From: Paolo Bonzini <pbonzini@redhat.com>
To: David Laight <David.Laight@ACULAB.COM>,
'Sean Christopherson' <seanjc@google.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"syzbot+e87846c48bf72bc85311@syzkaller.appspotmail.com"
<syzbot+e87846c48bf72bc85311@syzkaller.appspotmail.com>
Subject: Re: [PATCH] KVM: x86: fix shift out of bounds reported by UBSAN
Date: Tue, 22 Dec 2020 23:49:15 +0100 [thread overview]
Message-ID: <64932096-22a8-27dd-a8d6-1e40f3119db4@redhat.com> (raw)
In-Reply-To: <01b7c21e3a864c0cb89fd036ebe03ccf@AcuMS.aculab.com>
On 22/12/20 19:31, David Laight wrote:
>> /*
>> * Use 2ULL to incorporate the necessary +1 in the shift; adding +1 in
>> * the shift count will overflow SHL's max shift of 63 if s=0 and e=63.
>> */
> A comment of the desired output value would be more use.
> I think it is:
> return 'e-s' ones followed by 's' zeros without shifting by 64.
>
What about a mix of the two:
/*
* Return 'e-s' ones followed by 's' zeros. Note that the
* apparently obvious 1ULL << (e - s + 1) can shift by 64 if
* s=0 and e=63, which is undefined behavior.
*/
Paolo
next prev parent reply other threads:[~2020-12-22 22:51 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-22 10:21 Paolo Bonzini
2020-12-22 18:13 ` Sean Christopherson
2020-12-22 18:31 ` David Laight
2020-12-22 22:49 ` Paolo Bonzini [this message]
2020-12-23 16:59 ` Sean Christopherson
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