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@xxxxxxxxxx> 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
