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

[Edouard Klein <edou@rdklein.fr>]

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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