From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752930Ab2HBN4c (ORCPT ); Thu, 2 Aug 2012 09:56:32 -0400 Received: from mx1.redhat.com ([209.132.183.28]:23694 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750972Ab2HBN4b (ORCPT ); Thu, 2 Aug 2012 09:56:31 -0400 Date: Thu, 2 Aug 2012 15:53:13 +0200 From: Oleg Nesterov To: Srikar Dronamraju Cc: Ingo Molnar , Anton Arapov , "Frank Ch. Eigler" , Peter Zijlstra , William Cohen , linux-kernel@vger.kernel.org Subject: Re: [PATCH] uprobes: Ignore unsupported instructions in uprobe_mmap Message-ID: <20120802135313.GA4334@redhat.com> References: <20120728163157.GA22719@redhat.com> <20120731064730.GB5087@linux.vnet.ibm.com> <20120731124805.GA485@redhat.com> <20120802100515.GC5782@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120802100515.GC5782@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/02, Srikar Dronamraju wrote: > > uprobe_mmap()->install_breakpoint() can fail if the probed insn is not > supported But there are other reasons why it can fail, > However failing mmap_region()/do_fork() because of a probe on an > unsupported instruction is wrong. Srikar, I strongly, absolutely disagree. Please correct me, but.. Do you realize how much uprobes hooks in mmap_region/dup_mmap are broken? (cough, can't resist, vma_adjust()->uprobe_mmap() is not right too, but this is another story). OK, lets start with dup_mmap: // retval == 0 if (file && uprobe_mmap(tmp)) goto out; out: up_write(&mm->mmap_sem); flush_tlb_mm(oldmm); up_write(&oldmm->mmap_sem); return retval; Given that retval == 0, what do you think dup_mmap() returns if uprobe_mmap() fails? And note that we didn't copy all vmas. OK, at least this can't crash (afaics), and easy to fix. But mmap_region() is worse, much worse. It simply can _not_ fail after uprobe_mmap (of course, I am not saying this is unfixable) without the crash. And note that the crash is "delayed". And btw, like dup_mmap(), mmap_region() doesn't return the error too. Srikar, I strongly believe this horror must not exist. Either we should teach mmap_region() and dup_mmap() (and vma_adjust!) to fail correctly, or we should ignore the error code. It is that simple, isn't it? Whatever you do with uprobe_mmap(), even if you change it to always return 0, the code in mmap_region() is absolutely, absolutely broken. Do you agree? And once again. I agree, in the long term we should reconsider this change. But we need a simple fix for now/stable. > Hence change uprobe_mmap() to ignore unsupported instructions. OK. Now suppose that mmap_region()-> uprobe_mmap() fails because the caller is SIGKILL'ed (so __get_user_pages fails). Given that mmap_region() can't handle the error correctly, the kernel can crash. Oleg.