Re: [PATCH] KVM: arm64: ptdump: Fix exec attribute printing

[Sebastian Ene <sebastianene@google.com>]

On Mon, Aug 04, 2025 at 08:41:35PM +0800, Wei-Lin Chang wrote:

Hi Wei-Lin,

> Hi Anshuman,
> 
> On Sun, Aug 03, 2025 at 07:33:04PM +0530, Anshuman Khandual wrote:
> > 
> > 
> > On 02/08/25 4:10 PM, Wei-Lin Chang wrote:
> > > Currently the guest stage-2 page table dump has the executable attribute
> > > printed in reverse, showing "X" for a non-executable region and showing
> > > " " for an executable one. This is caused by misjudgement of which
> > > string gets printed for the executable and non-executable case. Fix it
> > > by swapping the two strings.
> > > 
> > > Signed-off-by: Wei-Lin Chang <r09922117@csie.ntu.edu.tw>
> > > ---
> > >  arch/arm64/kvm/ptdump.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/arch/arm64/kvm/ptdump.c b/arch/arm64/kvm/ptdump.c
> > > index 098416d7e5c25..99fc13f1c11fb 100644
> > > --- a/arch/arm64/kvm/ptdump.c
> > > +++ b/arch/arm64/kvm/ptdump.c
> > > @@ -44,8 +44,8 @@ static const struct ptdump_prot_bits stage2_pte_bits[] = {
> > >  	}, {
> > >  		.mask	= KVM_PTE_LEAF_ATTR_HI_S2_XN | PTE_VALID,
> > >  		.val	= PTE_VALID,
> > > -		.set	= " ",
> > > -		.clear	= "X",
> > > +		.set	= "X",
> > > +		.clear	= " ",
> > >  	}, {
> > >  		.mask	= KVM_PTE_LEAF_ATTR_LO_S2_AF | PTE_VALID,
> > >  		.val	= KVM_PTE_LEAF_ATTR_LO_S2_AF | PTE_VALID,
> > 
> > Is not KVM_PTE_LEAF_ATTR_HI_S2_XN already in the reverse semantics aka
> > XN (Execute Never). Hence when KVM_PTE_LEAF_ATTR_HI_S2_XN macro is set
> > that means the entry is not executable and vice versa.
> 
> Yes you are correct. However in dump_prot() we have:
> 
> if ((st->current_prot & bits->mask) == bits->val)
> 	s = bits->set;
> else
> 	s = bits->clear;
> 
> Analysis:
> 
> 1. region is executable:
>     - st->current_prot == PTE_VALID (ignore other bits)
>     - st->current_prot & bits->mask gets PTE_VALID
>     - if condition is true (.val is PTE_VALID)
>     - prints bits->set
> 
> 2. region is not executable:
>     - st->current_prot == KVM_PTE_LEAF_ATTR_HI_S2_XN | PTE_VALID
>     - st->current_prot & bits->mask gets (KVM_PTE_LEAF_ATTR_HI_S2_XN | PTE_VALID)
>     - if condition is false
>     - prints bits->clear
> 
> Therefore we want .set = "X", and .clear = " ".


This seems correct but it will produce a different output than the
stage-1 ptdump. What if we drop the PTE_VALID from the mask & value as
Mark and Anshuman are suggesting and print NX when the bit is set and x
otherwise as the stage-1 ptdump does ?

> 
> Thanks,
> Wei-Lin Chang

Thanks,
Sebastian