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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3a568a547ecsm12251433f8f.17.2025.06.16.14.26.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Jun 2025 14:26:30 -0700 (PDT) Date: Mon, 16 Jun 2025 22:26:29 +0100 From: David Laight To: Nicolas Pitre Cc: Andrew Morton , Thomas =?UTF-8?B?V2Vpw59z?= =?UTF-8?B?Y2h1aA==?= , Oleg Nesterov , Uwe =?UTF-8?B?S2xlaW5lLUvDtm5pZw==?= , Peter Zijlstra , Biju Das , linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH v2] mul_u64_u64_div_u64: fix the division-by-zero behavior Message-ID: <20250616222629.620830f6@pumpkin> In-Reply-To: References: X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Mon, 16 Jun 2025 15:22:44 -0400 (EDT) Nicolas Pitre wrote: > The current implementation forces a compile-time 1/0 division, which > generates an undefined instruction (ud2 on x86) rather than a proper > runtime division-by-zero exception. > > Change to trigger an actual div-by-0 exception at runtime, consistent > with other division operations. Use a non-1 dividend to prevent the > compiler from optimizing the division into a comparison. Given there is a definite 'plan' to avoid adding more BUG() to code I'm not at all sure generating UB here is right at all. I don't know the best solution though. To my mind returning zero for divide by zero and ~0 for overflow is least likely to cause real grief later on in the called code. David > > Signed-off-by: Nicolas Pitre > --- > > Change from v1 (http://lore.kernel.org/all/q2o7r916-5601-11pn-30pn-8n5ns6p079o7@onlyvoer.pbz): > - use OPTIMIZER_HIDE_VAR() in place of the open coded incantation. > > diff --git a/lib/math/div64.c b/lib/math/div64.c > index 5faa29208bdb..bf77b9843175 100644 > --- a/lib/math/div64.c > +++ b/lib/math/div64.c > @@ -212,12 +212,13 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c) > > #endif > > - /* make sure c is not zero, trigger exception otherwise */ > -#pragma GCC diagnostic push > -#pragma GCC diagnostic ignored "-Wdiv-by-zero" > - if (unlikely(c == 0)) > - return 1/0; > -#pragma GCC diagnostic pop > + /* make sure c is not zero, trigger runtime exception otherwise */ > + if (unlikely(c == 0)) { > + unsigned long zero = 0; > + > + OPTIMIZER_HIDE_VAR(zero); > + return ~0UL/zero; > + } > > int shift = __builtin_ctzll(c); >