On 2/8/24 00:59, Andrii Nakryiko wrote: > On Tue, Jul 30, 2024 at 8:31 PM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote: >> >> >> >> On 31/7/24 01:28, Andrii Nakryiko wrote: >>> On Mon, Jul 29, 2024 at 8:32 PM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote: >>>> >>>> >>>> >>>> On 30/7/24 05:01, Andrii Nakryiko wrote: >>>>> On Fri, Jul 26, 2024 at 9:04 PM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote: >>>>>> >>>>>> >>>>>> >>>>>> On 2024/7/27 08:12, Andrii Nakryiko wrote: >>>>>>> On Thu, Jul 25, 2024 at 7:57 PM Leon Hwang <hffilwlqm@xxxxxxxxx> wrote: >>>>>>>> >>>>>>>> >>>>>>>> >>>> >>>> [...] >>>> >>>>>>>> >>>>>>>> Is it OK to add a tracepoint here? I think tracepoint is more generic >>>>>>>> than retsnoop-like way. >>>>>>> >>>>>>> I personally don't see a problem with adding tracepoint, but how would >>>>>>> it look like, given we are talking about vararg printf-style function >>>>>>> calls? I'm not sure how that should be represented in such a way as to >>>>>>> make it compatible with tracepoints and not cause any runtime >>>>>>> overhead. >>>>>> >>>>>> The tracepoint is not about vararg printf-style function calls. >>>>>> >>>>>> It is to trace the reason why it fails to bpf_check_attach_target() at >>>>>> attach time. >>>>>> >>>>> >>>>> Oh, that changes things. I don't think we can keep adding extra >>>>> tracepoints for various potential reasons that BPF prog might be >>>>> failing to verify. >>>>> >>>>> But there is usually no need either. This particular code already >>>>> supports emitting extra information into verifier log, you just have >>>>> to provide that. This is done by libbpf automatically, can't your >>>>> library of choice do the same (if BPF program failed). >>>>> >>>>> Why go to all this trouble if we already have a facility to debug >>>>> issues like this. Note every issue is logged into verifier log, but in >>>>> this case it is. >>>>> >>>> >>>> Yeah, it is unnecessary to add tracepoint here, as we are able to trace >>>> the log message in bpf_log() arguments with retsnoop. >>> >>> My point was that you don't even need retsnoop, you can just ask for >>> verifier log directly, that's the main way to understand and debug BPF >>> program verification/load failures. >>> >> >> Nope. It is not about BPF program verification/load failures. It is >> about freplace program attach failures instead. > > Ah, my bad, it's at an attach time. Still, I don't think a tracepoint > for every possible failure will ever work. Perhaps the right approach > is to wire up bpf_log into attach commands (LINK_CREATE, at least), so > that the kernel can report back what's the reason for declining > attachment? > OK. Let me take a try. Thanks, Leon
