linux-kernel - Re: [PATCH v3] KVM: x86: fix for missing initialization of return status variable

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Message-ID: <20211206184031.GA143655@hamza-OptiPlex-7040>
Date:   Mon, 6 Dec 2021 23:40:31 +0500
From:   Ameer Hamza <amhamza.mgc@...il.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     vkuznets@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com,
        wanpengli@...cent.com, jmattson@...gle.com, joro@...tes.org,
        tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com
Subject: Re: [PATCH v3] KVM: x86: fix for missing initialization of return
 status variable

On Mon, Dec 06, 2021 at 06:01:05PM +0000, Sean Christopherson wrote:
> On Mon, Dec 06, 2021, Ameer Hamza wrote:
> > On Mon, Dec 06, 2021 at 05:02:01PM +0000, Sean Christopherson wrote:
> > > On Mon, Dec 06, 2021, Ameer Hamza wrote:
> > > > If undefined ioctl number is passed to the kvm_vcpu_ioctl_device_attr
> > > > ioctl, we should trigger KVM_BUG_ON() and return with EIO to silent
> > > > coverity warning.
> > > > 
> > > > Addresses-Coverity: 1494124 ("Uninitialized scalar variable")
> > > > Signed-off-by: Ameer Hamza <amhamza.mgc@...il.com>
> > > > ---
> > > > Changes in v3:
> > > > Added KVM_BUG_ON() as default case and returned -EIO
> > > > ---
> > > >  arch/x86/kvm/x86.c | 3 +++
> > > >  1 file changed, 3 insertions(+)
> > > > 
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index e0aa4dd53c7f..b37068f847ff 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -5019,6 +5019,9 @@ static int kvm_vcpu_ioctl_device_attr(struct kvm_vcpu *vcpu,
> > > >  	case KVM_SET_DEVICE_ATTR:
> > > >  		r = kvm_arch_tsc_set_attr(vcpu, &attr);
> > > >  		break;
> > > > +	default:
> > > > +		KVM_BUG_ON(1, vcpu->kvm);
> > > > +		r = -EIO;
> > > 
> > > At least have a
> > > 
> > > 		break;
> > > 
> > > if we're going to be pedantic about things.
> > I just started as a contributer in this community and trying
> > to fix issues found by static analyzer tools. If you think that's
> > not necessary, its totally fine :)
> 
> (Most) Static analyzers are great, they definitely find real bugs.  But they also
> have a fair number of false positives, e.g. this is a firmly a false positive, so
> the results of any static analyzer needs to thought about critically, not blindly
> followed.  It's completely understandable that Coverity got tripped up in this
> case, but that's exactly why having a human vet the bug report is necessary.
> 
> There is arguably value in having a default statement to ensure future KVM code
> doesn't end up adding a bad call, which is why I'm not completely opposed to the
> above addition.
> 
> Where folks, myself included, get a bit grumpy is when patches are sent to "fix"
> bug reports from static analyzers without evidence that the submitter has done
> their due dilegence to understand the code they are changing, e.g. even without
> any understanding of KVM, a search of kvm_vcpu_ioctl_device_attr() in the code
> base and reading of the function would have shown that the report was a false
> positive, albeit a somewhat odd one, and that returning -EINVAL was likely the
> wrong thing to do.  If you're unsure if something is a real bug, please ask a
> question.
> 
> Rapid firing patches at the list also makes reviewers grumpy as it again suggests
> a lack of due dilegence, especially when the patches have typos ("EINV" in v2)
> and/or have obvious shortcomings (missing "break" in v3).
> 
> TL;DR: I have no objection whatsover to fixing (potential) bugs found by static
> analyzers, but please slow down and (a) make sure that it's actually a bug, (b)
> ask if you're unsure, and (c) do your best to ensure that what you're sending is
> an overall improvement.
Totally agreed with you. Thank you so much for your insights on this. I will keep
this into consideration moving forward.

Best Regards,
Hamza.