From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757382Ab2E3SVP (ORCPT ); Wed, 30 May 2012 14:21:15 -0400 Received: from casper.infradead.org ([85.118.1.10]:50442 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757181Ab2E3SVO convert rfc822-to-8bit (ORCPT ); Wed, 30 May 2012 14:21:14 -0400 Message-ID: <1338402061.28384.22.camel@twins> Subject: Re: [PATCH 1/3] uprobes: install_breakpoint() should fail if is_swbp_insn() == T From: Peter Zijlstra To: Srikar Dronamraju Cc: Oleg Nesterov , Ingo Molnar , Ananth N Mavinakayanahalli , Anton Arapov , Linus Torvalds , Masami Hiramatsu , linux-kernel@vger.kernel.org Date: Wed, 30 May 2012 20:21:01 +0200 In-Reply-To: <20120530180409.GO15587@linux.vnet.ibm.com> References: <20120530165757.GA8077@redhat.com> <20120530165816.GA8085@redhat.com> <1338398931.28384.7.camel@twins> <20120530173717.GM15587@linux.vnet.ibm.com> <1338400142.28384.12.camel@twins> <1338400450.28384.16.camel@twins> <20120530180409.GO15587@linux.vnet.ibm.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 2012-05-30 at 23:34 +0530, Srikar Dronamraju wrote: > Nit: {,un}egister_uprobe should have been uprobe_{,un}register at > couple of places. Ah indeed.. very silly.. hopefully last version. --- Subject: uprobe: Document uprobe_register() vs uprobe_mmap() race Because the mind is treacherous and makes us forget we need to write stuff down. Signed-off-by: Peter Zijlstra --- kernel/events/uprobes.c | 29 ++++++++++++++++++++++++++++- 1 file changed, 28 insertions(+), 1 deletion(-) diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c index 985be4d..fd6fb30 100644 --- a/kernel/events/uprobes.c +++ b/kernel/events/uprobes.c @@ -45,6 +45,23 @@ static DEFINE_SPINLOCK(uprobes_treelock); /* serialize rbtree access */ #define UPROBES_HASH_SZ 13 +/* + * We need separate register/unregister and mmap/munmap lock hashes because of + * mmap_sem nesting. + * + * uprobe_register() needs to install probes on (potentially) all processes + * and thus needs to acquire multiple mmap_sems (consequtively, not + * concurrently), whereas uprobe_mmap() is called while holding mmap_sem + * for the particular process doing the mmap. + * + * uprobe_register()->register_for_each_vma() needs to drop/acquire mmap_sem + * because of lock order against i_mmap_mutex. This means there's a hole in the + * register vma iteration where a mmap() can happen. + * + * Thus uprobe_register() can race with uprobe_mmap() and we can try and + * install a probe where one is already installed. + */ + /* serialize (un)register */ static struct mutex uprobes_mutex[UPROBES_HASH_SZ]; @@ -356,6 +373,9 @@ int __weak set_swbp(struct arch_uprobe *auprobe, struct mm_struct *mm, unsigned { int result; + /* + * See the comment near uprobes_hash(). + */ result = is_swbp_at_addr(mm, vaddr); if (result == 1) return -EEXIST; @@ -870,6 +890,10 @@ static int register_for_each_vma(struct uprobe *uprobe, bool is_register) up_read(&mm->mmap_sem); mmput(mm); if (is_register) { + /* + * We can race against uprobe_mmap() see the comment + * near uprobe_hash(). + */ if (ret && ret == -EEXIST) ret = 0; if (ret) @@ -1080,7 +1104,10 @@ int uprobe_mmap(struct vm_area_struct *vma) ret = install_breakpoint(uprobe, vma->vm_mm, vma, vaddr); - /* Ignore double add: */ + /* + * We can race against uprobe_register(), see the + * comment near uprobe_hash(). + */ if (ret == -EEXIST) { ret = 0;