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Date:   Thu, 24 Jun 2021 20:42:08 +1000
From:   Nicholas Piggin <npiggin@gmail.com>
Subject: Re: [PATCH 2/6] KVM: mmu: also return page from gfn_to_pfn
To:     Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        Marc Zyngier <maz@kernel.org>,
        Paul Mackerras <paulus@ozlabs.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        David Stevens <stevensd@chromium.org>,
        Zhenyu Wang <zhenyuw@linux.intel.com>,
        Zhi Wang <zhi.a.wang@intel.com>
Cc:     Alexandru Elisei <alexandru.elisei@arm.com>,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        intel-gvt-dev@lists.freedesktop.org,
        James Morse <james.morse@arm.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>, kvmarm@lists.cs.columbia.edu,
        kvm-ppc@vger.kernel.org, kvm@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        Sean Christopherson <seanjc@google.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Will Deacon <will@kernel.org>
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Excerpts from Paolo Bonzini's message of June 24, 2021 8:21 pm:
> On 24/06/21 12:17, Nicholas Piggin wrote:
>>> If all callers were updated that is one thing, but from the changelog
>>> it sounds like that would not happen and there would be some gfn_to_pfn
>>> users left over.
>>>
>>> But yes in the end you would either need to make gfn_to_pfn never retur=
n
>>> a page found via follow_pte, or change all callers to the new way. If
>>> the plan is for the latter then I guess that's fine.
>>
>> Actually in that case anyway I don't see the need -- the existence of
>> gfn_to_pfn is enough to know it might be buggy. It can just as easily
>> be grepped for as kvm_pfn_page_unwrap.
>=20
> Sure, but that would leave us with longer function names=20
> (gfn_to_pfn_page* instead of gfn_to_pfn*).  So the "safe" use is the one=20
> that looks worse and the unsafe use is the one that looks safe.

The churn isn't justified because of function name length. Choose g2pp()=20
if you want a non-descriptive but short name.

The existing name isn't good anyway because it not only looks up a pfn=20
but also a page, and more importantly it gets a ref on the page. The
name should be changed if you introduce a new API.

>> And are gfn_to_page cases also
>> vulernable to the same issue?
>=20
> No, they're just broken for the VM_IO|VM_PFNMAP case.

No they aren't vulnerable, or they are vunlerable but also broken in=20
other cases?

Thanks,
Nick