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Message-ID: <aJHBf4KCUPpWbyU5@google.com>
Date: Tue, 5 Aug 2025 08:31:59 +0000
From: Sebastian Ene <sebastianene@...gle.com>
To: Wei-Lin Chang <r09922117@...e.ntu.edu.tw>
Cc: Anshuman Khandual <anshuman.khandual@....com>,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
Joey Gouly <joey.gouly@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>
Subject: Re: [PATCH] KVM: arm64: ptdump: Fix exec attribute printing
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@...e.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
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