Re: [PATCH v3 00/25] user_namespace: introduce fsid mappings

On Wed, Feb 19, 2020 at 03:48:37PM -0600, Serge E. Hallyn wrote:

On Wed, Feb 19, 2020 at 01:35:58PM -0600, Serge E. Hallyn wrote:

quoted

On Tue, Feb 18, 2020 at 03:33:46PM +0100, Christian Brauner wrote:

quoted

With fsid mappings we can solve this by writing an id mapping of 0
100000 100000 and an fsid mapping of 0 300000 100000. On filesystem
access the kernel will now lookup the mapping for 300000 in the fsid
mapping tables of the user namespace. And since such a mapping exists,
the corresponding files will have correct ownership.

So if I have

/proc/self/uid_map: 0 100000 100000
/proc/self/fsid_map: 1000 1000 1

Oh, sorry.  Your explanation in 20/25 i think set me straight, though I need
to think through a few more examples.

...

quoted

3. If I create a new file, as nsuid 1000, what will be the inode owning kuid?

(Note - I edited the quoted txt above to be more precise)

I'm still not quite clear on this.  I believe the fsid mapping will take
precedence so it'll be uid 1000 ?  Per mount behavior would be nice there,
but perhaps unwieldy.

The is_userns_visible() bits seems to be an attempt at understanding
what people would want per-mount, with a policy hard coded in the
kernel.

But maybe per-mount behavior can be solved more elegantly with shifted
bind mounts, so we can drop all that from this series, and ignore
per-mount settings here?

Tycho