Subject: Re: [PATCH bpf-next v2 0/3] bpf: implement bpf_send_signal() helper

From: Stanislav Fomichev <[email-masked]>
Date: Wed, 22 May 2019 09:38:54 -0700

On 05/21, Yonghong Song wrote:
> This patch tries to solve the following specific use case.
> 
> Currently, bpf program can already collect stack traces
> through kernel function get_perf_callchain()
> when certain events happens (e.g., cache miss counter or
> cpu clock counter overflows). But such stack traces are
> not enough for jitted programs, e.g., hhvm (jited php).
> To get real stack trace, jit engine internal data structures
> need to be traversed in order to get the real user functions.
> 
> bpf program itself may not be the best place to traverse
> the jit engine as the traversing logic could be complex and
> it is not a stable interface either.
> 
> Instead, hhvm implements a signal handler,
> e.g. for SIGALARM, and a set of program locations which
> it can dump stack traces. When it receives a signal, it will
> dump the stack in next such program location.
> 

[..]
> This patch implements bpf_send_signal() helper to send
> a signal to hhvm in real time, resulting in intended stack traces.
Series looks good. One minor nit/question: maybe rename bpf_send_signal
to something like bpf_send_signal_to_current/bpf_current_send_signal/etc?
bpf_send_signal is too generic now that you send the signal
to the current process..

> Patch #1 implemented the bpf_send_helper() in the kernel,
> Patch #2 synced uapi header bpf.h to tools directory.
> Patch #3 added a self test which covers tracepoint
> and perf_event bpf programs.
> 
> Changelogs:
>   RFC v1 => v2:
>     . previous version allows to send signal to an arbitrary
>       pid. This version just sends the signal to current
>       task to avoid unstable pid and potential races between
>       sending signals and task state changes for the pid.
> 
> Yonghong Song (3):
>   bpf: implement bpf_send_signal() helper
>   tools/bpf: sync bpf uapi header bpf.h to tools directory
>   tools/bpf: add a selftest for bpf_send_signal() helper
> 
>  include/uapi/linux/bpf.h                      |  17 +-
>  kernel/trace/bpf_trace.c                      |  67 ++++++
>  tools/include/uapi/linux/bpf.h                |  17 +-
>  tools/testing/selftests/bpf/Makefile          |   3 +-
>  tools/testing/selftests/bpf/bpf_helpers.h     |   1 +
>  .../bpf/progs/test_send_signal_kern.c         |  51 +++++
>  .../selftests/bpf/test_send_signal_user.c     | 212 ++++++++++++++++++
>  7 files changed, 365 insertions(+), 3 deletions(-)
>  create mode 100644 tools/testing/selftests/bpf/progs/test_send_signal_kern.c
>  create mode 100644 tools/testing/selftests/bpf/test_send_signal_user.c
> 
> -- 
> 2.17.1
>