linux-kernel - Re: 2.6.25.3: su gets stuck for root

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Date:	Mon, 2 Jun 2008 12:16:56 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"David Newall" <davidn@...idnewall.com>,
	"Willy Tarreau" <w@....eu>,
	"Harald Dunkel" <harald.dunkel@...nline.de>,
	"Joe Peterson" <joe@...rush.com>, linux-kernel@...r.kernel.org,
	"Alan Cox" <alan@...hat.com>
Subject: Re: 2.6.25.3: su gets stuck for root

On Mon, Jun 2, 2008 at 11:20 AM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> On Mon, 02 Jun 2008 18:31:34 +0930
> David Newall <davidn@...idnewall.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

View attachment "reproduce.c" of type "text/x-csrc" (1077 bytes)