Re: arm64 syzbot instances

[Dmitry Vyukov]

On Mon, Mar 15, 2021 at 11:31 AM John Garry <john.garry@xxxxxxxxxx> wrote:
>
> On 15/03/2021 10:01, Dmitry Vyukov wrote:
> > On Mon, Mar 15, 2021 at 10:45 AM John Garry<john.garry@xxxxxxxxxx>  wrote:
> >>>> It does not happen too often on syzbot so far, so let's try to do the
> >>>> right thing first.
> >>>> I've filed:https://bugs.launchpad.net/qemu/+bug/1918917
> >>>> with a link to this thread. To be fair, I don't fully understand what
> >>>> I am talking about, I hope I proxied your description properly.
> >>> Thanks, looks good. I provided a little more detail in a comment there.
> >>>
> >>>           Arnd
> >>> .
> >>>
> >>   From looking at the bug report, my impression is that this is a qemu
> >> issue, as the logical IO space is mapped to the PCI host bridge IO
> >> space, and qemu does not handle accesses to that CPU addressable region
> >> at all. As Arnd said.
> >>
> >> However, we really should not be accessing logical IO ports 0 or 0x2f8
> >> at all via ttyS3 if not enumerated from PCI device at that logical IO
> >> port. That is what I think anyway, as who knows what device - if any -
> >> really exists at that location. That is why I had this patch to just
> >> stop accesses to legacy IO port regions on arm64:
> >>
> >> https://lore.kernel.org/lkml/1610729929-188490-2-git-send-email-john.garry@xxxxxxxxxx/
> > Hi John,
> >
> > Thanks for the info.
> >
> > The patch is from January, but it's not merged yet, right?
> > It will fix the crash we see, right?
> > .
>
> It's not merged, and it probably would solve this issue. But following
> discussion with Arnd when it was originally posted, I still need to do
> some analysis whether it is the proper thing to do.

OK, I will tell syzbot about the fixing patch.

> However, as mentioned, the fundamental issue looks like qemu IO port
> access, so it would be good to check that first.

I've filed https://bugs.launchpad.net/qemu/+bug/1918917 for qemu.

> On a related topic, I will cc colleague Jiahui Cen, who I think was
> doing some work arm on qemu support in a related area, so may share some
> experience here.
>
> Jiahui Cen did have a patch to fix logical PIO code from this work [0],
> which is not merged, but I don't think would help here. I will cc you on it.
>
> Thanks,
> John
>
> [0]
> https://lore.kernel.org/lkml/006ad6ce-d6b2-59cb-8209-aca3f6e53fec@xxxxxxxxxx/