Re: [PATCH bpf] bpf: Don't WARN_ON_ONCE in bpf_bprintf_prepare

[Andrii Nakryiko]

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.

>
> > > 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