linux-kernel - Re: um: TTY fixes (?)

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Message-ID: <4FCD4161.7060409@nod.at>
Date:	Tue, 05 Jun 2012 01:14:41 +0200
From:	Richard Weinberger <richard@....at>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	jslaby@...e.cz, user-mode-linux-devel@...ts.sourceforge.net,
	gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
	viro@...iv.linux.org.uk, alan@...ux.intel.com, kzak@...hat.com
Subject: Re: um: TTY fixes (?)

Am 04.06.2012 23:17, schrieb Alan Cox:
>> 	On all other ttys login works but bash dies because of of -EIO.
>> 	After vhangup() the tty returns -EIO upon read()/write().
> 
> You can't re-open the tty because a process is holding on to it, not
> closing it and not killable. Fedora shouldn't be holding these devices
> open this way. The behaviour we have of refusing to reopen them why this
> is the case is both a) what the spec seems to say b) good security.

Hmm, there seems to be a bug in util-linux's login.
login-utils/login.c::init_tty() does:
...
       /* Kill processes left on this tty */
        tcsetattr(0, TCSAFLUSH, &ttt);

        signal(SIGHUP, SIG_IGN);        /* so vhangup() wont kill us */
        vhangup();
        signal(SIGHUP, SIG_DFL);

        /* open stdin,stdout,stderr to the tty */
        open_tty(cxt->tty_path);

        /* restore tty modes */
        tcsetattr(0, TCSAFLUSH, &tt);
...

By calling vhangup() it kills all other programs on the current tty.
open_tty() opens the tty again but it's still open because stdin, stdout and stderr
belongs to it.

If I add:
        fclose(stdin);
        fclose(stdout);
        fclose(stderr);
before the call to vhangup() login works like charm. :-)

Karel, what do you think?

Thanks,
//richard


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