linux-kernel - Re: [BUG] signal: multithread program returns with wrong errno on receiving SIGSTOP

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Date:	Mon, 28 May 2007 18:58:50 +0900
From:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Satoru Takeuchi <takeuchi_satoru.jp@...itsu.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [BUG] signal: multithread program returns with wrong errno on receiving SIGSTOP

Hi Oleg,

At Mon, 28 May 2007 11:07:53 +0400,
Oleg Nesterov wrote:
> 
> Satoru Takeuchi wrote:
> >
> > 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.
> 
> It doesn't matter in this particular case, but in general it is good to
> report the kernel version as well.
> 
> > 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.
> 
> Wild guess,
> 
> 	drivers/char/n_tty.c:job_control()
> 
> 		kill_pgrp(task_pgrp(current), SIGTTIN, 1);
> 		return -ERESTARTSYS;
> 
> This is wrong. kill_pgrp()->__group_send_sig_info() sends SIGTTIN
> to the thread group, but can choose any sub-thread as a target for
> signal_wake_up().
> 
> This means we return ERESTARTSYS, but signal_pending() != true, bug.
> 
> Satoru, could you re-test with the untested patch below?

I tested this patch on my ia64 box and it works fine.

# Since I don't know anything about tty driver, I can't determine its
# adequacy immediately, sorry.

Satoru

> 
> Oleg.
> 
> --- t/drivers/char/n_tty.c~	2007-04-05 12:18:26.000000000 +0400
> +++ t/drivers/char/n_tty.c	2007-05-28 10:57:58.000000000 +0400
> @@ -1191,6 +1191,7 @@ static int job_control(struct tty_struct
>  			    is_current_pgrp_orphaned())
>  				return -EIO;
>  			kill_pgrp(task_pgrp(current), SIGTTIN, 1);
> +			set_thread_flag(TIF_SIGPENDING);
>  			return -ERESTARTSYS;
>  		}
>  	}
> 
> -
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