Re: [PATCHv5 06/13] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Mon, Jul 18, 2022 at 07:47:44PM +0200, Alexander Potapenko wrote:
> On Wed, Jul 13, 2022 at 1:13 AM Kirill A. Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > Add a couple of arch_prctl() handles:
> >
> >  - ARCH_ENABLE_TAGGED_ADDR enabled LAM. The argument is required number
> >    of tag bits. It is rounded up to the nearest LAM mode that can
> >    provide it. For now only LAM_U57 is supported, with 6 tag bits.
> >
> >  - ARCH_GET_UNTAG_MASK returns untag mask. It can indicates where tag
> >    bits located in the address.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> >  arch/x86/include/uapi/asm/prctl.h |  3 ++
> >  arch/x86/kernel/process_64.c      | 60 ++++++++++++++++++++++++++++++-
> >  2 files changed, 62 insertions(+), 1 deletion(-)
> 
> 
> > +
> > +static int prctl_enable_tagged_addr(struct mm_struct *mm, unsigned long nr_bits)
> > +{
> > +       int ret = 0;
> > +
> > +       if (!cpu_feature_enabled(X86_FEATURE_LAM))
> > +               return -ENODEV;
> 
> Hm, I used to think ENODEV is specific to devices, and -EINVAL is more
> appropriate here.
> On the other hand, e.g. prctl(PR_SET_SPECULATION_CTRL) can also return ENODEV...

I'm fine either way. Although there are way too many -EINVALs around, so
it does not communicate much to user.

> >  long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
> >  {
> >         int ret = 0;
> > @@ -829,7 +883,11 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
> >         case ARCH_MAP_VDSO_64:
> >                 return prctl_map_vdso(&vdso_image_64, arg2);
> >  #endif
> > -
> > +       case ARCH_GET_UNTAG_MASK:
> > +               return put_user(task->mm->context.untag_mask,
> > +                               (unsigned long __user *)arg2);
> 
> Can we have ARCH_GET_UNTAG_MASK return the same error value (ENODEV or
> EINVAL) as ARCH_ENABLE_TAGGED_ADDR in the case the host doesn't
> support LAM?
> After all, the mask does not make much sense in this case.

I'm not sure about this.

As it is ARCH_GET_UNTAG_MASK returns -1UL mask if LAM is not present or
not enabled. Applying this mask will give correct result for both.

Why is -ENODEV better here? Looks like just more work for userspace.

> 
> > +       case ARCH_ENABLE_TAGGED_ADDR:
> > +               return prctl_enable_tagged_addr(task->mm, arg2);
> >         default:
> >                 ret = -EINVAL;
> >                 break;
> > --
> > 2.35.1
> >
> 
> 
> -- 
> Alexander Potapenko
> Software Engineer
> 
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-- 
 Kirill A. Shutemov