From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753566AbYI0QNQ (ORCPT ); Sat, 27 Sep 2008 12:13:16 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752595AbYI0QNB (ORCPT ); Sat, 27 Sep 2008 12:13:01 -0400 Received: from casper.infradead.org ([85.118.1.10]:52170 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752576AbYI0QNB (ORCPT ); Sat, 27 Sep 2008 12:13:01 -0400 Date: Sat, 27 Sep 2008 09:13:04 -0700 From: Arjan van de Ven To: Avi Kivity Cc: Andrew Morton , viro@ZenIV.linux.org.uk, linux-kernel@vger.kernel.org Subject: Re: [PATCH 0/3][RFC] ioctl dispatcher Message-ID: <20080927091304.12b8c4c3@infradead.org> In-Reply-To: <1222530242-1272-1-git-send-email-avi@redhat.com> References: <1222530242-1272-1-git-send-email-avi@redhat.com> Organization: Intel X-Mailer: Claws Mail 3.5.0 (GTK+ 2.12.12; i386-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-SRS-Rewrite: SMTP reverse-path rewritten from by casper.infradead.org See http://www.infradead.org/rpr.html Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 27 Sep 2008 18:43:59 +0300 Avi Kivity wrote: > While ioctls are officially ugly, they are the best choice for some > use cases, not to mention compatibility issues. Currently ioctl > writers face the following hurdles: > > - if the ioctl uses a data buffer, the ioctl handler must allocate > kernel memory for this buffer > - the memory may be allocated on the heap or on the stack, > depending on the buffer size > - handle any errors from the operation > - copy the data from userspace, if necessary > - handle any errors from the operation > - actually perform the operation > - copy the data back to userspace, if necessary > - handle any errors from the operation > - free the buffer, if allocated from the heap > > The first patch automates these operations, only requiring the caller > to supply the ioctl number and a callback in a table. > > The second patch addresses another problem with ioctls: they are > brittle. Once written, an ioctl cannot be extended, since the buffer > sizes used for transferring data are encoded in the ioctl number. > This is addressed by allowing the user-supplied size and the > kernel-visible size of the data buffer to be different; the kernel > will zero fill or truncate appropriately. With the new mechanism, it > is easy to write forward- and backward- compatible ioctl handlers. > > The third patch demonstrates the effectiveness of the first patch; it > converts some of kvm's ioctl handlers to the new mechanism, removing > around 90 lines in the process. > this doesn't seem to be much different from the way the DRM code deals with ioctls. Or the V4L code. Personally I hate that code though..... There is a fine balance here; between driver writers screwing something up they shouldn't be doing in the first place and us being able to clearly see what the code is doing; your patch kinda hides some key elements of the ioctl path... I'm afraid it gives a false sense of security though. Not having to deal with one aspect of security just to have to deal with the rest.... Lets put it this way: if the driver author has to type "copy_from_user" there's a chance that he'll remember that the data comes from the user and isn't to be trusted on face value.