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=-2.3 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,USER_AGENT_SANE_1 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 D7669C38A29 for ; Tue, 14 Apr 2020 14:53:01 +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 A8FF520578 for ; Tue, 14 Apr 2020 14:53:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="jecAZG75" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A8FF520578 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:Date: Message-ID:From:References:To:Subject:Reply-To:Content-ID:Content-Description :Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=fhqLkgHAn6h80PHT+bmtf1ODSm4sHndL+ZzmbkRgkqc=; b=jecAZG755Zyl5E 1+h1Aihv+woJn4CYbel632KEU934bfgbQqSMaRud3aX7m0PKRoWLjueE9KMBS0HbNgxNX8J6baLmM x2x0iUCGo6XtZmHzvEP75frtTQUryQb1qMFk9zGmCg07pPchi3pXw+uxes+oH9Fdh6Mnwpx64EAx0 bSmW3ieiKYa1EZX1WMdpP8Jw5UqbrZJXv8YqYjzv7ZLSo08hJsIhnNWZco6FzzN1jpZ/m6lJPJzKL glrVCdCGgALBEbKS8tqRJEcMqfGGBo31y91MhK2lq80m5/EhycskWJ/2Aa1R6Jk8F6Ctn09hWtCG5 2B4BwjzoXAzd4g90LCwQ==; 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 1jOMvt-0008Gc-6i; Tue, 14 Apr 2020 14:53:01 +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 1jOMvq-0008G7-FG for linux-arm-kernel@lists.infradead.org; Tue, 14 Apr 2020 14:52:59 +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 DFBC130E; Tue, 14 Apr 2020 07:52:56 -0700 (PDT) Received: from [192.168.1.172] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EE19B3F73D; Tue, 14 Apr 2020 07:52:55 -0700 (PDT) Subject: Re: [PATCH 1/5] arm64: vdso: don't free unallocated pages To: Mark Rutland References: <20200414104252.16061-1-mark.rutland@arm.com> <20200414104252.16061-2-mark.rutland@arm.com> <20200414132751.GF2486@C02TD0UTHF1T.local> From: Vincenzo Frascino Message-ID: <8681c958-0fd9-130e-f7bb-99bfd3a027cb@arm.com> Date: Tue, 14 Apr 2020 15:53:45 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.9.0 MIME-Version: 1.0 In-Reply-To: <20200414132751.GF2486@C02TD0UTHF1T.local> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200414_075258_556024_D4762031 X-CRM114-Status: GOOD ( 20.89 ) 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 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. From what I can see the implementation of the page allocator honors this assumption. Since you say it is a bug (providing evidence), we might have to investigate because probably there is an issue somewhere else. > > * 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. > > * It hinders comprehension of the code. When a developer sees the > free_page() they will assume that the page was allocated somewhere, > but there is no corresponding allocation as the pointers are never > assigned to. Even if the code in question is not harmful to the > functional correctness of the code, it is an unnecessary burden to > developers. > > * page_to_virt(NULL) does not have a well-defined result, and > page_to_virt() should only be called for a valid struct page pointer. > The result of page_to_virt(NULL) may not be a pointer into the linear > map as would be expected. > Do you know why this is the case? To be compliant with what the page allocator expects page_to_virt(NULL) should be equal to NULL. > * 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. [...] -- Regards, Vincenzo _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel