Bug#841935: pbuilder: incorrect permissions on /dev/ptmx breaks openpty()

[Yuriy M. Kaminskiy]

On 06.11.2016 23:41, James Clarke wrote:
>> On 6 Nov 2016, at 20:34, Yuriy M. Kaminskiy <yumkam at gmail.com> wrote:
>>
>> Andreas Henriksson <andreas at fatal.se> writes:
>>
>>> It seems /dev/ptmx has incorrect permissions in a pbuilder chroot:
>>>
>>> # ls -l /dev/ptmx
>>> lrwxrwxrwx 1 root root 8 Oct  4 06:43 /dev/ptmx -> pts/ptmx
>>> # ls -l /dev/pts/ptmx
>>> c--------- 1 root root 5, 2 Oct 24 14:46 /dev/pts/ptmx
>>>
>>> Please compare to what's stated in
>>> https://www.kernel.org/doc/Documentation/filesystems/devpts.txt
>>>
>>> For comparison this is from my regular system:
>>> $ ls -l /dev/ptmx /dev/pts/ptmx
>>> crw-rw-rw- 1 root tty  5, 2 okt 24 17:03 /dev/ptmx
>>> c--------- 1 root root 5, 2 okt 11 12:35 /dev/pts/ptmx
>>>
>>> I've hacked up my pbuilder installation and confirmed that appending
>>> ",ptmxmode=0666" to the /dev/pts mount flags fixes the issue.
>>
>> IIRC, it would also need `,newinstance` option by then (otherwise
>> it will clobber "host system" devpts options).
>
> newinstance seems to be a world of pain, as then it can’t access the TTY
> for std{in,out,err}. I tried many months ago and couldn’t get it to work,
> but would love to be proved wrong.

Then, probably, it should `mount --bind` "host system" /dev/pts and 
/dev/ptmx? (as mounting devpts again with different options has 
undesirable side effects)