Invalid argument when mapping from QEMU 9p virtio share

[ktprograms]

Hi, I found out about ID-mapped mounts from LWN and found this tool.

I was able to make it work mounting a regular directory, but when I tried to ID-map a directory mounted from QEMU 9p virtio share, I get an Invalid argument error. I'm on Alpine aarch64, with kernel 5.15.29. Commands ran and output below:

~/mount-idmapped (master)> doas mount -t 9p -o trans=virtio virt9pshare /mnt/ -oversion=9p2000.L
~/mount-idmapped (master)> doas ./mount-idmapped --map-mount=u:501:1000:1 --map-mount=g:20:1000:1 /mnt /mnt-mapped
Invalid argument - Failed to change mount attributes

[brauner]

Hey @ktprograms creating idmapped mounts requires filesystem support and 9p simply hasn't been converted to support idmapped mounts yet.

[ktprograms]

Ah, I see. From reading the description of it I thought it didn't require filesystem support.

Would there be any way to do ID-mapping for 9pfs?

[brauner]

In principle 9pfs can support this going forward.

[ktprograms]

What changes would need to be made?

Could you please link to the implementation for other filesystems so I can take a look? I wouldn't mind looking at trying to implement it.

[brauner]

Yeah, I can certainly help you there. Is it ok if I wait until after next week. I won't have time before that.

[ktprograms]

On Thu Jun 16, 2022 at 8:47 PM +08, Christian Brauner wrote: Yeah, I can certainly help you there. Is it ok if I wait until after next week. I won't have time before that.

Thanks. Sure, I'm in no hurry to get it done.

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[ktprograms]

Hi @brauner, do you have some links to documentation/other implementations?