[BUG] signal: multithread program returns with wrong errno on receiving SIGSTOP

[Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>]

Hi,

I found a bug on signal subsystem. If there is some multithread program
and one of the thread is blocking on the system call, it returns with
wrong errno on receiving SIGSTOP and following SIGCONT.

Arch dependency
===============

succeed to reproduce: i386, ia64
Unknown:              any other arch

# I suspect that this problem occur on any arch...

How to reproduce
================

This process needs 2 terminals, term A and term B.

1. issue the test program (attached on this mail) from term A.

   $ ./mt-wrong-errno

2. send SIGSTOP to test program from term B.

   $ ps a
   ...
    9685 pts/3    Sl+    0:00 ./mt-wrong-errno
    9688 pts/4    R+     0:00 ps a
   $ kill -STOP 9685

3. send SIGCONT to test program from term B.

   $ kill -CONT 9685

Expected Result
===============

`./mt-wrong-errno' restarts read syscall (and stop again by receiving
SIGTTIN).

Actual Result
=============

`./mt-wrong-errno' returns with wrong errno 512 means ERESTARTSYS.
The comment on include/linux/errno.h says "this errno should never be
seen by user program." So it's definitely a bug.

  output of term A
  ----------------

    $ ./mt-wrong-errno

    [1]+  Stopped                 ./mt-wrong-errno
    $ errno = 512

  output of term B
  ----------------

    $ ps a
    ...
     9685 pts/3    Sl+    0:00 ./mt-wrong-errno
     9688 pts/4    R+     0:00 ps a
    $ kill -STOP 9685
    $ kill -CONT 9685


This problem can always reproduce, so it's no timing issue. I suspect
that group_stop_count counting mechanism has something wrong and
signal handling is canceled erroneously. 

Any hints or patch itself are welcome.

Thanks,

Satoru

-------------------------------------------------------------------------------
/*
 * mt-wrong-errno.c
 * 
 * Copyright 2007 Satoru Takeuchi <takeuchi_satoru@jp.futjisu.com>
 *
 * This software may be used and distributed according to the terms
 * of the GNU General Public License, incorporated herein by reference.
 * 
 */

#include <sys/types.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <err.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>

void *thread_fn(void *arg)
{
	char c;
	int ret;

	ret = read(STDIN_FILENO, &c, 1);
	if (ret < 0)
		printf("errno = %d\n", errno);

	return NULL;
}

int main(int argc, char *argv[])
{
	pthread_t t;
	
	if (pthread_create(&t, NULL, thread_fn, NULL))
		err(EXIT_FAILURE, "pthread_create() failed\n");
	
	if (pthread_join(t, NULL))
		warn("pthread_join() failed\n");
}