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

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Message-ID: <20190606151346.6a9ed27e@cakuba.netronome.com>
Date:   Thu, 6 Jun 2019 15:13:46 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Matt Mullins <mmullins@...com>
Cc:     <hall@...com>, <ast@...nel.org>, <daniel@...earbox.net>,
        <bpf@...r.kernel.org>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Ingo Molnar" <mingo@...hat.com>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH bpf] bpf: fix nested bpf tracepoints with per-cpu data

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...