Re: [PATCH bpf] bpf: preallocate a perf_sample_data per event fd

[Matt Mullins]

On Thu, 2019-05-30 at 23:28 +0000, Song Liu wrote:
> > On May 30, 2019, at 3:55 PM, Matt Mullins <mmullins@xxxxxx> 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@xxxxxx>
> > ---
> > 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;
> };

Yeah, that totally works.  I was mostly doing this so we had only one
kmalloc allocation, but I'm not too worried about having an extra
object in kmalloc-64 if it simplifies the code a lot.

> 
> > -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?

You mean xchg a null-pointer or something in there while it's in-use,
then xchg the slab back?  Makes sense to me.  I'll try that and see
where it gets me.

> 
> 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
> > 
> 
>