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Date: Thu, 10 Jan 2013 15:50:58 +0000
From: Alan Cox <alan@linux.intel.com>
To: Florian Westphal <fw@strlen.de>
Cc: linux-kernel@vger.kernel.org, gaowanlong@cn.fujitsu.com,
        gregkh@linuxfoundation.org, frank@lichtenheld.de
Subject: Re: regression, bisected: openpty fails from 3.7 onwards without
 devpts
Message-ID: <20130110155058.12f66e73@bob.linux.org.uk>
In-Reply-To: <20130110144626.GA26279@breakpoint.cc>
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On Thu, 10 Jan 2013 15:46:26 +0100
Florian Westphal <fw@strlen.de> wrote:

> Frank Lichtenheld discovered that openpty() doesn't work anymore when
> /dev/pts is not present.
> 
> We bisected this down to
> 
> commit bbb63c514a3464342967237a51a21ea8f61ab951
> Author: Wanlong Gao <gaowanlong@cn.fujitsu.com>
> Subject: drivers:tty:fix up ENOIOCTLCMD error handling
> 
> The original program triggering the error was pptpd, but
> the test program below is sufficient:
> ----
> #include <stdio.h>
> #include <pty.h>
> int main(void) {
>  int pty_fd, tty_fd;
>  if (openpty(&pty_fd, &tty_fd, NULL, NULL, NULL) != 0) {
>   perror("openpty");
>   return 1;
>  }
> return 0;
> }
> ----
> [ compile with cc -lutil pty.c -o pty ]
> 
> If devpts is available or above commit reverted openpty works again.

The commit is fairly general - what we need to do here is to figure out
which specific thing trips up openpty so we can put the error on that
back as it was (or find a better way) so it still works.

Can you attach an strace of the working/failing cases without /dev/pts 

Alan