_mprotect_pkey()?

I think that makes more sense, will do in the next version, thanks!

>
> I think that's an overall design question.  I kinda asked the same thing
> about patching call sites vs. VMA lookup functions.

Replied in the other thread.