Re: [PATCH] environment: container: Do not remount network files as read-only.

[Drew C]

On Sat, Mar 26, 2016 at 7:06 AM, Thompson, David <address@hidden> wrote:

On Fri, Mar 18, 2016 at 4:51 PM, Ludovic Courtès <address@hidden> wrote:
> "Thompson, David" <address@hidden> skribis:
>
>> I noticed that 'guix environment --container --network' didn't work on
>> an Ubuntu machine I was on, and the culprit was remounting things like
>> /etc/resolv.conf read-only after the initial bind mount.

One thing that I have run across is the following :

$ cat /etc/resolv.conf 

# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)

#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN

nameserver 127.0.1.1

It re-writes that file every time a new network connection is made.

$ man resolvconf

[...]

DESCRIPTION

       The resolvconf package comprises a simple database for  run-time  name‐

       server information and a simple framework for notifying applications of

       changes in that information.  Resolvconf thus sets  itself  up  as  the

       intermediary  between  programs  that supply nameserver information and

       applications that use that information.

[...]

I am not at all sure if this is the cause of the issue, but I have run into it many times before with WiFi and me trying to edit it to 8.8.8.8, so I figure this is a decent time to bring it up.

Cheers, 

Drew Crampsie

 

>
> [...]
>
>>                                              (file-system-mapping
>>                                               (source file)
>>                                               (target file)
>> -                                             (writable? #f))))
>> +                                             ;; An unpriviliged user might not
>> +                                             ;; be able to remount
>> +                                             ;; /etc/resolv.conf as read-only,
>> +                                             ;; so we say that it is writable
>> +                                             ;; here, even though in practice
>> +                                             ;; it is not.
>> +                                             (writable? #t))))
>>                                       %network-configuration-files)
>
> Not sure I understand: why would bind-mounting /etc/resolv.conf
> read-only fail?

I haven't figured out the exact reason yet, but here's a strace
snippet as proof:

[pid 11334] mount("/etc/resolv.conf",
"/tmp/guix-directory.Rc4nc6//etc/resolv.conf", 0x23da000,
MS_RDONLY|MS_BIND, NULL) = 0
[pid 11334] mount("/etc/resolv.conf",
"/tmp/guix-directory.Rc4nc6//etc/resolv.conf", 0x23e4080,
MS_RDONLY|MS_REMOUNT|MS_BIND, NULL) = -1 EPERM (Operation not
permitted)

Another Ubuntu user was able to reproduce this as well.  I find it
kind of silly to mount these files read-only because an unprivileged
user couldn't write to them anyway.  WDYT?

- Dave