Re: [9fans] Re: Mounting a 9P filesystem under a Linux "user namespace"

[Edouard Klein <edou@rdklein.fr>]

Again for the record, if anybody is looking for a 9P2000.L FUSE
implementation, I had to write one, I used github.com/hugelgupf/p9 as a
base:

git clone git@the-dam.org:f29p

With that, one can mount a 9P2000.L server from inside a linux 'mount
namespace'.

I'll talk about that if my paper passes the IWP9 review.

Cheers,

Edouard.


Edouard Klein <edou@rdklein.fr> writes:

> For the record here is the lkml post
> https://lkml.org/lkml/2023/10/28/155
> Edouard Klein <edou@rdklein.fr> writes:
>
>> Thanks Moody for the nudge in a direction I hadn't explored.
>>
>> It seems that Linux does not see 9p as been safe to mount without
>> privilege. From what I understand, only FS with the FS_USERNS_MOUNT flag
>> can be mounted in a user namespace. It seems that v9fs is not one of
>> them:
>>
>> For example, tmpfs is a safe FS, and I can do:
>> unshare --user --map-root-user --mount
>> mount -t tmpfs tmpfs mnt/mnt1/
>>
>> and it works.
>>
>> However, if I do:
>> unshare --user --map-root-user --mount
>> mount -t 9p -o trans=unix /run/9p/srv4 mnt/mnt1
>>
>> I get  mount: /home/edouard/mnt/mnt1: permission denied.
>>
>>
>> I've sent an email to the linux kernel mailing list to see if somebody
>> there has any up to date information.
>>
>> Somebody tried the same thing in 2018:
>> https://lore.kernel.org/all/39b08c53-3449-3164-c1b1-44ac587dd4ea@metux.net/T/
>> Seemingly without succeeding.
>>
>> The end of the above thread is a bit worrying:
>>>  plan9fs would
>>> also be a candidate for that kind of treatment if it had a maintainer.
>>
>> I did not know v9fs was unmaintained, I find that a bit surprising. It
>> does work very reliably.
>>
>> I'll keep this list updated as I make progress.
>>
>> Cheers,
>>
>> Edouard
>>
>> moody@posixcafe.org writes:
>>
>>> Edouard,
>>>
>>> I am no Linux expert, but I think if you create a mount namespace as part of
>>> the user namespace you will be allowed to execute mounts without root. In
>>> terms of clients, I am not aware
>>> of any other then the one within the linux kernel.
>>>
>>> Regards,
>>> Moody
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