Re: [PATCH bpf-next 4/9] samples: bpf: refactor task_fd_query program with libbpf

On Tue, Nov 17, 2020 at 6:57 AM Daniel T. Lee <[email protected]> wrote:
>
> This commit refactors the existing kprobe program with libbpf bpf
> loader. To attach bpf program, this uses generic bpf_program__attach()
> approach rather than using bpf_load's load_bpf_file().
>
> To attach bpf to perf_event, instead of using previous ioctl method,
> this commit uses bpf_program__attach_perf_event since it manages the
> enable of perf_event and attach of BPF programs to it, which is much
> more intuitive way to achieve.
>
> Also, explicit close(fd) has been removed since event will be closed
> inside bpf_link__destroy() automatically.
>
> DEBUGFS macro from trace_helpers has been used to control uprobe events.
> Furthermore, to prevent conflict of same named uprobe events, O_TRUNC
> flag has been used to clear 'uprobe_events' interface.
>
> Signed-off-by: Daniel T. Lee <[email protected]>
> ---
>  samples/bpf/Makefile             |   2 +-
>  samples/bpf/task_fd_query_user.c | 101 ++++++++++++++++++++++---------
>  2 files changed, 74 insertions(+), 29 deletions(-)
>

[...]

>  static int test_debug_fs_uprobe(char *binary_path, long offset, bool is_return)
>  {
> +       char buf[256], event_alias[sizeof("test_1234567890")];
>         const char *event_type = "uprobe";
>         struct perf_event_attr attr = {};
> -       char buf[256], event_alias[sizeof("test_1234567890")];
>         __u64 probe_offset, probe_addr;
>         __u32 len, prog_id, fd_type;
> -       int err, res, kfd, efd;
> +       int err = -1, res, kfd, efd;
> +       struct bpf_link *link;
>         ssize_t bytes;
>
> -       snprintf(buf, sizeof(buf), "/sys/kernel/debug/tracing/%s_events",
> -                event_type);
> -       kfd = open(buf, O_WRONLY | O_APPEND, 0);
> +       snprintf(buf, sizeof(buf), DEBUGFS "%s_events", event_type);
> +       kfd = open(buf, O_WRONLY | O_TRUNC, 0);

O_TRUNC will also remove other events, created by users. Not a great
experience. Let's leave the old behavior?

>         CHECK_PERROR_RET(kfd < 0);
>
>         res = snprintf(event_alias, sizeof(event_alias), "test_%d", getpid());
> @@ -240,8 +252,8 @@ static int test_debug_fs_uprobe(char *binary_path, long offset, bool is_return)
>         close(kfd);
>         kfd = -1;
>
> -       snprintf(buf, sizeof(buf), "/sys/kernel/debug/tracing/events/%ss/%s/id",
> -                event_type, event_alias);
> +       snprintf(buf, sizeof(buf), DEBUGFS "events/%ss/%s/id", event_type,

I'd leave the string verbatim here (and above), I think it's better
that way and easier to figure out what's written where. And then no
need to expose DEBUGFS.

> +                event_alias);
>         efd = open(buf, O_RDONLY, 0);
>         CHECK_PERROR_RET(efd < 0);
>

[...]