From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3vbjGL0kjSzDqBV for ; Sun, 5 Mar 2017 23:41:29 +1100 (AEDT) Date: Sun, 5 Mar 2017 06:37:37 -0600 From: Segher Boessenkool To: Michael Ellerman Cc: Anton Blanchard , benh@kernel.crashing.org, paulus@samba.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH] powerpc: Avoid panic during boot due to divide by zero in init_cache_info() Message-ID: <20170305123737.GM31469@gate.crashing.org> References: <1488671674-20833-1-git-send-email-anton@ozlabs.org> <87pohw3tp4.fsf@concordia.ellerman.id.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <87pohw3tp4.fsf@concordia.ellerman.id.au> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Sun, Mar 05, 2017 at 09:26:47PM +1100, Michael Ellerman wrote: > > I see a panic in early boot when building with a recent gcc toolchain. > > The issue is a divide by zero, which is undefined. Older toolchains > > let us get away with it: > > > > int foo(int a) { return a / 0; } > > > > foo: > > li 9,0 > > divw 3,3,9 > > extsw 3,3 > > blr > > > > But newer ones catch it: > > > > foo: > > trap > > > > Add a check to avoid the divide by zero. > > Erk sorry. One of the static checkers spotted it, but I hadn't got > around to fixing it because it seemed to not actually blow up, guess > not. The PowerPC divw etc. instructions do not trap by themselves, but recent GCC inserts trap instructions on code paths that are always undefined behaviour (like, dividing by zero). Segher