---
- branch: MAIN
  date: Mon Oct 21 17:07:01 UTC 2019
  files:
  - new: '1.376'
    old: '1.375'
    path: src/sys/kern/kern_sig.c
    pathrev: src/sys/kern/kern_sig.c@1.376
    type: modified
  - new: '1.138'
    old: '1.137'
    path: src/tests/lib/libc/sys/t_ptrace_wait.c
    pathrev: src/tests/lib/libc/sys/t_ptrace_wait.c@1.138
    type: modified
  id: 20191021T170701Z.8a9ea96460e07ba93b261585714b2bae601d82a6
  log: |
    Fix a race condition when handling concurrent LWP signals and add a test

    Fix a race condition that caused PT_GET_SIGINFO to return incorrect
    information when multiple signals were delivered concurrently
    to different LWPs.  Add a regression test that verifies that when 50
    threads concurrently use pthread_kill() on themselves, the debugger
    receives all signals with correct information.

    The kernel uses separate signal queues for each LWP.  However,
    the signal context used to implement PT_GET_SIGINFO is stored in 'struct
    proc' and therefore common to all LWPs in the process.  Previously,
    this member was filled in kpsignal2(), i.e. when the signal was sent.
    This meant that if another LWP managed to send another signal
    concurrently, the data was overwritten before the process was stopped.

    As a result, PT_GET_SIGINFO did not report the correct LWP and signal
    (it could even report a different signal than wait()).  This can be
    quite reliably reproduced with the number of 20 LWPs, however it can
    also occur with 10.

    This patch moves setting of signal context to issignal(), just before
    the process is actually stopped.  The data is taken from per-LWP
    or per-process signal queue.  The added test confirms that the debugger
    correctly receives all signals, and PT_GET_SIGINFO reports both correct
    LWP and signal number.

    Reviewed by kamil.
  module: src
  subject: 'CVS commit: src'
  unixtime: '1571677621'
  user: mgorny