[PATCH] arm64: bpf: Fix branch offset in JIT

[Ilias Apalodimas]

On Mon, Sep 14, 2020 at 11:52:16AM -0700, Xi Wang wrote:
> On Mon, Sep 14, 2020 at 11:28 AM Ilias Apalodimas
> <ilias.apalodimas at linaro.org> wrote:
> > Even if that's true, is any reason at all why we should skip the first element
> > of the array, that's now needed since 7c2e988f400 to jump back to the first
> > instruction?
> > Introducing 2 extra if conditions and hotfix the array on the fly (and for
> > every future invocation of that), seems better to you?
> 
> My point was that there's no inherently correct/wrong way to construct
> offsets.  As Luke explained in his email, 1) there are two different
> strategies used by the JITs and 2) there are likely similar bugs
> beyond arm64.
> 
> Each strategy has pros and cons, and I'm fine with either.  I like the
> strategy used in your patch because it's more intuitive (offset[i] is
> the start of the emitted instructions for BPF instruction i, rather
> than the end), though the changes to the construction process are
> trickier.
> 

Well the arm64 was literally a 'save the idx before building the instruction',
and add another element on the array.  So it's not that trickier, especially
if we document it properly.

I haven't checked the rest of the architectures tbh (apart from x86). 
I assumed the tracking used in arm64 at that point, was a result of how 
eBPF worked before bounded loops were introduced. Maybe I was wrong.
It felt a bit more natural to track the beginning of the emitted 
instructions rather than the end.

> If we decide to patch the arm64 JIT the way you proposed, we should
> consider whether to change other JITs consistently.

I think this is a good idea. Following the code is not exactly a stroll in the
park, so we can at least make it consistent across architectures.

Thanks
/Ilias