On Thu, Jan 9, 2020 at 5:16 PM Yonghong Song <yhs@xxxxxx> wrote:
>
> Commit 8b401f9ed244 ("bpf: implement bpf_send_signal() helper")
> added helper bpf_send_signal() which permits bpf program to
> send a signal to the current process.
>
> We found a use case where sending the signal to the current
> thread is more preferable.
> - A bpf program will collect the stack trace and then
> send signal to the user application.
> - The user application will add some thread specific
> information to the just collected stack trace for
> later analysis.
>
> If bpf_send_signal() is used, user application will need
> to check whether the thread receiving the signal matches
> the thread collecting the stack by checking thread id.
> If not, it will need to send signal to another thread
> through pthread_kill().
>
> This patch proposed a new helper bpf_send_signal_thread(),
> which sends the signal to the current thread. This way,
> user space is guaranteed that bpf_program execution context
> and user space signal handling context are the same thread.
>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
> include/uapi/linux/bpf.h | 18 ++++++++++++++++--
> kernel/trace/bpf_trace.c | 27 ++++++++++++++++++++++++---
> 2 files changed, 40 insertions(+), 5 deletions(-)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 52966e758fe5..3320f8bdfe7e 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2714,7 +2714,7 @@ union bpf_attr {
> *
> * int bpf_send_signal(u32 sig)
> * Description
> - * Send signal *sig* to the current task.
> + * Send signal *sig* to the process of the current task.
> * Return
> * 0 on success or successfully queued.
> *
> @@ -2850,6 +2850,19 @@ union bpf_attr {
> * Return
> * 0 on success, or a negative error in case of failure.
> *
> + * int bpf_send_signal_thread(u32 sig)
> + * Description
> + * Send signal *sig* to the current task.
This all makes sense and looks good, but I think it's very unclear why
the distinction between sending signal to process vs thread. Could you
extend bpf_send_signal and bpf_send_signal_thread descriptions
explaining the difference (e.g., that, according to POSIX, when
sending signal to a process, any thread within that process can get
signal delivered, while sending to a specific thread will ensure that
that specific thread will receive desired signal).
[...]
