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

[Ilias Apalodimas]

Hi Will,

On Mon, Sep 14, 2020 at 03:01:15PM +0100, Will Deacon wrote:
> Hi Ilias,
> 

[...]

> > > > 
> > > > No Fixes: tag?
> > > 
> > > I'll re-spin and apply one 
> > > 
> > Any suggestion on any Fixes I should apply? The original code was 'correct' and
> > broke only when bounded loops and their self-tests were introduced.
> 
> Ouch, that's pretty bad as it means nobody is regression testing BPF on
> arm64 with mainline. Damn.

That might not be entirely true. Since offset is a pointer, there's a chance
(and a pretty high one according to my reproducer) that the offset[-1] value 
happens to be 0. In that case the tests will pass fine. I can reproduce the bug
approximately 1 every 6-7 passes here.

I'll send a v2 shortly fixing the tags and adding a few comments on the code,
which will hopefully make future reading easier.

Cheers
/Ilias