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

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Date:   Thu, 6 Jun 2019 22:39:03 +0000
From:   Matt Mullins <mmullins@...com>
To:     "jakub.kicinski@...ronome.com" <jakub.kicinski@...ronome.com>
CC:     Song Liu <songliubraving@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "ast@...nel.org" <ast@...nel.org>, Andrew Hall <hall@...com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Martin Lau" <kafai@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf] bpf: fix nested bpf tracepoints with per-cpu data

On Thu, 2019-06-06 at 15:13 -0700, Jakub Kicinski wrote:
> 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.

Oops, I totally am.  I'll give it some more time for opinions to roll
in, and I'll fix that before I resubmit :)

> 
> > ---
> > 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...
> 
>