Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030442AbWHIDcp (ORCPT ); Tue, 8 Aug 2006 23:32:45 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030441AbWHIDco (ORCPT ); Tue, 8 Aug 2006 23:32:44 -0400 Received: from smtp.osdl.org ([65.172.181.4]:56451 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1030434AbWHIDcn (ORCPT ); Tue, 8 Aug 2006 23:32:43 -0400 Date: Tue, 8 Aug 2006 20:31:59 -0700 From: Andrew Morton To: Andi Kleen Cc: Kirill Korotaev , Linux Kernel Mailing List , haveblue@us.ibm.com, linux-arch@vger.kernel.org Subject: Re: [PATCH] sys_getppid oopses on debug kernel Message-Id: <20060808203159.a317f5d3.akpm@osdl.org> In-Reply-To: References: <44D865FD.1040806@sw.ru> <1155050817.19249.42.camel@localhost.localdomain> <44D8B12C.40200@sw.ru> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1865 Lines: 51 On 09 Aug 2006 05:09:11 +0200 Andi Kleen wrote: > Kirill Korotaev writes: > > [adding linux-arch] > > > > Accessing freed memory is a bug, always, not just *only* when slab > > > debugging is on, right? Doesn't this mean we could get junk, or that > > > the reader could potentially run off a bad pointer? > > no, read the comment in sys_getppid. > > It is a valid optimization. _safe_ and alowing to bypass taking the lock. > > BUT! This optimization relies on the fact that kernel memory (DMA + normal zone) > > is always mapped into virtual address space. > > Which is invalid for debug kernels only. > > In x86 arch code we would use __get_user for this (and we do in a couple > of places). But it wouldn't be portable because sometimes _user is > in a different address space. > > Maybe it would be time to make a similar facility (read/write_kernel_safe() or similar) > with error return available to generic code? > > It should be easy to implement - iirc near all architectures already > use the exception handling frame work and it is a simple extension > of that. x86 could just define it to __put/get_user > I just did something like that: Similar to ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.18-rc3/2.6.18-rc3-mm2/broken-out/add-probe_kernel_address.patch Although I'm not sure it's needed for this problem. A getppid() which does asmlinkage long sys_getppid(void) { int pid; read_lock(&tasklist_lock); pid = current->group_leader->real_parent->tgid; read_unlock(&tasklist_lock); return pid; } seems like a fine implementation to me ;) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/