On Mon, Aug 04, 2025 at 08:41:35PM +0800, Wei-Lin Chang wrote:
> 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 at 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 = " ",
> > > }, {
I think the big problem here is that we've included the 'PTE_VALID' bit
in the mask. We don't bother with that for the Stage-1 ptdump code, e.g.
{
.mask = PTE_PXN,
.val = PTE_PXN,
.set = "NX",
.clear = "x ",
},
.... so do we actually need to take the PTE_VALID bit into account here? Do
invalid Stage-2 entries have anything we don't want to report?
... or can we change the Stage-2 ptdump code to have:
{
.mask = KVM_PTE_LEAF_ATTR_HI_S2_XN,
.val = KVM_PTE_LEAF_ATTR_HI_S2_XN,
.set = "NX",
.clear = "x ",
},
... and match the Stage-1 code?
Otherwise, maybe we can add a separate valid-only filter.
> > > .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
As above, this is because the PTE_VALID bit has been placed into the
mask, and that's not how the ptdump code was intended to be used.
> Therefore we want .set = "X", and .clear = " ".
That'll work around the problem, but I'm not sure that's the right fix.
If nothing else, it's *very* confusing.
Mark.