A fifo and signal bug

H.J. Lu (hjl@lucon.org)
Fri, 20 Nov 1998 11:04:55 -0800 (PST)

Hi, Linus,

The POSIX testsuite found another Linux kernel bug. This time is the
interaction between fifo open and signal:

# gcc fifo.c
# a.out 1
# a.out
parent: open: Device not configured

The problem is the child is waiting in fifo_open in kernel and is
stopped by a signal. As the result, the parant fifo open returns
ENXIO. It is incorrect. The kernel should notice that there a
process doing fifo_open but was stopped. I don't think it is too
hard to fix.

Thanks.

-- 
H.J. Lu (hjl@gnu.org)
----
#include <signal.h>
#include <errno.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <sys/stat.h>


#define WAITTIME	5


#define FIFO_FILE       "test.fifo"


static int fd = -1;


static
void
cleanup ()
{
  if (fd >= 0)
    close (fd);
  unlink (FIFO_FILE);
}


static
void
do_open ()
{
  int fd;


  fd = open(FIFO_FILE, O_RDONLY);
  if (fd < 0)
    perror ("child: open");
}


static
void
child ()
{
  if(setpgid((pid_t)0, (pid_t)0) < 0)
    {
      perror ("setpgid");
      exit (1);
    }
  do_open ();
  exit (0);
}


int
main (int argc, char **argv)
{
  int ret;
  int status;
  int sel;
  pid_t child_pid;


  cleanup ();
  if (mkfifo (FIFO_FILE, S_IRWXU|S_IRWXG|S_IRWXO) != 0)
    {
      perror ("mkfifo");
      exit (1);
    }


  sel = argc == 1 ? 0 : atoi (argv [1]);


  switch (child_pid = fork ())
    {
    case 0:
      /* Child */
      child ();
      break;


    case -1:
      perror ("fork");
      cleanup (sel);
      exit (1);
      break;


    default:
      break;
    }


  sleep (WAITTIME);


  if (sel == 0)
    {
      if(kill(child_pid, SIGTSTP))
        {
	  perror ("kill");
	  cleanup ();
	  exit (1);
	}
      sleep (WAITTIME);
    }


  fd = open(FIFO_FILE, O_WRONLY|O_NONBLOCK);
  if (sel == 0)
    {
      if (fd < 0)
        {
	  perror ("parent: open");
	  if(kill(child_pid, SIGKILL))
	    {
	      perror ("kill");
	      cleanup ();
	      exit (1);
	    }
	}
      else
	{
	  if(kill(child_pid, SIGCONT))
	    {
	      perror ("kill");
	      cleanup ();
	      exit (1);
	    }
	}
    }
  
  ret = waitpid (child_pid, &status, WUNTRACED);
  if (ret < 0)
    {
      perror ("waitpid");
      exit (1);
    }


  cleanup ();
  return ret;
}


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