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

[Gabriel L. Somlo]

On Tue, Sep 29, 2015 at 12:08:14AM +0200, Laszlo Ersek wrote:
> On 09/28/15 23:45, Gabriel L. Somlo wrote:
> > On Mon, Sep 28, 2015 at 11:05:32PM +0200, Laszlo Ersek wrote:
> >> 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.
> > 
> > Maybe something like this, then:
> > 
> > Both <item_name> and, if applicable, the content <string> will be
> > passed through by QEMU without any interpretation, expansion, or 
> > further processing. Any such processing (potentially performed by
> > e.g., the shell) is outside QEMU's responsibility; as such, using
> > plain ASCII characters is recommended.
> > 
> > Let me know what you think.
> 
> Sounds good to me. Thanks for your patience. :)

Cool, v3 coming right up. Not 100% sure about etiquette, but I'm
inclined to assume your earlier Reviewed-by is still valid :)

Thanks,
--Gabriel

PS. You have a *very* *large* reserve of patience credits with me, so
no worries on that :)