Re: 2.6.25.3: su gets stuck for root

["Vegard Nossum" <vegard.nossum@gmail.com>]

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On Mon, Jun 2, 2008 at 11:20 AM, Alan Cox <alan@lxorguk.ukuu.org.uk> wrote:
> On Mon, 02 Jun 2008 18:31:34 +0930
> David Newall <davidn@davidnewall.com> wrote:
>
>> Alan Cox wrote:
>> > Not really. The task would get suspended if it attempted to change the
>> > tty settings while not being session leader. This is part of the POSIX
>> > and BSD job control.
>>
>> I haven't heard about this new restriction, but it begs the observation
>> that stty, when forked from a shell (the usual case), is never a session
>> leader.
>
> Sorry I mean part of the current session. I was thinking about the
> specific case of bash or the ssh->bash setup where the question would be
> whether the shell was session leader.
>
> Someone who can dup this needs to instrument it in tty_ioctl really.

Hi,

I have written a short test program that seems to reproduce it for me
(see attachment), even though the original su/stty stuff wouldn't.

Basically, the strace shows this:
ioctl(0, SNDCTL_TMR_START or TCSETS, {B38400 opost isig icanon echo
...}) = ? ERESTARTSYS (To be restarted)
--- SIGTTOU (Stopped (tty output)) @ 0 (0) ---
--- SIGTTOU (Stopped (tty output)) @ 0 (0) ---
ioctl(0, SNDCTL_TMR_START or TCSETS, {B38400 opost isig icanon echo
...}) = ? ERESTARTSYS (To be restarted)
--- SIGTTOU (Stopped (tty output)) @ 0 (0) ---
--- SIGTTOU (Stopped (tty output)) @ 0 (0) ---
... (repeating)

The exact code path triggering this seems to be:

tcsetattr() -> ioctl(TCSETS) -> set_termios() -> tty_check_change()

This is on a 2.6.24.5-85.fc8 kernel.

I don't know what's wrong, but I hope this helps.


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036

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#include <sys/types.h>
#include <sys/wait.h>

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <termios.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
	pid_t child;

	printf("pgid = %d\n", getpgrp());

	child = fork();
	if (child == 0) {
		struct termios termios_p;

		printf("forked, pgid = %d\n", getpgrp());

		if (setpgrp() == -1) {
			printf("error: setpgid: %s\n", strerror(errno));
			exit(EXIT_FAILURE);
		}

		printf("new pgid = %d\n", getpgrp());

		if (tcgetattr(STDIN_FILENO, &termios_p) == -1) {
			printf("error: tcgetattr: %s\n", strerror(errno));
			exit(EXIT_FAILURE);
		}

		if (tcsetattr(STDIN_FILENO, 0, &termios_p) == -1) {
			printf("error: tcsetattr: %s\n", strerror(errno));
			exit(EXIT_FAILURE);
		}

		exit(EXIT_SUCCESS);
	}

	printf("forked, child = %d\n", child);

	while (1) {
		pid_t pid;
		int status;

		pid = wait(&status);
		if (pid == -1) {
			printf("error: wait: %s\n", strerror(errno));
			exit(EXIT_FAILURE);
		}

		printf("pid %d status %d\n", pid, status);
	}

	return EXIT_SUCCESS;
}