From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D8553C38A29 for ; Tue, 14 Apr 2020 15:13:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BDA6720768 for ; Tue, 14 Apr 2020 15:13:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2437077AbgDNPM6 (ORCPT ); Tue, 14 Apr 2020 11:12:58 -0400 Received: from foss.arm.com ([217.140.110.172]:57920 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2437512AbgDNPMv (ORCPT ); Tue, 14 Apr 2020 11:12:51 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0E2E930E; Tue, 14 Apr 2020 08:12:51 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.30.4]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0BBC83F73D; Tue, 14 Apr 2020 08:12:49 -0700 (PDT) Date: Tue, 14 Apr 2020 16:12:47 +0100 From: Mark Rutland To: Vincenzo Frascino Cc: linux-arm-kernel@lists.infradead.org, catalin.marinas@arm.com, will@kernel.org, stable@vger.kernel.org Subject: Re: [PATCH 1/5] arm64: vdso: don't free unallocated pages Message-ID: <20200414151247.GJ2486@C02TD0UTHF1T.local> References: <20200414104252.16061-1-mark.rutland@arm.com> <20200414104252.16061-2-mark.rutland@arm.com> <20200414132751.GF2486@C02TD0UTHF1T.local> <8681c958-0fd9-130e-f7bb-99bfd3a027cb@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8681c958-0fd9-130e-f7bb-99bfd3a027cb@arm.com> Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On Tue, Apr 14, 2020 at 03:53:45PM +0100, Vincenzo Frascino wrote: > > On 4/14/20 2:27 PM, Mark Rutland wrote: > > On Tue, Apr 14, 2020 at 01:50:38PM +0100, Vincenzo Frascino wrote: > >> Hi Mark, > >> > >> On 4/14/20 11:42 AM, Mark Rutland wrote: > >>> The aarch32_vdso_pages[] array never has entries allocated in the C_VVAR > >>> or C_VDSO slots, and as the array is zero initialized these contain > >>> NULL. > >>> > >>> However in __aarch32_alloc_vdso_pages() when > >>> aarch32_alloc_kuser_vdso_page() fails we attempt to free the page whose > >>> struct page is at NULL, which is obviously nonsensical. > >> > >> Could you please explain why do you think that free(NULL) is "nonsensical"? > > > > Regardless of the below, can you please explain why it is sensical? I'm > > struggling to follow your argument here. > > free(NULL) is a no-operation ("no action occurs") according to the C standard > (ISO-IEC 9899 paragraph 7.20.3.2). Hence this should not cause any bug if the > allocator is correctly implemented. This is true, but irrelevant. The C standard does not define free_page(), which is a Linnux kernel internal function, and does not have the same semantics as free(). > > * It serves no legitimate purpose. One cannot free a page without a > > corresponding struct page. > > > > * It is redundant. Removing the code does not detract from the utility > > of the remainging code, or make that remaing code more complex. > > * free_page(x) calls free_pages(x, 0), which checks virt_addr_valid(x). > > As page_to_virt(NULL) is not a valid linear map address, this can > > trigger a VM_BUG_ON() > > > > free_pages(x, 0) checks virt_addr_valid(x) only if "addr != 0" (as per C > standard) which makes me infer what I stated above. But maybe I am missing > something. Regardless, this is all academic unless you disagree with the first two bullets above. You don't randomly sprinkle a program with free(NULL) for the fun of it. Similarly, and regardless of how obfuscated, one should not do the same here. Thanks, Mark. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-0.8 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EC3CCC3815B for ; Tue, 14 Apr 2020 15:12:55 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id B539020768 for ; Tue, 14 Apr 2020 15:12:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="Xvdyn5fu" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org B539020768 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=JA5tl8aaSPxHSAUFoQ7FphHcbVqBPLlypgFH76CAkMo=; b=Xvdyn5fuj8SzHe FdkATF5yx98WsqSm9YizFYAHwgjmu4cwqKmPyf9V4pWuW1kTa3a6nOXPgD3SjGdwA3PMTnxMR9sse B/plsc6w12bTZH5VWtYp8fqchQ42DYKSoC89M3ydM5UXg5GTwZpUZ4PEbVizVpHAp1mWWlktMeWYR K6PABjkAxZDqwTd/M7/c7P7YUdmb8ahkrNrhxlHsid+/T5Mf5JwZwHL5vptchq+BDAwgc3SrAR1Hq dMAETH1osxaa/rQMwaTIswMAUd+VMbEQEfIlh+ydynPAlvn7pEsTo0zSUxXnoVhoGxj2LgEv5ZmaR vrDnMOE0sMVvc+Nnx4Qw==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jONF9-0006Ty-6g; Tue, 14 Apr 2020 15:12:55 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jONF5-0006TL-TA for linux-arm-kernel@lists.infradead.org; Tue, 14 Apr 2020 15:12:53 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0E2E930E; Tue, 14 Apr 2020 08:12:51 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.30.4]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0BBC83F73D; Tue, 14 Apr 2020 08:12:49 -0700 (PDT) Date: Tue, 14 Apr 2020 16:12:47 +0100 From: Mark Rutland To: Vincenzo Frascino Subject: Re: [PATCH 1/5] arm64: vdso: don't free unallocated pages Message-ID: <20200414151247.GJ2486@C02TD0UTHF1T.local> References: <20200414104252.16061-1-mark.rutland@arm.com> <20200414104252.16061-2-mark.rutland@arm.com> <20200414132751.GF2486@C02TD0UTHF1T.local> <8681c958-0fd9-130e-f7bb-99bfd3a027cb@arm.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <8681c958-0fd9-130e-f7bb-99bfd3a027cb@arm.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200414_081252_002194_8988871C X-CRM114-Status: GOOD ( 17.84 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: catalin.marinas@arm.com, will@kernel.org, stable@vger.kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Apr 14, 2020 at 03:53:45PM +0100, Vincenzo Frascino wrote: > > On 4/14/20 2:27 PM, Mark Rutland wrote: > > On Tue, Apr 14, 2020 at 01:50:38PM +0100, Vincenzo Frascino wrote: > >> Hi Mark, > >> > >> On 4/14/20 11:42 AM, Mark Rutland wrote: > >>> The aarch32_vdso_pages[] array never has entries allocated in the C_VVAR > >>> or C_VDSO slots, and as the array is zero initialized these contain > >>> NULL. > >>> > >>> However in __aarch32_alloc_vdso_pages() when > >>> aarch32_alloc_kuser_vdso_page() fails we attempt to free the page whose > >>> struct page is at NULL, which is obviously nonsensical. > >> > >> Could you please explain why do you think that free(NULL) is "nonsensical"? > > > > Regardless of the below, can you please explain why it is sensical? I'm > > struggling to follow your argument here. > > free(NULL) is a no-operation ("no action occurs") according to the C standard > (ISO-IEC 9899 paragraph 7.20.3.2). Hence this should not cause any bug if the > allocator is correctly implemented. This is true, but irrelevant. The C standard does not define free_page(), which is a Linnux kernel internal function, and does not have the same semantics as free(). > > * It serves no legitimate purpose. One cannot free a page without a > > corresponding struct page. > > > > * It is redundant. Removing the code does not detract from the utility > > of the remainging code, or make that remaing code more complex. > > * free_page(x) calls free_pages(x, 0), which checks virt_addr_valid(x). > > As page_to_virt(NULL) is not a valid linear map address, this can > > trigger a VM_BUG_ON() > > > > free_pages(x, 0) checks virt_addr_valid(x) only if "addr != 0" (as per C > standard) which makes me infer what I stated above. But maybe I am missing > something. Regardless, this is all academic unless you disagree with the first two bullets above. You don't randomly sprinkle a program with free(NULL) for the fun of it. Similarly, and regardless of how obfuscated, one should not do the same here. Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel