Re: [Qemu-devel] [PATCH] fw_cfg: insert string blobs via qemu cmdline

[Laszlo Ersek]

On 09/28/15 22:56, Laszlo Ersek wrote:
> On 09/28/15 22:00, Eric Blake wrote:
>> On 09/28/2015 01:51 PM, Gabriel L. Somlo wrote:
>>> On Mon, Sep 28, 2015 at 01:46:33PM -0600, Eric Blake wrote:
>>>> On 09/28/2015 07:30 AM, Laszlo Ersek wrote:
>>>>
>>>>>>  
>>>>>> +Small enough items may be provided directly as strings on the command
>>>>>> +line, using the syntax:
>>>>>> +
>>>>>> +    -fw_cfg [name=]<item_name>,content=<string>
>>>>>> +
>>>>>
>>>>> Please consider spelling out that these blobs will NOT be NUL-terminated
>>>>> when viewed on the guest. (It kinda follows from all the other fw_cfg
>>>>> things, but once we leave host-side files for qemu command line strings,
>>>>> it might become non-obvious to users.)
>>>>
>>>> Or else GUARANTEE that it will be NUL-terminated (and the only way to
>>>> get blobs that are not NUL terminated is to use files rather than 
>>>> content=).
>>>
>>> I went with the first suggestion (leave out the trailing '\0' from the
>>> blob payload, and say so in docs/specs/fw_cfg.txt) in v2 of the patch.
>>>
>>> Do you feel strongly about including the \0 ? Otherwise, we're already
>>> there :)
>>
>> I don't know what users are more likely to want to push through. A
>> trailing NUL implies a binary file (as text files cannot contain \0),
>> but even without a trailing NUL, a file is not a text file (per the
>> POSIX definition) unless it is either empty or ends in a newline.  Use
>> of content=... is unlikely to have users remembering to place a newline
>> there if examples don't suggest it.  And I don't know if guests are
>> expecting text data from these blobs, or are okay with binary blobs.
> 
> fw_cfg blobs are considered binary, unless a specific selector key or
> fw_cfg file name makes different arrangements. (Described in QEMU docs,
> or elsewhere.) See more below.
> 
>> That's a long-winded way of stating that omitting the NUL is fine by me,
>> as long as you document it, and as long as you are catering to the most
>> common user usage of the feature.
> 
> The main consumer of the -fw_cfg switch is guest firmware (and, perhaps
> soon, the guest kernel too); the idea is to pass down firmware config
> items without QEMU being aware of their actual meaning. Therefore I'd
> like to see as little smarts as possible in QEMU wrt. the content passed
> down with -fw_cfg.
> 
>>
>> Either that, or it's my way of dreaming about alternative 3: guarantee a
>> trailing newline (rather than NUL), so that 'content="abc"' on the
>> command line results in the 4-byte blob "abc\n" in the guest.
>>
> 
> Please don't :)
> 
> The current client code in OVMF (in effect for two specific fw_cfg file
> names) recognizes the following content pattern:
> 
>   [0nN1yY](\n|\r\n)?
> 
> E.g., QEMU may pass in a simple host-side file as an fw_cfg entry on a
> Windows host too. If you edited that file with "notepad.exe", it'll have
> \r\n, or maybe no line terminator at all. Other (really binary) blobs
> (passed in with file=...) may have embedded \0 characters.
> 
> I think such flexibility is best left to the firmware, or else should be
> restricted in specifications living outside of QEMU, and QEMU should be
> dumb and transparent here, in accordance with the original goal of this
> feature.
> 
> Re: policy vs. mechanism, the opt/ prefix is also strongly recommended
> (for the names), but we don't enforce it.

... This made me think of the following language that Gabriel added in
v2 (at my request, and to my acceptance):

> Both <item_name> and, if applicable, the content <string> are assumed
> to consist exclusively of plain ASCII characters.

Now I think that this could be improved. I think we should say "should
consist" rather than "are assumed to consist", because neither the QEMU
nor the firmware(s) "assume" anything in general here -- that would be
policy --, we just want to help the user avoid shooting himself in the
foot (and reporting a bug), lest he pass non-ASCII UTF-8 on the command
line, and the firmware do surprising things.

Maybe I should even retract my request for spelling out ASCII... That's
really not a requirement, just a high-level recommendation for humans
who develop guest code for this interface, to save their sanity.

Thanks
Laszlo