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

[Thompson, David]

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.
>
> [...]
>
>>                                              (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