Re: [PATCH bpf v2] bpf: preallocate a perf_sample_data per event fd

On Fri, May 31, 2019 at 6:28 PM Song Liu [off-list ref] wrote:

quoted

On May 31, 2019, at 3:37 PM, Matt Mullins [off-list ref] 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 <redacted>

This looks great. Thanks for the fix.

Acked-by: Song Liu <redacted>

quoted

---
v1->v2:
      keep a pointer to the struct perf_sample_data rather than directly
      embedding it in the structure, avoiding the circular include and
      removing the need for in_use.  Suggested by Song.

include/linux/bpf.h      |  1 +
kernel/bpf/arraymap.c    |  3 ++-
kernel/trace/bpf_trace.c | 29 ++++++++++++++++-------------
3 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 4fb3aa2dc975..47fd85cfbbaf 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h

@@ -472,6 +472,7 @@ struct bpf_event_entry {
      struct file *perf_file;
      struct file *map_file;
      struct rcu_head rcu;
+     struct perf_sample_data *sd;

};

bool bpf_prog_array_compatible(struct bpf_array *array, const struct bpf_prog *fp);

diff --git a/kernel/bpf/arraymap.c b/kernel/bpf/arraymap.c
index 584636c9e2eb..c7f5d593e04f 100644
--- a/kernel/bpf/arraymap.c
+++ b/kernel/bpf/arraymap.c

@@ -654,11 +654,12 @@ static struct bpf_event_entry *bpf_event_entry_gen(struct file *perf_file,

{
      struct bpf_event_entry *ee;

-     ee = kzalloc(sizeof(*ee), GFP_ATOMIC);
+     ee = kzalloc(sizeof(*ee) + sizeof(struct perf_sample_data), GFP_ATOMIC);
      if (ee) {
              ee->event = perf_file->private_data;
              ee->perf_file = perf_file;
              ee->map_file = map_file;
+             ee->sd = (void *)ee + sizeof(*ee);

This bit looks quite weird, but I don't have better ideas
to avoid circular .h pain.

Applied to bpf tree. Thanks