Re: hw/usb/hcd-ohci: Fix #1510, #303: pid not IN or OUT

[Cord Amfmgm <dmamfmgm@gmail.com>]

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On Thu, May 30, 2024 at 2:12 PM Alex Bennée <alex.bennee@linaro.org> wrote:

> Cord Amfmgm <dmamfmgm@gmail.com> writes:
>
> > On Thu, May 30, 2024 at 3:33 AM Alex Bennée <alex.bennee@linaro.org>
> wrote:
> >
> >  Cord Amfmgm <dmamfmgm@gmail.com> writes:
> >
> >  > On Tue, May 28, 2024 at 11:32 AM Peter Maydell <
> peter.maydell@linaro.org> wrote:
> >  >
> >  >  On Tue, 28 May 2024 at 16:37, Cord Amfmgm <dmamfmgm@gmail.com>
> wrote:
> >  >  >
> >  >  > On Tue, May 28, 2024 at 9:03 AM Peter Maydell <
> peter.maydell@linaro.org> wrote:
> >  >  >>
> >  >  >> On Mon, 20 May 2024 at 23:24, Cord Amfmgm <dmamfmgm@gmail.com>
> wrote:
> >  >  >> > On Mon, May 20, 2024 at 12:05 PM Peter Maydell <
> peter.maydell@linaro.org> wrote:
> >  <snip>
> >  >  >> > And here's an example buffer of length 0 -- you probably
> already know what I'm going to do here:
> >  >  >> >
> >  >  >> > char buf[0];
> >  >  >> > char * CurrentBufferPointer = &buf[0];
> >  >  >> > char * BufferEnd = &buf[-1]; // "address of the last byte in
> the buffer"
> >  >  >> > // The OHCI Host Controller than advances CurrentBufferPointer
> like this: CurrentBufferPointer += 0
> >  >  >> > // After the transfer:
> >  >  >> > // CurrentBufferPointer = &buf[0];
> >  >  >> > // BufferEnd = &buf[-1];
> >  >  >>
> >  >  >> Right, but why do you think this is valid, rather than
> >  >  >> being a guest software bug? My reading of the spec is that it's
> >  >  >> pretty clear about how to say "zero length buffer", and this
> >  >  >> isn't it.
> >  >  >>
> >  >  >> Is there some real-world guest OS that programs the OHCI
> >  >  >> controller this way that we're trying to accommodate?
> >  >  >
> >  >  >
> >  >  > qemu versions 4.2 and before allowed this behavior.
> >  >
> >  >  So? That might just mean we had a bug and we fixed it.
> >  >  4.2 is a very old version of QEMU and nobody seems to have
> >  >  complained in the four years since we released 5.0 about this,
> >  >  which suggests that generally guest OS drivers don't try
> >  >  to send zero-length buffers in this way.
> >  >
> >  >  > I don't think it's valid to ask for a *popular* guest OS as a
> proof-of-concept because I'm not an expert on those.
> >  >
> >  >  I didn't ask for "popular"; I asked for "real-world".
> >  >  What is the actual guest code you're running that falls over
> >  >  because of the behaviour change?
> >  >
> >  >  More generally, why do you want this behaviour to be
> >  >  changed? Reasonable reasons might include:
> >  >   * we're out of spec based on reading the documentation
> >  >   * you're trying to run some old Windows VM/QNX/etc image,
> >  >     and it doesn't work any more
> >  >   * all the real hardware we tested behaves this way
> >  >
> >  >  But don't necessarily include:
> >  >   * something somebody wrote and only tested on QEMU happens to
> >  >     assume the old behaviour rather than following the hw spec
> >  >
> >  >  QEMU occasionally works around guest OS bugs, but only as
> >  >  when we really have to. It's usually better to fix the
> >  >  bug in the guest.
> >  >
> >  > It's not, and I've already demonstrated that real hardware is
> consistent with the fix in this patch.
> >  >
> >  > Please check your tone.
> >
> >  I don't think that is a particularly helpful comment for someone who is
> >  taking the time to review your patches. Reading through the thread I
> >  didn't see anything that said this is how real HW behaves but I may well
> >  have missed it. However you have a number of review comments to address
> >  so I suggest you spin a v2 of the series to address them and outline the
> >  reason to accept an out of spec transaction.
> >
> > I did a rework of the patch -- see my email from May 20, quoted below --
> and I was under the impression it addressed all the
> > review comments. Did I miss something? I apologize if I did.
>
> Ahh I see - I'd only seen this thread continue so wasn't aware a new
> version had been posted. For future patches consider using -vN when
> sending them so we can clearly see a new revision is available.
>
> >
> >> index acd6016980..71b54914d3 100644
> >> --- a/hw/usb/hcd-ohci.c
> >> +++ b/hw/usb/hcd-ohci.c
> >> @@ -941,8 +941,8 @@ static int ohci_service_td(OHCIState *ohci, struct
> ohci_ed *ed)
> >>          if ((td.cbp & 0xfffff000) != (td.be & 0xfffff000)) {
> >>              len = (td.be & 0xfff) + 0x1001 - (td.cbp & 0xfff);
> >>          } else {
> >> -            if (td.cbp > td.be) {
> >> -                trace_usb_ohci_iso_td_bad_cc_overrun(td.cbp, td.be);
> >> +            if (td.cbp - 1 > td.be) {  /* rely on td.cbp != 0 */
> >
> >> Reading through the thread I didn't see anything that said this is how
> real HW behaves but I may well have missed it.
> >
> > This is what I wrote regarding real HW:
> >
> > Results are:
> >
> >  qemu 4.2   | qemu HEAD  | actual HW
> > ------------+------------+------------
> >  works fine | ohci_die() | works fine
> >
> > Would additional verification of the actual HW be useful?
> >
> > Peter posted the following which is more specific than "qemu 4.2" -- I
> agree this is most likely the qemu commit where this
> > thread is focused:
> >
> >> Almost certainly this was commit 1328fe0c32d54 ("hw: usb: hcd-ohci:
> >> check len and frame_number variables"), which added these bounds
> >> checks. Prior to that we did no bounds checking at all, which
> >> meant that we permitted cbp=be+1 to mean a zero length, but also
> >> that we permitted the guest to overrun host-side buffers by
> >> specifying completely bogus cbp and be values. The timeframe is
> >> more or less right (2020), at least.
> >>
> >> -- PMM
> >
> > Where does the conversation go from here? I'm under the impression I
> have provided objective answers to all the questions
> > and resolved all review comments on the code. I receive the feedback
> > that I missed something - please restate the question?
>
> I can see patch 1/2 has been queued and 2/2 is still outstanding. I'm
> having trouble finding the referenced entry in the OHCI spec. The only
> one I can see is Release 1.1, January 6th, 2000 and that doesn't have a
> section 4.3.1.2.
>
> I think discussion should continue on that thread.
>

Yes, agreed.


>
> --
> Alex Bennée
> Virtualisation Tech Lead @ Linaro
>

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