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

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Message-ID: <z2hmwwmtgbrio2wv3sj2pc4zhxdjioorlhnm45o2arcsjahjni@xod435q26jqq>
Date: Mon, 4 Aug 2025 20:41:35 +0800
From: Wei-Lin Chang <r09922117@...e.ntu.edu.tw>
To: Anshuman Khandual <anshuman.khandual@....com>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org
Cc: Marc Zyngier <maz@...nel.org>, Oliver Upton <oliver.upton@...ux.dev>, 
	Joey Gouly <joey.gouly@....com>, Sebastian Ene <sebastianene@...gle.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

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 = " ".

Thanks,
Wei-Lin Chang