On Thu, May 6, 2021 at 11:38 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote: > > On 5/6/21 10:17 PM, Florent Revest wrote: > > On Thu, May 6, 2021 at 8:52 PM Andrii Nakryiko > > <andrii.nakryiko@xxxxxxxxx> wrote: > >> On Wed, May 5, 2021 at 3:29 PM Florent Revest <revest@xxxxxxxxxxxx> wrote: > >>> On Wed, May 5, 2021 at 10:52 PM Andrii Nakryiko > >>> <andrii.nakryiko@xxxxxxxxx> wrote: > >>>> On Wed, May 5, 2021 at 1:48 PM Andrii Nakryiko > >>>> <andrii.nakryiko@xxxxxxxxx> wrote: > >>>>> On Wed, May 5, 2021 at 1:00 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote: > >>>>>> On 5/5/21 8:55 PM, Andrii Nakryiko wrote: > >>>>>>> On Wed, May 5, 2021 at 9:23 AM Florent Revest <revest@xxxxxxxxxxxx> wrote: > >>>>>>>> > >>>>>>>> The bpf_seq_printf, bpf_trace_printk and bpf_snprintf helpers share one > >>>>>>>> per-cpu buffer that they use to store temporary data (arguments to > >>>>>>>> bprintf). They "get" that buffer with try_get_fmt_tmp_buf and "put" it > >>>>>>>> by the end of their scope with bpf_bprintf_cleanup. > >>>>>>>> > >>>>>>>> If one of these helpers gets called within the scope of one of these > >>>>>>>> helpers, for example: a first bpf program gets called, uses > >>>>>>> > >>>>>>> Can we afford having few struct bpf_printf_bufs? They are just 512 > >>>>>>> bytes, so can we have 3-5 of them? Tracing low-level stuff isn't the > >>>>>>> only situation where this can occur, right? If someone is doing > >>>>>>> bpf_snprintf() and interrupt occurs and we run another BPF program, it > >>>>>>> will be impossible to do bpf_snprintf() or bpf_trace_printk() from the > >>>>>>> second BPF program, etc. We can't eliminate the probability, but > >>>>>>> having a small stack of buffers would make the probability so > >>>>>>> miniscule as to not worry about it at all. > >>>>>>> > >>>>>>> Good thing is that try_get_fmt_tmp_buf() abstracts all the details, so > >>>>>>> the changes are minimal. Nestedness property is preserved for > >>>>>>> non-sleepable BPF programs, right? If we want this to work for > >>>>>>> sleepable we'd need to either: 1) disable migration or 2) instead of > >>>>> > >>>>> oh wait, we already disable migration for sleepable BPF progs, so it > >>>>> should be good to do nestedness level only > >>>> > >>>> actually, migrate_disable() might not be enough. Unless it is > >>>> impossible for some reason I miss, worst case it could be that two > >>>> sleepable programs (A and B) can be intermixed on the same CPU: A > >>>> starts&sleeps - B starts&sleeps - A continues&returns - B continues > >>>> and nestedness doesn't work anymore. So something like "reserving a > >>>> slot" would work better. > >>> > >>> Iiuc try_get_fmt_tmp_buf does preempt_enable to avoid that situation ? > >>> > >>>>>>> assuming a stack of buffers, do a loop to find unused one. Should be > >>>>>>> acceptable performance-wise, as it's not the fastest code anyway > >>>>>>> (printf'ing in general). > >>>>>>> > >>>>>>> In any case, re-using the same buffer for sort-of-optional-to-work > >>>>>>> bpf_trace_printk() and probably-important-to-work bpf_snprintf() is > >>>>>>> suboptimal, so seems worth fixing this. > >>>>>>> > >>>>>>> Thoughts? > >>>>>> > >>>>>> Yes, agree, it would otherwise be really hard to debug. I had the same > >>>>>> thought on why not allowing nesting here given users very likely expect > >>>>>> these helpers to just work for all the contexts. > >>>>>> > >>>>>> Thanks, > >>>>>> Daniel > >>> > >>> What would you think of just letting the helpers own these 512 bytes > >>> buffers as local variables on their stacks ? Then bpf_prepare_bprintf > >>> would only need to write there, there would be no acquire semantic > >>> (like try_get_fmt_tmp_buf) and the stack frame would just be freed on > >>> the helper return so there would be no bpf_printf_cleanup either. We > >>> would also not pre-reserve static memory for all CPUs and it becomes > >>> trivial to handle re-entrant helper calls. > >>> > >>> I inherited this per-cpu buffer from the pre-existing bpf_seq_printf > >>> code but I've not been convinced of its necessity. > >> > >> I got the impression that extra 512 bytes on the kernel stack is quite > >> a lot and that's why we have per-cpu buffers. Especially that > >> bpf_trace_printk() can be called from any context, including NMI. > > > > Ok, I understand. > > > > What about having one buffer per helper, synchronized with a spinlock? > > Actually, bpf_trace_printk already has that, not for the bprintf > > arguments but for the bprintf output so this wouldn't change much to > > the performance of the helpers anyway: > > https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/kernel/trace/bpf_trace.c?id=9d31d2338950293ec19d9b095fbaa9030899dcb4#n385 > > > > These helpers are not performance sensitive so a per-cpu stack of > > buffers feels over-engineered to me (and is also complexity I feel a > > bit uncomfortable with). > > But wouldn't this have same potential of causing a deadlock? Simple example > would be if you have a tracing prog attached to bstr_printf(), and one of > the other helpers using the same lock called from a non-tracing prog. If Ah, right, I see :/ > it can be avoided fairly easily, I'd also opt for per-cpu buffers as Andrii > mentioned earlier. We've had few prior examples with similar issues [0]. > > [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9594dc3c7e71b9f52bee1d7852eb3d4e3aea9e99 Ok it's not as bad as I imagined, thank you Daniel :) I'll look into it beginning of next week.
