Re: [PATCH bpf] bpf: fix nested bpf tracepoints with per-cpu data

From: Jakub Kicinski
Date: Thu Jun 06 2019 - 18:18:13 EST

On Thu, 6 Jun 2019 11:54:27 -0700, Matt Mullins wrote:
> BPF_PROG_TYPE_RAW_TRACEPOINTs can be executed nested on the same CPU, as
> they do not increment bpf_prog_active while executing.
> This enables three levels of nesting, to support
> - a kprobe or raw tp or perf event,
> - another one of the above that irq context happens to call, and
> - another one in nmi context
> (at most one of which may be a kprobe or perf event).
> Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")

No comment on the code, but you're definitely missing a sign-off.

> ---
> This is more lines of code, but possibly less intrusive than the
> per-array-element approach.
> I don't necessarily like that I duplicated the nest_level logic in two
> places, but I don't see a way to unify them:
> - kprobes' bpf_perf_event_output doesn't use bpf_raw_tp_regs, and does
> use the perf_sample_data,
> - raw tracepoints' bpf_get_stackid uses bpf_raw_tp_regs, but not
> the perf_sample_data, and
> - raw tracepoints' bpf_perf_event_output uses both...