Re: [PATCH 5/8] host/rootfs: create a per-VM user namespace

Alyssa Ross
2025-12-12T17:56:21Z

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Demi Marie Obenour <demiobenour@gmail.com> writes:

> On 12/11/25 07:41, Alyssa Ross wrote:
>> Demi Marie Obenour <demiobenour@gmail.com> writes:
>>
>>> On 12/10/25 07:47, Alyssa Ross wrote:
>>>> The document portal has to be root to mount its fuse filesystem. This
>>>> needs to be a shared namespace because virtiofsd needs to be in the
>>>> same mount namespace as the document portal so that it sees the fuse
>>>> filesystem, so we create a per-VM persistent user namespace.
>>>>
>>>> Signed-off-by: Alyssa Ross <hi@alyssa.is>
>>>
>>> (snip)
>>>
>>>> diff --git a/host/rootfs/image/usr/bin/run-appimage b/host/rootfs/image/usr/bin/run-appimage
>>>> index 47cab4c5..5e8e29fa 100755
>>>> --- a/host/rootfs/image/usr/bin/run-appimage
>>>> +++ b/host/rootfs/image/usr/bin/run-appimage
>>>> @@ -44,4 +44,6 @@ if { s6-instance-delete /run/service/vm-services $id }
>>>>
>>>> if { umount ${dir}/mount } # mount namespace
>>>> if { umount ${dir}/mount } # private bind mount
>>>> +if { umount ${dir}/user } # user namespace
>>>> +if { umount ${dir}/user } # private bind mount
>>>> rm -r $dir /run/configs/${id}
>>>> diff --git a/host/rootfs/image/usr/bin/run-flatpak b/host/rootfs/image/usr/bin/run-flatpak
>>>> index bb366735..86ccc12a 100755
>>>> --- a/host/rootfs/image/usr/bin/run-flatpak
>>>> +++ b/host/rootfs/image/usr/bin/run-flatpak
>>>> @@ -46,4 +46,6 @@ if { s6-instance-delete -- /run/service/vm-services $id }
>>>>
>>>> if { umount ${dir}/mount } # mount namespace
>>>> if { umount ${dir}/mount } # private bind mount
>>>> +if { umount ${dir}/user } # user namespace
>>>> +if { umount ${dir}/user } # private bind mount
>>>> rm -r $dir /run/configs/${id}
>>>
>>> Why is it necessary to unmount twice here?
>>
>> Because we mount twice, just like the comments say. It is a bit
>> counterintuitive though. Namespaces can only be mounted onto a
>> mountpoint that itself has private propagation. This is why we create a
>> private bind mount at the start of create-vm-dependencies before unshare
>> bind mounts the namespaces. You can also see this in an example in
>> unshare(1).
>>
>> Perhaps it would be better and clearer to have /run/vm/by-id/${VM}/ns as
>> a private mountpoint, with the nsfs files inside? That way, each of
>> them only needs to be unmounted once, and then the ns directory also has
>> to be unmounted once.
>
> I think it would be easier to understand.

Then you will like v2!

https://spectrum-os.org/lists/archives/spectrum-devel/20251211162145.124509-10-hi@alyssa.is/

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