linux-kernel - Re: [PATCH] kdesu broken

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Date:	Tue, 28 Jul 2009 08:48:02 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Ray Lee <ray-lk@...rabbit.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] kdesu broken

On Tue, 28 Jul 2009, OGAWA Hirofumi wrote:
> 
> Just a quick hack though. Is this wrong/unpreferable way?

That seems to be right regardless of any other issues.

> n_tty_read() checks the pending buffer and consume it before
> input_available_p().

Why not move this _inside_ "input_available_p()"? There are only two 
call-sites, and strictly speaking they both want it.

Look at n_tty_poll(), for example:

        if (input_available_p(tty, TIME_CHAR(tty) ? 0 : MIN_CHAR(tty)))
                mask |= POLLIN | POLLRDNORM;
        if (tty->packet && tty->link->ctrl_status)
                mask |= POLLPRI | POLLIN | POLLRDNORM;
        if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
                mask |= POLLHUP;
        if (tty_hung_up_p(file))
                mask |= POLLHUP;

and notice what happens to somebody who uses select/poll when there might 
be pending data that hasn't been handled yet, and the tty has been marked 
TTY_OTHER_CLOSED or hung up. It would return only POLLHUP, and as a 
result, that side would never even try to read the pending data because 
poll implies that there is no data and it's EOF. Which is just wrong.

So _any_ time you check "is there input available?" you should always 
check if there are other buffers. No?

		Linus
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