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Re: [Qemu-devel] [PATCH v2] vl.c: disallow command line fw cfg without o

From: Laszlo Ersek
Subject: Re: [Qemu-devel] [PATCH v2] vl.c: disallow command line fw cfg without opt/
Date: Wed, 16 Mar 2016 20:15:07 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.6.0

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" <address@hidden> 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 <address@hidden>
>>>>> 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).


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