From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:46946) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1agGuZ-000280-9B for qemu-devel@nongnu.org; Wed, 16 Mar 2016 15:15:16 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1agGuV-0004xJ-7e for qemu-devel@nongnu.org; Wed, 16 Mar 2016 15:15:15 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53971) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1agGuU-0004xC-Vy for qemu-devel@nongnu.org; Wed, 16 Mar 2016 15:15:11 -0400 References: <20160316181541.GG12454@HEDWIG.INI.CMU.EDU> <56E9A75D.60603@redhat.com> <20160316204150-mutt-send-email-mst@redhat.com> From: Laszlo Ersek Message-ID: <56E9B0BB.8040700@redhat.com> Date: Wed, 16 Mar 2016 20:15:07 +0100 MIME-Version: 1.0 In-Reply-To: <20160316204150-mutt-send-email-mst@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH v2] vl.c: disallow command line fw cfg without opt/ List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Michael S. Tsirkin" Cc: pbonzini@redhat.com, "Gabriel L. Somlo" , qemu-devel@nongnu.org, armbru@redhat.com, kraxel@redhat.com On 03/16/16 19:43, Michael S. Tsirkin wrote: > On Wed, Mar 16, 2016 at 07:35:09PM +0100, Laszlo Ersek wrote: >> On 03/16/16 19:15, Gabriel L. Somlo wrote: >>> On Wed, 16 Mar 2016 at 18:50:57 +0200, Michael S. Tsirkin wrote: >>>> On Wed, Mar 16, 2016 at 05:29:45PM +0100, Markus Armbruster wrote: >>>>> "Michael S. Tsirkin" writes: >>>>> >>>>>> Allowing arbitary file names on command line is setting us up for >>>>>> failure: future guests will look for a specific QEMU-specified name and >>>>>> will get confused finding a user file there. >>>>>> >>>>>> We do warn but people are conditioned to ignore warnings by now, >>>>>> so at best that will help users debug problem, not avoid it. >>>>>> >>>>>> Disable this by default, so distros don't get to deal with it, >>>>>> but leave an option for developers to configure this in, >>>>>> with scary warnings so people only do it if they know >>>>>> what they are doing. >>>>>> >>>>>> Signed-off-by: Michael S. Tsirkin >>>>> >>>>> I'm having a hard time to see the point. >>>> >>>> Frankly, I am having a hard time to see the point of exposing fw cfg to >>>> users at all. It was designed as an internal interface between QEMU PC >>>> hardware and firmware. As a PC maintainer, I do not like it that users >>>> get to poke at PC internals. >>>> >>>> So it is yet another way to pass binaries to Linux guests. Don't we >>>> have enough of these? But Gerd likes it for some reason, and merged it. >>>> OK. >>> >>> As the author of the feature, I feel I should jump back in and clarify: >>> >>> It's a way to pass arbitrary blobs to any type of guest, with two >>> important properties: 1. asynchronous, and 2. out-of-band. When I >>> started looking, all existing methods involved either having the host >>> start polling for the guest to bring up qga and be ready to accept an >>> out-of-band connection (i.e., *not* asynchronous), or have the guest >>> mount some special cdrom or floppy image prepared by the host (i.e., >>> *not* out of band). >>> >>> fw_cfg is both asynchronous and out-of-band, so it appeared to be the >>> perfect choice. >>> >>>> But please find a way to make sure it does not conflict with its current >>>> usage in PC. Asking that all files have an "opt/" prefix is one way >>>> but only if it is enforced. >>> >>> Enforcing the "opt/" prefix was clearly on the table when I submitted >>> the feature (and totally acceptable for my own needs). At the time, however, >>> most of the advice I received on the list was to leave it as a warning >>> only (i.e., "mechanism, not policy"), especially since other respondents >>> expressed interest in passing in non-"/opt" blobs for easier development >>> and debugging of alternative firmware (such as OVMF, iirc). >>> >>> Having a mis-use of this feature become "institutionalized" over time was >>> seen as a low/negligible risk at the time. Do we have any new reasons >>> to worry about it ? >> >> OVMF uses this feature for a few flags. They are all called >> "opt/ovmf/...". I followed the advice in "docs/specs/fw_cfg.txt" (which >> shouldn't be surprising since I seem to have reviewed every patch for >> that file): >> >>> NOTE: Users *SHOULD* choose item names beginning with the prefix "opt/" >>> when using the "-fw_cfg" command line option, to avoid conflicting with >>> item names used internally by QEMU. For instance: >>> >>> -fw_cfg name=opt/my_item_name,file=./my_blob.bin >>> >>> Similarly, QEMU developers *SHOULD NOT* use item names prefixed with >>> "opt/" when inserting items programmatically, e.g. via fw_cfg_add_file(). >> >> It says "should", not "must". > > should means "might be ok". Yes, if there is a pressing reason. And even then, "you have been warned". > So change it to must then? I didn't suggest that. (For consistency, your patch should be attempting to modify the code and the docs together -- but this doesn't mean that I agree with the patch.) >> I liked (and like) the "mechanism, not >> policy" thing. Letting developers pass in whatever they want, for >> development / debugging / testing purposes, is a plus to me. >> >> Thanks >> Laszlo > > Could you flesh out the development / debugging / testing and > how is inserting files in PC namespace useful for that? I don't know -- which is part of the argument. Lack of a ready example doesn't imply (to me) that the possibility should be excluded. As Paolo said, I believe we've been through this discussion. I have no new arguments; the old ones are valid to me still. I won't try to influence this discussion any further; I only chimed in because OVMF was mentioned (and because I noticed that the docs would get out of sync with the code). Thanks Laszlo