On Tue, 2019-06-04 at 02:43 +0200, Daniel Borkmann wrote: > On 06/04/2019 01:54 AM, Alexei Starovoitov wrote: > > On Mon, Jun 3, 2019 at 4:48 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote: > > > On 06/04/2019 01:27 AM, Alexei Starovoitov wrote: > > > > On Mon, Jun 3, 2019 at 3:59 PM Matt Mullins <mmullins@xxxxxx> wrote: > > > > > > > > > > If these are invariably non-nested, I can easily keep bpf_misc_sd when > > > > > I resubmit. There was no technical reason other than keeping the two > > > > > codepaths as similar as possible. > > > > > > > > > > What resource gives you worry about doing this for the networking > > > > > codepath? > > > > > > > > my preference would be to keep tracing and networking the same. > > > > there is already minimal nesting in networking and probably we see > > > > more when reuseport progs will start running from xdp and clsbpf > > > > > > > > > > Aside from that it's also really bad to miss events like this as exporting > > > > > > through rb is critical. Why can't you have a per-CPU counter that selects a > > > > > > sample data context based on nesting level in tracing? (I don't see a discussion > > > > > > of this in your commit message.) > > > > > > > > > > This change would only drop messages if the same perf_event is > > > > > attempted to be used recursively (i.e. the same CPU on the same > > > > > PERF_EVENT_ARRAY map, as I haven't observed anything use index != > > > > > BPF_F_CURRENT_CPU in testing). > > > > > > > > > > I'll try to accomplish the same with a percpu nesting level and > > > > > allocating 2 or 3 perf_sample_data per cpu. I think that'll solve the > > > > > same problem -- a local patch keeping track of the nesting level is how > > > > > I got the above stack trace, too. > > > > > > > > I don't think counter approach works. The amount of nesting is unknown. > > > > imo the approach taken in this patch is good. > > > > I don't see any issue when event_outputs will be dropped for valid progs. > > > > Only when user called the helper incorrectly without BPF_F_CURRENT_CPU. > > > > But that's an error anyway. > > > > > > My main worry with this xchg() trick is that we'll miss to export crucial > > > data with the EBUSY bailing out especially given nesting could increase in > > > future as you state, so users might have a hard time debugging this kind of > > > issue if they share the same perf event map among these programs, and no > > > option to get to this data otherwise. Supporting nesting up to a certain > > > level would still be better than a lost event which is also not reported > > > through the usual way aka perf rb. Tracing can already be lossy: trace_call_bpf() silently simply doesn't call the prog and instead returns zero if bpf_prog_active != 1. > > > > I simply don't see this 'miss to export data' in all but contrived conditions. > > Say two progs share the same perf event array. > > One prog calls event_output and while rb logic is working > > another prog needs to start executing and use the same event array > > Correct. > > > slot. Today it's only possible for tracing prog combined with networking, > > but having two progs use the same event output array is pretty much > > a user bug. Just like not passing BPF_F_CURRENT_CPU. > > I don't see the user bug part, why should that be a user bug? It's the same > as if we would say that sharing a BPF hash map between networking programs > attached to different hooks or networking and tracing would be a user bug > which it is not. One concrete example would be cilium monitor where we > currently expose skb trace and drop events a well as debug data through > the same rb. This should be usable from any type that has perf_event_output > helper enabled (e.g. XDP and tc/BPF) w/o requiring to walk yet another per > cpu mmap rb from user space. Neither of these solutions would affect the behavior of sharing the perf array between networking programs -- since they're never called in a nested fashion, then you'll never hit the "xchg() returned NULL" at all. That said, I think I can logically limit nesting in tracing to 3 levels: - a kprobe or raw tp or perf event, - another one of the above that irq context happens to run, and - if we're really unlucky, the same in nmi at most one of which can be a kprobe or perf event. There is also a comment /* * bpf_raw_tp_regs are separate from bpf_pt_regs used from skb/xdp * to avoid potential recursive reuse issue when/if tracepoints are added * inside bpf_*_event_output, bpf_get_stackid and/or bpf_get_stack */ that suggests that one day bpf_perf_event_output might grow a static tracepoint. However, if the program attached to such a hypothetical tracepoint were to call bpf_perf_event_output, that would infinitely recurse ... it seems fine to let that case return -EBUSY as well. It does make me wonder if I should do the same nesting for the pt_regs. I've now got an experiment running with the counter approach, and the workload that I hit the original crash with seems to be fine with 2 layers' worth -- though if we decide that's the one I should move forward with, I'll probably bump it to a third to be safe for an NMI.
