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specify cgroup ownership semantics #1123

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Nov 23, 2021
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specify cgroup ownership semantics #1123

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45 changes: 45 additions & 0 deletions config-linux.md
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Expand Up @@ -196,6 +196,51 @@ For example, to run a new process in an existing container without updating limi

Runtimes MAY attach the container process to additional cgroup controllers beyond those necessary to fulfill the `resources` settings.

### Cgroup ownership

Runtimes MAY, according to the following rules, change (or cause to
be changed) the owner of the container's cgroup to the host uid that
maps to the value of `process.user.uid` in the [container
namespace](glossary.md#container-namespace); that is, the user that
will execute the container process.

Runtimes SHOULD NOT change the ownership of container cgroups when
cgroups v1 is in use. Cgroup delegation is not secure in cgroups
v1.

A runtime SHOULD NOT change the ownership of a container cgroup
unless it will also create a new cgroup namespace for the container.
Typically this occurs when the `linux.namespaces` array contains an
object with `type` equal to `"cgroup"` and `path` unset.

Runtimes SHOULD change the cgroup ownership if and only if the
cgroup filesystem is to be mounted read/write; that is, when the
configuration's `mounts` array contains an object where:

- The `source` field is equal to `"cgroup"`
- The `destination` field is equal to `"/sys/fs/cgroup"`
- The `options` field does not contain the value `"ro"`

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If the configuration does not specify such a mount, the runtime
SHOULD NOT change the cgroup ownership.

A runtime that changes the cgroup ownership SHOULD only change the
ownership of the container's cgroup directory and files within that
directory that are listed in `/sys/kernel/cgroup/delegate` (see
`cgroups(7)` for details about this file). If the
`/sys/kernel/cgroup/delegate` file does not exist, the runtime MUST
fall back to using the following list of files:

```
cgroup.procs
cgroup.subtree_control
cgroup.threads
```

The runtime SHOULD NOT change the ownership of any other files.
Changing other files may allow the container to elevate its own
resource limits or perform other unwanted behaviour.

### Example

```json
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