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18d35b6122
Since we do not want the mapping to be applied to uids, we should use the `g` flag in the mapping in the example as well. Follow up of #18173 Signed-off-by: Sergio Oller <sergioller@gmail.com>
197 lines
8.5 KiB
Markdown
197 lines
8.5 KiB
Markdown
####> This option file is used in:
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####> podman create, run
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####> If file is edited, make sure the changes
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####> are applicable to all of those.
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#### **--uidmap**=*[flags]container_uid:from_uid[:amount]*
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Run the container in a new user namespace using the supplied UID mapping. This
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option conflicts with the **--userns** and **--subuidname** options. This
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option provides a way to map host UIDs to container UIDs. It can be passed
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several times to map different ranges.
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The possible values of the optional *flags* are discussed further down on this page.
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The *amount* value is optional and assumed to be **1** if not given.
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The *from_uid* value is based upon the user running the command, either rootful or rootless users.
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* rootful user: [*flags*]*container_uid*:*host_uid*[:*amount*]
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* rootless user: [*flags*]*container_uid*:*intermediate_uid*[:*amount*]
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`Rootful mappings`
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When **podman <<subcommand>>** is called by a privileged user, the option **--uidmap**
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works as a direct mapping between host UIDs and container UIDs.
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host UID -> container UID
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The _amount_ specifies the number of consecutive UIDs that is mapped.
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If for example _amount_ is **4** the mapping looks like:
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| host UID | container UID |
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| ---------- | ---------------- |
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| *from_uid* | *container_uid* |
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| *from_uid* + 1 | *container_uid* + 1 |
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| *from_uid* + 2 | *container_uid* + 2 |
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| *from_uid* + 3 | *container_uid* + 3 |
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`Rootless mappings`
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When **podman <<subcommand>>** is called by an unprivileged user (i.e. running rootless),
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the value *from_uid* is interpreted as an "intermediate UID". In the rootless
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case, host UIDs are not mapped directly to container UIDs. Instead the mapping
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happens over two mapping steps:
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host UID -> intermediate UID -> container UID
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The **--uidmap** option only influences the second mapping step.
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The first mapping step is derived by Podman from the contents of the file
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_/etc/subuid_ and the UID of the user calling Podman.
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First mapping step:
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| host UID | intermediate UID |
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| -------- | ---------------- |
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| UID for Podman user | 0 |
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| 1st subordinate UID | 1 |
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| 2nd subordinate UID | 2 |
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| 3rd subordinate UID | 3 |
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| nth subordinate UID | n |
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To be able to use intermediate UIDs greater than zero, the user needs to have
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subordinate UIDs configured in _/etc/subuid_. See **subuid**(5).
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The second mapping step is configured with **--uidmap**.
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If for example _amount_ is **5** the second mapping step looks like:
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| intermediate UID | container UID |
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| ------------------ | ---------------- |
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| *from_uid* | *container_uid* |
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| *from_uid* + 1 | *container_uid* + 1 |
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| *from_uid* + 2 | *container_uid* + 2 |
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| *from_uid* + 3 | *container_uid* + 3 |
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| *from_uid* + 4 | *container_uid* + 4 |
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When running as rootless, Podman uses all the ranges configured in the _/etc/subuid_ file.
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The current user ID is mapped to UID=0 in the rootless user namespace.
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Every additional range is added sequentially afterward:
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| host | rootless user namespace | length |
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| ------ | ----------------------- | ------ |
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| $UID | 0 | 1 |
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| 1 | $FIRST_RANGE_ID | $FIRST_RANGE_LENGTH |
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| 1+$FIRST_RANGE_LENGTH | $SECOND_RANGE_ID | $SECOND_RANGE_LENGTH|
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`Referencing a host ID from the parent namespace`
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As a rootless user, the given host ID in **--uidmap** or **--gidmap**
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is mapped from the *intermediate namespace* generated by Podman. Sometimes
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it is desirable to refer directly at the *host namespace*. It is possible
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to manually do so, by running `podman unshare cat /proc/self/gid_map`,
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finding the desired host id at the second column of the output, and getting
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the corresponding intermediate id from the first column.
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Podman can perform all that by preceding the host id in the mapping
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with the `@` symbol. For instance, by specifying `--gidmap 100000:@2000:1`,
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podman will look up the intermediate id corresponding to host id `2000` and
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it will map the found intermediate id to the container id `100000`. The
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given host id must have been subordinated (otherwise it would not be mapped
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into the intermediate space in the first place).
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If the length is greater than one, for instance with `--gidmap 100000:@2000:2`,
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Podman will map host ids `2000` and `2001` to `100000` and `100001`, respectively,
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regardless of how the intermediate mapping is defined.
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`Extending previous mappings`
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Some mapping modifications may be cumbersome. For instance, a user
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starts with a mapping such as `--gidmap="0:0:65000"`, that needs to be
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changed such as the parent id `1000` is mapped to container id `100000`
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instead, leaving container id `1` unassigned. The corresponding `--gidmap`
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becomes `--gidmap="0:0:1" --gidmap="2:2:65534" --gidmap="100000:1:1"`.
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This notation can be simplified using the `+` flag, that takes care of
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breaking previous mappings removing any conflicting assignment with
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the given mapping. The flag is given before the container id
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as follows: `--gidmap="0:0:65000" --gidmap="+100000:1:1"`
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Flag | Example | Description
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-----------|---------------|-------------
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`+` | `+100000:1:1` | Extend the previous mapping
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This notation leads to gaps in the assignment, so it may be convenient to
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fill those gaps afterwards: `--gidmap="0:0:65000" --gidmap="+100000:1:1" --gidmap="1:65001:1"`
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One specific use case for this flag is in the context of rootless
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users. A rootless user may specify mappings with the `+` flag as
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in `--gidmap="+100000:1:1"`. Podman will then "fill the gaps" starting
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from zero with all the remaining intermediate ids. This is convenient when
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a user wants to map a specific intermediate id to a container id, leaving
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the rest of subordinate ids to be mapped by Podman at will.
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`Passing only one of --uidmap or --gidmap`
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Usually, subordinated user and group ids are assigned simultaneously, and
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for any user the subordinated user ids match the subordinated group ids.
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For convenience, if only one of **--uidmap** or **--gidmap** is given,
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podman assumes the mapping refers to both UIDs and GIDs and applies the
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given mapping to both. If only one value of the two needs to be changed,
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the mappings should include the `u` or the `g` flags to specify that
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they only apply to UIDs or GIDs and should not be copied over.
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flag | Example | Description
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---------|-----------------|-----------------
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`u` | `u20000:2000:1` |The mapping only applies to UIDs
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`g` | `g10000:1000:1` |The mapping only applies to GIDs
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For instance given the command
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podman <<subcommand>> --gidmap "0:0:1000" --gidmap "g2000:2000:1"
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Since no **--uidmap** is given, the **--gidmap** is copied to **--uidmap**,
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giving a command equivalent to
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podman <<subcommand>> --gidmap "0:0:1000" --gidmap "2000:2000:1" --uidmap "0:0:1000"
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The `--gidmap "g2000:2000:1"` used the `g` flag and therefore it was
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not copied to **--uidmap**.
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`Rootless mapping of additional host GIDs`
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A rootless user may desire to map a specific host group that has already been
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subordinated within _/etc/subgid_ without specifying the rest of the mapping.
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This can be done with **--gidmap "+g*container_gid*:@*host_gid*"**
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Where:
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- The host GID is given through the `@` symbol
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- The mapping of this GID is not copied over to **--usermap** thanks to the `g` flag.
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- The rest of the container IDs will be mapped starting from 0 to n,
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with all the remaining subordinated GIDs, thanks to the `+` flag.
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For instance, if a user belongs to the group `2000` and that group is
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subordinated to that user (with `usermod --add-subgids 2000-2000 $USER`),
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the user can map the group into the container with: **--gidmap=+g100000:@2000**.
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If this mapping is combined with the option, **--group-add=keep-groups**, the
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process in the container will belong to group `100000`, and files belonging
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to group `2000` in the host will appear as being owned by group `100000`
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inside the container.
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podman run --group-add=keep-groups --gidmap="+g100000:@2000" ...
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`No subordinate UIDs`
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Even if a user does not have any subordinate UIDs in _/etc/subuid_,
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**--uidmap** can be used to map the normal UID of the user to a
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container UID by running `podman <<subcommand>> --uidmap $container_uid:0:1 --user $container_uid ...`.
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`Pods`
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The **--uidmap** option cannot be called in conjunction with the **--pod** option as a uidmap cannot be set on the container level when in a pod.
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