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

[Yonghong Song]

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]);