uid/gid mapping flags

Motivation
===========

This feature aims to make --uidmap and --gidmap easier to use, especially in rootless podman setups.

(I will focus here on the --gidmap option, although the same applies for --uidmap.)

In rootless podman, the user namespace mapping happens in two steps, through an intermediate mapping.

See https://docs.podman.io/en/latest/markdown/podman-run.1.html#uidmap-container-uid-from-uid-amount
for further detail, here is a summary:

First the user GID is mapped to 0 (root), and all subordinate GIDs (defined at /etc/subgid, and
usually >100000) are mapped starting at 1.

One way to customize the mapping is through the `--gidmap` option, that maps that intermediate mapping
to the final mapping that will be seen by the container.

As an example, let's say we have as main GID the group 1000, and we also belong to the additional GID 2000,
that we want to make accessible inside the container.

We first ask the sysadmin to subordinate the group to us, by adding "$user:2000:1" to /etc/subgid.

Then we need to use --gidmap to specify that we want to map GID 2000 into some GID inside the container.

And here is the first trouble:

Since the --gidmap option operates on the intermediate mapping, we first need to figure out where has
podman placed our GID 2000 in that intermediate mapping using:

    podman unshare cat /proc/self/gid_map

Then, we may see that GID 2000 was mapped to intermediate GID 5. So our --gidmap option should include:

    --gidmap 20000:5:1

This intermediate mapping may change in the future if further groups are subordinated to us (or we stop
having its subordination), so we are forced to verify the mapping with
`podman unshare cat /proc/self/gid_map` every time, and parse it if we want to script it.

**The first usability improvement** we agreed on #18333 is to be able to use:

    --gidmap 20000:@2000:1

so podman does this lookup in the parent user namespace for us.

But this is only part of the problem. We must specify a **full** gidmap and not only what we want:

    --gidmap 0:0:5 --gidmap 5:6:15000 --gidmap 20000:5:1

This is becoming complicated. We had to break the gidmap at 5, because the intermediate 5 had to
be mapped to another value (20000), and then we had to keep mapping all other subordinate ids... up to
close to the maximum number of subordinate ids that we have (or some reasonable value). This is hard
to explain to someone who does not understand how the mappings work internally.

To simplify this, **the second usability improvement** is to be able to use:

   --gidmap "+20000:@2000:1"

where the plus flag (`+`) states that the given mapping should extend any previous/default mapping,
overriding any previous conflicting assignment.

Podman will set that mapping and fill the rest of mapped gids with all other subordinated gids, leading
to the same (or an equivalent) full gidmap that we were specifying before.

One final usability improvement related to this is the following:

By default, when podman  gets a --gidmap argument but not a --uidmap argument, it copies the mapping.
This is convenient in many scenarios, since usually subordinated uids and gids are assigned in chunks
simultaneously, and the subordinated IDs in /etc/subuid and /etc/subgid for a given user match.

For scenarios with additional subordinated GIDs, this map copying is annoying, since it forces the user
to provide a --uidmap, to prevent the copy from being made. This means, that when the user wants:

    --gidmap 0:0:5 --gidmap 5:6:15000 --gidmap 20000:5:1

The user has to include a uidmap as well:

    --gidmap 0:0:5 --gidmap 5:6:15000 --gidmap 20000:5:1 --uidmap 0:0:65000

making everything even harder to understand without proper context.

For this reason, besides the "+" flag, we introduce the "u" and "g" flags. Those flags applied to a
mapping tell podman that the mapping should only apply to users or groups, and ignored otherwise.

Therefore we can use:

   --gidmap "+g20000:@2000:1"

So the mapping only applies to groups and is ignored for uidmaps. If no "u" nor "g" flag is assigned
podman assumes the mapping applies to both users and groups as before, so we preserve backwards compatibility.

Co-authored-by: Tom Sweeney <tsweeney@redhat.com>
Signed-off-by: Sergio Oller <sergioller@gmail.com>

docs/tutorials/rootless_tutorial.md

@@ -87,6 +87,34 @@ grep johndoe /etc/subuid /etc/subgid
 /etc/subgid:johndoe:100000:65536
 ```
 
+#### Giving access to additional groups
+
+Users can fully map additional groups to a container namespace if
+those groups subordinated to the user:
+
+```
+usermod --add-subgids 2000-2000 johndoe
+grep johndoe /etc/subgid
+```
+
+This means the user `johndoe` can "impersonate" the group `2000` inside the
+container. Note that it is usually not a good idea to subordinate active
+user ids to other users, because it would allow user impersonation.
+
+`johndoe` can use `--group-add keep-groups` to preserve the additional
+groups, and `--gidmap="+g102000:@2000"` to map the group `2000` in the host
+to the group `102000` in the container:
+
+```
+podman run \
+  --rm \
+  --group-add keep-groups \
+  --gidmap="+g102000:@2000" \
+  --volume "$PWD:/data:ro" \
+  --workdir /data \
+  alpine ls -lisa
+```
+
 ### Enable unprivileged `ping`
 
 Users running in a non-privileged container may not be able to use the `ping` utility from that container.