linux-kernel - Re: [kbuild] arch/x86/kvm/x86.c:4988 kvm_arch_tsc_set_attr() warn: check for integer overflow 'offset'

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Date:   Tue, 11 Oct 2022 15:02:28 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Dan Carpenter <dan.carpenter@...cle.com>
Cc:     kbuild@...ts.01.org, Oliver Upton <oupton@...gle.com>,
        lkp@...el.com, kbuild-all@...ts.01.org,
        linux-kernel@...r.kernel.org
Subject: Re: [kbuild] arch/x86/kvm/x86.c:4988 kvm_arch_tsc_set_attr() warn:
 check for integer overflow 'offset'

On 10/10/22 20:39, Sean Christopherson wrote:
>> 828ca89628bfcb Oliver Upton 2021-09-16 @4988  		tsc = kvm_scale_tsc(vcpu, rdtsc(), vcpu->arch.l1_tsc_scaling_ratio) + offset;
>>
>> Smatch hates obvious user triggerable integer overflows...  No checking
>> on offset.
>
> This is ok, and even necessary, e.g. if the host TSC > guest TSC.

(which in fact is the common case).  Also this is unsigned which is fine 
according to the C standard, though I understand that static analyzers 
want to be stricter.

> Is there anything
> we can do in KVM to help Smatch avoid false positives?  Or do you/Smatch already
> maintain a list of known false positives?

Seconded.

Paolo