[PATCH bpf-next v5 0/3] bpf: implement bpf_send_signal() helper

[Yonghong Song <yhs@fb.com>]

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. 

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:
  v4 => v5:
    . pass the "current" task struct to irq_work as well
      since the current task struct may change between
      nmi and subsequent irq_work_interrupt.
      Discovered by Daniel.
  v3 => v4:
    . fix one typo and declare "const char *id_path = ..."
      to avoid directly use the long string in the func body 
      in Patch #3.
  v2 => v3:
    . change the standalone test to be part of prog_tests.
  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 selftest in test_progs for bpf_send_signal() helper

 include/uapi/linux/bpf.h                      |  17 +-
 kernel/trace/bpf_trace.c                      |  72 +++++++
 tools/include/uapi/linux/bpf.h                |  17 +-
 tools/testing/selftests/bpf/bpf_helpers.h     |   1 +
 .../selftests/bpf/prog_tests/send_signal.c    | 198 ++++++++++++++++++
 .../bpf/progs/test_send_signal_kern.c         |  51 +++++
 6 files changed, 354 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/send_signal.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_send_signal_kern.c

-- 
2.17.1