From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933070AbcBZPKS (ORCPT ); Fri, 26 Feb 2016 10:10:18 -0500 Received: from userp1040.oracle.com ([156.151.31.81]:48749 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754318AbcBZPKQ (ORCPT ); Fri, 26 Feb 2016 10:10:16 -0500 Subject: Re: [Xen-devel] [PATCH v3 0/2] Clear .bss for VP guests To: Brian Gerst References: <1456413395-7679-1-git-send-email-boris.ostrovsky@oracle.com> <56D02EAA.4090200@citrix.com> <56D0587C.7080607@oracle.com> Cc: =?UTF-8?Q?Roger_Pau_Monn=c3=a9?= , Konrad Rzeszutek Wilk , David Vrabel , andrew.cooper3@citrix.com, Linux Kernel Mailing List , mcgrof@kernel.org, xen-devel@lists.xenproject.org From: Boris Ostrovsky Message-ID: <56D06AD9.6090200@oracle.com> Date: Fri, 26 Feb 2016 10:10:17 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit X-Source-IP: userv0022.oracle.com [156.151.31.74] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 02/26/2016 09:42 AM, Brian Gerst wrote: > On Fri, Feb 26, 2016 at 8:51 AM, Boris Ostrovsky > wrote: >> On 02/26/2016 05:53 AM, Roger Pau Monné wrote: >>> El 25/2/16 a les 16:16, Boris Ostrovsky ha escrit: >>>> PV guests need to have their .bss zeroed out since it is not guaranteed >>>> to be cleared by Xen's domain builder >>> I guess I'm missing something, but elf_load_image (in libelf-loader.c) >>> seems to be able to clear segments (it will zero the memory between >>> p_paddr + p_filesz and p_paddr + p_memsz) while loading the ELF into >>> memory, so if the program headers are correctly setup the .bss should be >>> zeroed out AFAICT. >> >> Right, but I don't think this is guaranteed. It's uninitialized data so in >> principle it can be anything. >> >> The ELF spec says "the system initializes the data with zero when the >> program begins to run" which I read as it's up to runtime and not the loader >> to do so. >> >> And since kernel does it explicitly on baremetal path I think it's a good >> idea for PV to do the same. > It does it on bare metal because bzImage is a raw binary image, not ELF. OK, I didn't think about this. But nevertheless, is it guaranteed that .bss is cleared by the loader? My reading of the spec is that it's not. -boris