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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx>
> > ---
> > 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
>
> Google Germany GmbH
> Erika-Mann-Straße, 33
> 80636 München
>
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--
Kirill A. Shutemov
