linux-kernel - Re: [PATCH] x86/cpu: fix undefined behavior in bit shift for intel_detect

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Message-ID: <Y1/AaJOcgIc/INtv@hirez.programming.kicks-ass.net>
Date:   Mon, 31 Oct 2022 13:32:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Gaosheng Cui <cuigaosheng1@...wei.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        tony.luck@...el.com, pawan.kumar.gupta@...ux.intel.com,
        pbonzini@...hat.com, chenyi.qiang@...el.com,
        jithu.joseph@...el.com, alexs@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/cpu: fix undefined behavior in bit shift for
 intel_detect_tlb

On Mon, Oct 31, 2022 at 01:30:57PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 31, 2022 at 07:43:40PM +0800, Gaosheng Cui wrote:
> > Shifting signed 32-bit value by 31 bits is undefined, so changing
> > significant bit to unsigned. The UBSAN warning calltrace like below:
> > 
> > UBSAN: shift-out-of-bounds in arch/x86/kernel/cpu/intel.c:948:21
> > left shift of 1 by 31 places cannot be represented in type 'int'
> 
> Is it really? Shouldn't -fstrict-overflow define this case?

-fno-strict-overflow that is, which implies -fwrapv which ensures 2s
complement, at which point shifting signed numbers is fully defined.