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

[Laszlo Ersek <lersek@redhat.com>]

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" <mst@redhat.com> 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 <mst@redhat.com>
>>>
>>> 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". 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