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Date:   Thu, 30 May 2019 23:28:14 +0000
From:   Song Liu <songliubraving@...com>
To:     Matt Mullins <mmullins@...com>
CC:     Andrew Hall <hall@...com>, Alexei Starovoitov <ast@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH bpf] bpf: preallocate a perf_sample_data per event fd



> On May 30, 2019, at 3:55 PM, Matt Mullins <mmullins@...com> 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@...com>
> ---
> It felt a bit overkill, but I had to split bpf_event_entry into its own
> header file to break an include cycle from perf_event.h -> cgroup.h ->
> cgroup-defs.h -> bpf-cgroup.h -> bpf.h -> (potentially) perf_event.h.
> 
> include/linux/bpf.h       |  7 -------
> include/linux/bpf_event.h | 20 ++++++++++++++++++++
> kernel/bpf/arraymap.c     |  2 ++
> kernel/trace/bpf_trace.c  | 30 +++++++++++++++++-------------
> 4 files changed, 39 insertions(+), 20 deletions(-)
> create mode 100644 include/linux/bpf_event.h
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 4fb3aa2dc975..13b253a36402 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -467,13 +467,6 @@ static inline bool bpf_map_flags_access_ok(u32 access_flags)
> 	       (BPF_F_RDONLY_PROG | BPF_F_WRONLY_PROG);
> }
> 

I think we can avoid the include cycle as:

+struct perf_sample_data *sd;
struct bpf_event_entry {
	struct perf_event *event;
	struct file *perf_file;
	struct file *map_file;
	struct rcu_head rcu;
+	struct perf_sample_data *sd;
};

> -struct bpf_event_entry {
> -	struct perf_event *event;
> -	struct file *perf_file;
> -	struct file *map_file;
> -	struct rcu_head rcu;
> -};
> -
> bool bpf_prog_array_compatible(struct bpf_array *array, const struct bpf_prog *fp);
> int bpf_prog_calc_tag(struct bpf_prog *fp);
> 
> diff --git a/include/linux/bpf_event.h b/include/linux/bpf_event.h
> new file mode 100644
> index 000000000000..9f415990f921
> --- /dev/null
> +++ b/include/linux/bpf_event.h
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_BPF_EVENT_H
> +#define _LINUX_BPF_EVENT_H
> +
> +#include <linux/perf_event.h>
> +#include <linux/types.h>
> +
> +struct file;
> +
> +struct bpf_event_entry {
> +	struct perf_event *event;
> +	struct file *perf_file;
> +	struct file *map_file;
> +	struct rcu_head rcu;
> +	struct perf_sample_data sd;
> +	atomic_t in_use;
> +};
> +
> +#endif /* _LINUX_BPF_EVENT_H */
> diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
> index 584636c9e2eb..08e5e486d563 100644
> --- a/kernel/bpf/arraymap.c
> +++ b/kernel/bpf/arraymap.c
> @@ -11,6 +11,7 @@
>  * General Public License for more details.
>  */
> #include <linux/bpf.h>
> +#include <linux/bpf_event.h>
> #include <linux/btf.h>
> #include <linux/err.h>
> #include <linux/slab.h>
> @@ -659,6 +660,7 @@ static struct bpf_event_entry *bpf_event_entry_gen(struct file *perf_file,
> 		ee->event = perf_file->private_data;
> 		ee->perf_file = perf_file;
> 		ee->map_file = map_file;

And do the kzalloc() or some other trick here. 

> +		atomic_set(&ee->in_use, 0);
> 	}
> 
> 	return ee;
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index f92d6ad5e080..a03e29957698 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -6,6 +6,7 @@
> #include <linux/types.h>
> #include <linux/slab.h>
> #include <linux/bpf.h>
> +#include <linux/bpf_event.h>
> #include <linux/bpf_perf_event.h>
> #include <linux/filter.h>
> #include <linux/uaccess.h>
> @@ -410,17 +411,17 @@ static const struct bpf_func_proto bpf_perf_event_read_value_proto = {
> 	.arg4_type	= ARG_CONST_SIZE,
> };
> 
> -static DEFINE_PER_CPU(struct perf_sample_data, bpf_trace_sd);
> -
> static __always_inline u64
> __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> -			u64 flags, struct perf_sample_data *sd)
> +			u64 flags, struct perf_raw_record *raw)
> {
> 	struct bpf_array *array = container_of(map, struct bpf_array, map);
> 	unsigned int cpu = smp_processor_id();
> 	u64 index = flags & BPF_F_INDEX_MASK;
> 	struct bpf_event_entry *ee;
> 	struct perf_event *event;
> +	struct perf_sample_data *sd;
> +	u64 ret;
> 
> 	if (index == BPF_F_CURRENT_CPU)
> 		index = cpu;
> @@ -439,13 +440,22 @@ __bpf_perf_event_output(struct pt_regs *regs, struct bpf_map *map,
> 	if (unlikely(event->oncpu != cpu))
> 		return -EOPNOTSUPP;
> 
> -	return perf_event_output(event, sd, regs);
> +	if (atomic_cmpxchg(&ee->in_use, 0, 1) != 0)
> +		return -EBUSY;

And we only need xchg() here, so we can eliminate in_use. 

Does this make sense?

Thanks,
Song

> +
> +	sd = &ee->sd;
> +	perf_sample_data_init(sd, 0, 0);
> +	sd->raw = raw;
> +
> +	ret = perf_event_output(event, sd, regs);
> +
> +	atomic_set(&ee->in_use, 0);
> +	return ret;
> }
> 
> BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
> 	   u64, flags, void *, data, u64, size)
> {
> -	struct perf_sample_data *sd = this_cpu_ptr(&bpf_trace_sd);
> 	struct perf_raw_record raw = {
> 		.frag = {
> 			.size = size,
> @@ -456,10 +466,8 @@ BPF_CALL_5(bpf_perf_event_output, struct pt_regs *, regs, struct bpf_map *, map,
> 	if (unlikely(flags & ~(BPF_F_INDEX_MASK)))
> 		return -EINVAL;
> 
> -	perf_sample_data_init(sd, 0, 0);
> -	sd->raw = &raw;
> 
> -	return __bpf_perf_event_output(regs, map, flags, sd);
> +	return __bpf_perf_event_output(regs, map, flags, &raw);
> }
> 
> static const struct bpf_func_proto bpf_perf_event_output_proto = {
> @@ -474,12 +482,10 @@ static const struct bpf_func_proto bpf_perf_event_output_proto = {
> };
> 
> static DEFINE_PER_CPU(struct pt_regs, bpf_pt_regs);
> -static DEFINE_PER_CPU(struct perf_sample_data, bpf_misc_sd);
> 
> u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
> 		     void *ctx, u64 ctx_size, bpf_ctx_copy_t ctx_copy)
> {
> -	struct perf_sample_data *sd = this_cpu_ptr(&bpf_misc_sd);
> 	struct pt_regs *regs = this_cpu_ptr(&bpf_pt_regs);
> 	struct perf_raw_frag frag = {
> 		.copy		= ctx_copy,
> @@ -497,10 +503,8 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
> 	};
> 
> 	perf_fetch_caller_regs(regs);
> -	perf_sample_data_init(sd, 0, 0);
> -	sd->raw = &raw;
> 
> -	return __bpf_perf_event_output(regs, map, flags, sd);
> +	return __bpf_perf_event_output(regs, map, flags, &raw);
> }
> 
> BPF_CALL_0(bpf_get_current_task)
> -- 
> 2.17.1
> 

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