On Fri, Aug 30, 2024 at 18:12:41PM +0800, Oleg Nesterov wrote: > The whole discussion was very confusing (yes, I too contributed to the > confusion ;), let me try to summarise. > > > U(ret)probes are designed to be filterable using the PID, which is the > > second parameter in the perf_event_open syscall. Currently, uprobe works > > well with the filtering, but uretprobe is not affected by it. > > And this is correct. But the CONFIG_BPF_EVENTS code in __uprobe_perf_func() > misunderstands the purpose of uprobe_perf_filter(). > > Lets forget about BPF for the moment. It is not that uprobe_perf_filter() > does the filtering by the PID, it doesn't. We can simply kill this function > and perf will work correctly. The perf layer in __uprobe_perf_func() does > the filtering when perf_event->hw.target != NULL. > > So why does uprobe_perf_filter() call uprobe_perf_filter()? Not to avoid > the __uprobe_perf_func() call (as the BPF code assumes), but to trigger > unapply_uprobe() in handler_chain(). > > Suppose you do, say, > > $ perf probe -x /path/to/libc some_hot_function > or > $ perf probe -x /path/to/libc some_hot_function%return > then > $perf record -e ... -p 1 > > to trace the usage of some_hot_function() in the init process. Everything > will work just fine if we kill uprobe_perf_filter()->uprobe_perf_filter(). > > But. If INIT forks a child C, dup_mm() will copy int3 installed by perf. > So the child C will hit this breakpoint and cal handle_swbp/etc for no > reason every time it calls some_hot_function(), not good. > > That is why uprobe_perf_func() calls uprobe_perf_filter() which returns > UPROBE_HANDLER_REMOVE when C hits the breakpoint. handler_chain() will > call unapply_uprobe() which will remove this breakpoint from C->mm. > > > We found that the filter function was not invoked when uretprobe was > > initially implemented, and this has been existing for ten years. > > See above, this is correct. > > Note also that if you only use perf-probe + perf-record, no matter how > many instances, you can even add BUG_ON(!uprobe_perf_filter(...)) into > uretprobe_perf_func(). IIRC, perf doesn't use create_local_trace_uprobe(). > Thanks for the detailed explanation above, I can understand the code now. Yes, I completely misunderstood the purpose of uprobe_perf_filter, sorry for the confusion. > --------------------------------------------------------------------------- > Now lets return to BPF and this particular problem. I won't really argue > with this patch, but > > - Please change the subject and update the changelog, > "Fixes: c1ae5c75e103" and the whole reasoning is misleading > and wrong, IMO. > > - This patch won't fix all problems because uprobe_perf_filter() > filters by mm, not by pid. The changelog/patch assumes that it > is a "PID filter", but it is not. > > See https://lore.kernel.org/linux-trace-kernel/20240825224018.GD3906@xxxxxxxxxx/ > If the traced process does clone(CLONE_VM), bpftrace will hit the > similar problem, with uprobe or uretprobe. > > - So I still think that the "right" fix should change the > bpf_prog_run_array_uprobe() paths somehow, but I know nothing > about bpf. I agree that this patch does not address the issue correctly. The PID filter should be implemented within bpf_prog_run_array_uprobe, or alternatively, bpf_prog_run_array_uprobe should be called after perf_tp_event_match to reuse the filtering mechanism provided by perf. Also, uretprobe may need UPROBE_HANDLER_REMOVE, similar to uprobe. For now, please forget the original patch as we need a new solution ;) Thanks for your time,
