Re: [PATCH] sample: bpf: syscall_tp_user: print result of verify_map

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On 3/31/22 8:37 PM, Song Chen wrote:


在 2022/4/1 11:01, Yonghong Song 写道:


On 3/31/22 6:41 PM, Song Chen wrote:
syscall_tp only prints the map id and messages when something goes wrong,
but it doesn't print the value passed from bpf map. I think it's better
to show that value to users.

What's more, i also added a 2-second sleep before calling verify_map,
to make the value more obvious.

Signed-off-by: Song Chen <chensong_2000@xxxxxx>
---
  samples/bpf/syscall_tp_user.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/samples/bpf/syscall_tp_user.c b/samples/bpf/syscall_tp_user.c
index a0ebf1833ed3..1faa7f08054e 100644
--- a/samples/bpf/syscall_tp_user.c
+++ b/samples/bpf/syscall_tp_user.c
@@ -36,6 +36,9 @@ static void verify_map(int map_id)
          fprintf(stderr, "failed: map #%d returns value 0\n", map_id);
          return;
      }
+
+    printf("verify map:%d val: %d\n", map_id, val);

I am not sure how useful it is or anybody really cares.
This is just a sample to demonstrate how bpf tracepoint works.
The error path has error print out already.

Considering we already have
   printf("prog #%d: map ids %d %d\n", i, map0_fds[i], map1_fds[i]);
I think your proposed additional printout
   printf("verify map:%d val: %d\n", map_id, val);
might be okay. The commit message should be rewritten
to justify this change something like:
   we already print out
     prog <some number>: map ids <..> <...>
   further print out
      verify map: ...
   will help user to understand the program runs successfully.

I think sleep(2) is unnecessary.


+
      val = 0;
      if (bpf_map_update_elem(map_id, &key, &val, BPF_ANY) != 0) {
          fprintf(stderr, "map_update failed: %s\n", strerror(errno));
@@ -98,6 +101,7 @@ static int test(char *filename, int num_progs)
      }
      close(fd);
+    sleep(2);

The commit message mentioned this sleep(2) is
to make the value more obvious. I don't know what does this mean.
sleep(2) can be added only if it fixed a bug.

The value in bpf map means how many times trace_enter_open_at are triggered with tracepoint,sys_enter_openat. Sleep(2) is to enlarge the result, tell the user how many files are opened in the last 2 seconds.

It shows like this:

sudo ./samples/bpf/syscall_tp
prog #0: map ids 4 5
verify map:4 val: 253
verify map:5 val: 252

If we work harder, we can also print those files' name and opened by which process.

It's just an improvement instead of a bug fix, i will drop it if reviewers think it's unnecessary.

Thanks.

BR

chensong

      /* verify the map */
      for (i = 0; i < num_progs; i++) {
          verify_map(map0_fds[i]);




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