linux-kernel - Re: [PATCH v5 perf, bpf-next 3/7] perf, bpf: introduce PERF_RECORD_BPF

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Date:   Tue, 8 Jan 2019 19:10:20 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     lkml <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "acme@...nel.org" <acme@...nel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH v5 perf, bpf-next 3/7] perf, bpf: introduce
 PERF_RECORD_BPF_EVENT



> On Jan 8, 2019, at 10:41 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Thu, Dec 20, 2018 at 10:29:00AM -0800, Song Liu wrote:
>> @@ -986,9 +987,35 @@ enum perf_event_type {
>> 	 */
>> 	PERF_RECORD_KSYMBOL			= 17,
>> 
>> +	/*
>> +	 * Record bpf events:
>> +	 *  enum perf_bpf_event_type {
>> +	 *	PERF_BPF_EVENT_UNKNOWN		= 0,
>> +	 *	PERF_BPF_EVENT_PROG_LOAD	= 1,
>> +	 *	PERF_BPF_EVENT_PROG_UNLOAD	= 2,
>> +	 *  };
>> +	 *
>> +	 * struct {
>> +	 *	struct perf_event_header	header;
>> +	 *	u16				type;
>> +	 *	u16				flags;
>> +	 *	u32				id;
>> +	 *	u8				tag[BPF_TAG_SIZE];
>> +	 *	struct sample_id		sample_id;
>> +	 * };
>> +	 */
>> +	PERF_RECORD_BPF_EVENT			= 18,
>> +
> 
> Elsewhere today, I raised the point that by the time (however short
> interval) userspace gets around to reading this event, the actual
> program could be gone again.
> 
> In this case the program has been with us for a very short period
> indeed; but it could still have generated some samples or otherwise
> generated trace data.

Since we already have the separate KSYMBOL events, BPF_EVENT is only 
required for advanced use cases, like annotation. So I guess missing 
it for very-short-living programs should not be a huge problem?

> It was suggested to allow pinning modules/programs to avoid this
> situation, but that of course has other undesirable effects, such as a
> trivial DoS.
> 
> A truly horrible hack would be to include an open filedesc in the event
> that needs closing to release the resource, but I'm sorry for even
> suggesting that **shudder**.
> 
> Do we have any sane ideas?

How about we gate the open filedesc solution with an option, and limit
that option for root only? If this still sounds hacky, maybe we should
just ignore when short-living programs are missed?

Thanks,
Song