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Subject: Re: [PATCH bpf v2] bpf: preallocate a perf_sample_data per event fd
From:   Daniel Borkmann <daniel@iogearbox.net>
To:     Matt Mullins <mmullins@fb.com>, hall@fb.com, ast@kernel.org,
        bpf@vger.kernel.org, netdev@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>
References: <20190531223735.4998-1-mmullins@fb.com>
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Message-ID: <68841715-4d5b-6ad1-5241-4e7199dd63da@iogearbox.net>
Date:   Mon, 3 Jun 2019 15:22:14 +0200
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On 06/03/2019 03:08 PM, Daniel Borkmann wrote:
> On 06/01/2019 12:37 AM, Matt Mullins wrote:
>> It is possible that a BPF program can be called while another BPF
>> program is executing bpf_perf_event_output.  This has been observed with
>> I/O completion occurring as a result of an interrupt:
>>
>> 	bpf_prog_247fd1341cddaea4_trace_req_end+0x8d7/0x1000
>> 	? trace_call_bpf+0x82/0x100
>> 	? sch_direct_xmit+0xe2/0x230
>> 	? blk_mq_end_request+0x1/0x100
>> 	? blk_mq_end_request+0x5/0x100
>> 	? kprobe_perf_func+0x19b/0x240
>> 	? __qdisc_run+0x86/0x520
>> 	? blk_mq_end_request+0x1/0x100
>> 	? blk_mq_end_request+0x5/0x100
>> 	? kprobe_ftrace_handler+0x90/0xf0
>> 	? ftrace_ops_assist_func+0x6e/0xe0
>> 	? ip6_input_finish+0xbf/0x460
>> 	? 0xffffffffa01e80bf
>> 	? nbd_dbg_flags_show+0xc0/0xc0 [nbd]
>> 	? blkdev_issue_zeroout+0x200/0x200
>> 	? blk_mq_end_request+0x1/0x100
>> 	? blk_mq_end_request+0x5/0x100
>> 	? flush_smp_call_function_queue+0x6c/0xe0
>> 	? smp_call_function_single_interrupt+0x32/0xc0
>> 	? call_function_single_interrupt+0xf/0x20
>> 	? call_function_single_interrupt+0xa/0x20
>> 	? swiotlb_map_page+0x140/0x140
>> 	? refcount_sub_and_test+0x1a/0x50
>> 	? tcp_wfree+0x20/0xf0
>> 	? skb_release_head_state+0x62/0xc0
>> 	? skb_release_all+0xe/0x30
>> 	? napi_consume_skb+0xb5/0x100
>> 	? mlx5e_poll_tx_cq+0x1df/0x4e0
>> 	? mlx5e_poll_tx_cq+0x38c/0x4e0
>> 	? mlx5e_napi_poll+0x58/0xc30
>> 	? mlx5e_napi_poll+0x232/0xc30
>> 	? net_rx_action+0x128/0x340
>> 	? __do_softirq+0xd4/0x2ad
>> 	? irq_exit+0xa5/0xb0
>> 	? do_IRQ+0x7d/0xc0
>> 	? common_interrupt+0xf/0xf
>> 	</IRQ>
>> 	? __rb_free_aux+0xf0/0xf0
>> 	? perf_output_sample+0x28/0x7b0
>> 	? perf_prepare_sample+0x54/0x4a0
>> 	? perf_event_output+0x43/0x60
>> 	? bpf_perf_event_output_raw_tp+0x15f/0x180
>> 	? blk_mq_start_request+0x1/0x120
>> 	? bpf_prog_411a64a706fc6044_should_trace+0xad4/0x1000
>> 	? bpf_trace_run3+0x2c/0x80
>> 	? nbd_send_cmd+0x4c2/0x690 [nbd]
>>
>> This also cannot be alleviated by further splitting the per-cpu
>> perf_sample_data structs (as in commit 283ca526a9bd ("bpf: fix
>> corruption on concurrent perf_event_output calls")), as a raw_tp could
>> be attached to the block:block_rq_complete tracepoint and execute during
>> another raw_tp.  Instead, keep a pre-allocated perf_sample_data
>> structure per perf_event_array element and fail a bpf_perf_event_output
>> if that element is concurrently being used.
>>
>> Fixes: 20b9d7ac4852 ("bpf: avoid excessive stack usage for perf_sample_data")
>> Signed-off-by: Matt Mullins <mmullins@fb.com>
> 
> You do not elaborate why is this needed for all the networking programs that
> use this functionality. The bpf_misc_sd should therefore be kept as-is. There
> cannot be nested occurrences there (xdp, tc ingress/egress). Please explain why
> non-tracing should be affected here...

Aside from that it's also really bad to miss events like this as exporting
through rb is critical. Why can't you have a per-CPU counter that selects a
sample data context based on nesting level in tracing? (I don't see a discussion
of this in your commit message.)