Re: [Qemu-devel] [Bug] MIPS code fails at branch instruction

[Paul Brook <paul@codesourcery.com>]

> >> So an emulation has several options:
> >>
> >> 1. Show undefined behaviour (this is what it does today).
> >> 2. Emulate the behaviour of existing CPUs as far as possible.
> >> As different CPUs behave different, this must depend on the
> >> current CPU.
> >> 3. Display an error message.
> >
> > (3) is bad, as it amounts to a DoS.
>
> DoS = Denial of Service? Then (1) is some kind of DoS, because QEMU hangs
> with code which works on real hardware. I don't understand why an
> error message (something printed to stdout or stderr like other boot
> messages of QEMU) amounts to a DoS.

It's not the same thing at all. In both cases buggy code crashes. I expect 
this could also happen on a fair proportion of real MIPS hardware. It may 
even happen on AR7 hardware is a interrupt or fault happens to trigger at the 
wrong time.

With (1) the buggy program crashes, and the rest of the machine keeps going.
With (3) an unprivileged user can effectively bring the whole machine down 
just by executing invalid code sequences.

Paul