Hi all, I'm trying to understand some of the internals of namespace-based Linux containers and I'm kindly asking you for help. When launching `podman run -it --rm -v ~/Downloads:/dwn docker.io/library/ubuntu /bin/bash`, the inside user is root. That is expected, and without any surprise the /proc/self/uid_map is: 0 1000 1 1 100000 65536 When launching `podman run -it --rm -v ~/Downloads:/dwn --userns keep-id docker.io/library/ubuntu /bin/bash` instead, the /proc/self/uid_map is: 0 1 1000 1000 0 1 1001 1001 64536 If I'm understanding it well, in the latter case there is a double mapping: to keep host UID and GID, podman fires two user namespaces, where the inner namespace maps its IDs the outer namespace, which finally maps to the host (that is, 1000 -> 0 -> 1000 again). The mechanism I don't get is how podman manages to make the rootfs owned by root inside the inner namespace, while assigning volumes to the unprivileged inner user: dr-xr-xr-x. 1 root root 18 May 4 06:33 . dr-xr-xr-x. 1 root root 18 May 4 06:33 .. lrwxrwxrwx. 1 root root 7 Mar 8 02:05 bin -> usr/bin drwxr-xr-x. 1 root root 0 Apr 18 2022 boot [...] drwxr-xr-x. 1 myuser 1000 2.1K May 3 15:07 dwn What is the algorithm here? I have a feeling there is some clever combination of syscalls here I don't get. When I tried to reproduce this double namespace situation, the rootfs of the inner namespace was all owned by 1000, not 0. Thank you so so much for your time if you're willing to help me, Fabio. _______________________________________________ Podman mailing list -- podman(a)lists.podman.io To unsubscribe send an email to podman-leave(a)lists.podman.io

On 5/4/23 04:29, Paul Holzinger wrote: > Hi Fabio, > > My understanding is that the image is copied and chown-ed to the correct uids when running rootless. > There is also the concept of idmapped mounts in the kernel but the kernel only allows this as root at the moment. > > Paul > > On Thu, May 4, 2023 at 8:56 AM Fabio <fabio(a)redaril.me&gt; wrote: > > Hi all, > > I'm trying to understand some of the internals of namespace-based > Linux > containers and I'm kindly asking you for help. > > When launching `podman run -it --rm -v ~/Downloads:/dwn > docker.io/library/ubuntu <http://docker.io/library/ubuntu> > /bin/bash`, the inside user is root. That is > expected, and without any surprise the /proc/self/uid_map is: >           0       1000          1 >           1     100000      65536 > > When launching `podman run -it --rm -v ~/Downloads:/dwn --userns > keep-id > docker.io/library/ubuntu <http://docker.io/library/ubuntu> > /bin/bash` instead, the /proc/self/uid_map is: >           0          1       1000 >        1000          0          1 >        1001       1001      64536 > > If I'm understanding it well, in the latter case there is a double > mapping: to keep host UID and GID, podman fires two user namespaces, > where the inner namespace maps its IDs the outer namespace, which > finally maps to the host (that is, 1000 -> 0 -> 1000 again). > Correct. > > The mechanism I don't get is how podman manages to make the rootfs > owned > by root inside the inner namespace, while assigning volumes to the > unprivileged inner user: > dr-xr-xr-x.   1 root   root      18 May  4 06:33 . > dr-xr-xr-x.   1 root   root      18 May  4 06:33 .. > lrwxrwxrwx.   1 root   root       7 Mar  8 02:05 bin -> usr/bin > drwxr-xr-x.   1 root   root       0 Apr 18  2022 boot > [...] > drwxr-xr-x.   1 myuser 1000    2.1K May  3 15:07 dwn > > What is the algorithm here? I have a feeling there is some clever > combination of syscalls here I don't get. When I tried to > reproduce this > double namespace situation, the rootfs of the inner namespace was all > owned by 1000, not 0. > > Thank you so so much for your time if you're willing to help me, > Fabio. > _______________________________________________ > Podman mailing list -- podman(a)lists.podman.io > To unsubscribe send an email to podman-leave(a)lists.podman.io > > > _______________________________________________ > Podman mailing list --podman(a)lists.podman.io > To unsubscribe send an email topodman-leave(a)lists.podman.io