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.
