Not sure why mysqld_safe is checking sendmail.

I believe this is caused by checking if a process from old mysqld pidfile is still running. In this particular case mysqld daemon probably finished with pidfile let existing and then it was restarted. At that time process sendmail had been started with the same PID as was stored in the old pidfile. mysqld_safe then checked if the process was mysqld_safe, which was false, so the pidfile was removed and start succeeded.

Using this design we probably don't have to care about potential AVC denials, because it doesn't have any bad consequences, since the checking code behaves the same like as if there is no such process.

However, is it possible in SELinux to define that "mysqld_safe can send SIGNULL to all processes"? 

Btw. the check is done by the following code:

# If there exists an old pid file, check if the daemon is already running
# Note: The switches to 'ps' may depend on your operating system
if test -f "$pid_file"
then
  PID=`cat "$pid_file"`
  if /usr/bin/kill -0 $PID > /dev/null 2> /dev/null
  then
    if /bin/ps wwwp $PID | grep -v " grep" | grep -v mysqld_safe | grep -- "$MYSQLD" > /dev/null
    then    # The pid contains a mysqld process
      log_error "A mysqld process already exists"
      exit 1
    fi
  fi
  rm -f "$pid_file"
  if test -f "$pid_file"
  then
    log_error "Fatal error: Can't remove the pid file:
$pid_file
Please remove it manually and start $0 again;
mysqld daemon not started"
    exit 1
  fi
fi

I think we should just ignore this, since receiving a signul, could cause an application to do something unexpected.  We could add a dontaudit.

Fixed in rawhide.

Hm, it seems unlikely that mysqld is the only thing that would probe for the existence of a process based on finding an old pid file.  It is a mistake to consider this problem as mysql-specific.

What will normally happen for such cases, and what upstream coders would be expecting, is that if the PID now belongs to someone else they would get EPERM on the grounds of the target PID belonging to some other UID.  Perhaps the correct fix is to move the kernel's SELinux check to only occur after that check.

Lets see what the kernel guys say.  I have seen something that looks similar from avahi.

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