Subject: 0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94 (posix-timers: Fix division by zero bug). Problems with glibc.

From: Alan Mackenzie <[email-masked]>
Date: Fri, 1 Feb 2019 21:47:00 +0000

Hello, Thomas, Hello Linux.

0e334db6bb4b1fd1e2d72c1f3d8f004313cd9f94
posix-timers: Fix division by zero bug
Committed: 2018-12-17 17:35:45 +0100

With this patch in place I am seeing problems with glibc's function
timer_create.  I am an Emacs maintainer, and saw these problems whilst
investigating Emacs bug #34235 "27.0.50; lisp profiler does not work".
Full details of this bug are at
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=34235.

Emacs's profiler fails in kernel 4.19.13, but works in a version of
4.19.13 with the patch reversed, otherwise unchanged.  My current version
of glibc is 2.27-r6 (I think the "-r6" comes from Gentoo, my distro).

The Emacs profiler works by a signal handler being repeatedly triggered
by the SIGPROF signal every 1 millisecond.  In the bug scenario, this
signal gets triggered precisely once each time the Emacs profiler is
started, rather than continually.

The core of the code in Emacs which initialises the glibc timer is:

      int i;
      struct sigevent sigev;
      sigev.sigev_value.sival_ptr = &profiler_timer;
      sigev.sigev_signo = SIGPROF;
      sigev.sigev_notify = SIGEV_SIGNAL;

      for (i = 0; i < ARRAYELTS (system_clock); i++)
        if (timer_create (system_clock[i], &sigev, &profiler_timer) == 0)
          {
            profiler_timer_ok = 1;
            break;
          }
    }

  if (profiler_timer_ok)
    {
      struct itimerspec ispec;
      ispec.it_value = ispec.it_interval = interval;
      if (timer_settime (profiler_timer, 0, &ispec, 0) == 0)
        return TIMER_SETTIME_RUNNING;
    }

The variable `interval' has been checked as non-zero.  This code is in
.../emacs/src/profiler.c

It seems either that the patch has uncovered some invalid call between
Emacs and glibc, or between glibc and Linux, or that there is some
intrinsic problem with the patch.

I have very little familiarity with glibc and Linux source code, so I
would be greatful if you could help me investigate the bug scenario.
Naturally, I will help as I can in this process.

Thanks in advance!

-- 
Alan Mackenzie (Nuremberg, Germany).