From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 46539332E1 for ; Fri, 2 Jun 2023 16:34:41 +0000 (UTC) Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [IPv6:2001:4d48:ad52:32c8:5054:ff:fe00:142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id D04201AB for ; Fri, 2 Jun 2023 09:34:39 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=JROnAh9uM+2iqyKp52V/ZLKTj/qwnj5KPDtEO4yRNyY=; b=xPtQ6YpISPHemnswmpZ2szvsxh apkT+jg6L2P+luE/0aqhyL9tJLMamoiCnWWrDHNxuBcyqWcL+nlBojkCjJzLLiKSCSXHGAgC5ZU6Z e6QQ2ihTa4i19RBkkV+ZBKanPjIejuuKMpaYxZawWwNRcc65O+Ww2UrsvJRLSHM+qV0VXLN6OF1bT OIRHddeKpmSyeHEBSdkv2tXYaCOncCHid9n1FgVgj6sumpwiyUJW2dvQ4GrJBjeBoBE0giLXFpQPG WAy64E4YbkNeXGMILxX4e7U2NVRXG9jNpo2HF7VxgRhBfUPssTOe94cuDbmU8ZiT33N8FUA+7TJzx 6pqLDyIw==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:37720) by pandora.armlinux.org.uk with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1q57jV-0008IB-Ij; Fri, 02 Jun 2023 17:34:33 +0100 Received: from linux by shell.armlinux.org.uk with local (Exim 4.94.2) (envelope-from ) id 1q57jT-00036L-V3; Fri, 02 Jun 2023 17:34:31 +0100 Date: Fri, 2 Jun 2023 17:34:31 +0100 From: "Russell King (Oracle)" To: Jakub Kicinski Cc: Andrew Lunn , Heiner Kallweit , "David S. Miller" , Eric Dumazet , Paolo Abeni , Dan Carpenter , Oleksij Rempel , netdev@vger.kernel.org Subject: Re: [PATCH net-next] net: phylib: fix phy_read*_poll_timeout() Message-ID: References: <20230601213345.3aaee66a@kernel.org> <20230601213509.7ef8f199@kernel.org> <20230602090539.6a4fa374@kernel.org> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Russell King (Oracle) X-Spam-Status: No, score=-4.4 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,SPF_HELO_NONE, SPF_NONE,T_SCC_BODY_TEXT_LINE,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net On Fri, Jun 02, 2023 at 05:17:59PM +0100, Russell King (Oracle) wrote: > On Fri, Jun 02, 2023 at 09:05:39AM -0700, Jakub Kicinski wrote: > > On Fri, 2 Jun 2023 09:53:09 +0100 Russell King (Oracle) wrote: > > > > Yes it is :) All this to save the single line of assignment > > > > after the read_poll_timeout() "call" ? > > > > > > Okay, so it seems you don't like it. We can't fix it then, and we'll > > > have to go with the BUILD_BUG_ON() forcing all users to use a signed > > > varable (which better be larger than a s8 so negative errnos can fit) > > > or we just rely on Dan to report the problems. > > > > Wait, did the version I proposed not work? > > > > https://lore.kernel.org/all/20230530121910.05b9f837@kernel.org/ > > If we're into the business of throwing web URLs at each other for > messages we've already read, here's my one for you which contains > the explanation why your one is broken, and proposing my solution. > > https://lore.kernel.org/all/ZHZmBBDSVMf1WQWI@shell.armlinux.org.uk/ > > To see exactly why yours is broken, see the paragraph starting > "The elephant in the room..." > > If it needs yet more explanation, which clearly it does, then let's > look at what genphy_loopback is doing: > > ret = phy_read_poll_timeout(phydev, MII_BMSR, val, > val & BMSR_LSTATUS, > 5000, 500000, true); > > Now, with your supposed "fix" of: > > + int __ret, __val; \ > + \ > + __ret = read_poll_timeout(phy_read, __val, __val < 0 || (cond), \ > sleep_us, timeout_us, sleep_before_read, phydev, regnum); \ > > This ends up being: > > int __ret, __val; > > __ret = read_poll_timeout(phy_read, __val, __val < 0 || (val & BMSR_LSTATUS), > sleep_us, timeout_us, sleep_before_read, phydev, regnum); > > and that expands to something that does this: > > __val = phy_read(phydev, regnum); > if (__val < 0 || (val & BMSR_LSTATUS)) > break; > > Can you spot the bug yet? Where does "val" for the test "val & BMSR_LSTATUS" > come from? > > A bigger hint. With the existing code, this would have been: > > val = phy_read(phydev, regnum); > if (val < 0 || (val & BMSR_LSTATUS)) > break; > > See the difference? val & BMSR_LSTATUS is checking the value that was > returned from phy_read() here, but in yours, it's checking an > uninitialised variable. > > With my proposal, this becomes: > > val = __val = phy_read(phydev, regnum); > if (__val < 0 || (val & BMSR_LSTATUS)) > break; > > where "val" is whatever type the user chose, which has absolutely _no_ > bearing what so ever on whether the test for __val < 0 can be correctly > evaluated, and makes that test totally independent of whatever type the > user chose. If you don't like my solution, then I suppose another possibility would be: #define __phy_poll_read(phydev, regnum, val) \ ({ \ int __err; \ __err = phy_read(phydev, regnum); \ if (__err >= 0) \ val = __err; \ __err; \ }) #define phy_read_poll_timeout(phydev, regnum, val, cond, sleep_us, \ timeout_us, sleep_before_read) \ ({ \ int __ret, __err; \ __ret = read_poll_timeout(__phy_poll_read, __err, \ __err < 0 || (cond), \ sleep_us, timeout_us, sleep_before_read, phydev, regnum, val); \ if (__err < 0) \ __ret = __err; \ ... but that brings with it the possibility of using an uninitialised "val" (e.g. if phy_read() returns an error on the first iteration.) and is way more horrid and even less easy to understand. Remember that we default to *not* warning about uninitialised variables when building the kernel, so this won't produce a warning - which I guess is probably why you didn't notice that your suggestion left "val" uninitialised. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!