config: add idmap and ridmap mount options

Adding new fields for MOUNT_ATTR_IDMAP had the flaw that users
specifying these fields with older runtimes would result in the fields
being ignored and incorrect mounts being configured. In addition, there
is no text in the specification indicating whether MOUNT_ATTR_IDMAP
should be applied with AT_RECURSIVE (which matters for rbind idmapped
mounts).

In retrospect, the addition of the fields should've included new (dummy)
mount options that would cause errors on older runtimes. Unfortunately,
we have had a runtime-spec release since then so we cannot MUST these
new mount options, but we can SHOULD them.

Fixes: 9d1130d ("IDMapping field for mount point")
Signed-off-by: Aleksa Sarai <[email protected]>

config.md

@@ -146,6 +146,8 @@ Runtimes MUST/SHOULD/MAY implement the following option strings for Linux:
  `sync` | MUST | [^1]
  `tmpcopyup` | MAY | copy up the contents to a tmpfs
  `unbindable` | MUST | [^2] (bind mounts)
+ `idmap` | SHOULD | Indicates that the mount has `uidMappings` and `gidMappings` specified. This option SHOULD NOT be passed to the underlying [`mount(2)`] call. If supported, the runtime MUST return an error if this option is provided and either of `uidMappings` or `gidMappings` are empty or not present.
+ `ridmap` | SHOULD | Indicates that the mount has `uidMappings` and `gidMappings` specified, and the mapping is applied recursively [^3]. This option SHOULD NOT be passed to the underlying [`mount(2)`] call. If supported, the runtime MUST return an error if this option is provided and either of `uidMappings` or `gidMappings` are empty or not present.
 
 [^1]: Corresponds to [`mount(8)` (filesystem-independent)][mount.8-filesystem-independent].
 [^2]: Corresponds to [`mount(8)` (filesystem-specific)][mount.8-filesystem-specific].
@@ -177,10 +179,14 @@ For POSIX platforms the `mounts` structure has the following fields:
 * **`type`** (string, OPTIONAL) The type of the filesystem to be mounted.
  * Linux: filesystem types supported by the kernel as listed in */proc/filesystems* (e.g., "minix", "ext2", "ext3", "jfs", "xfs", "reiserfs", "msdos", "proc", "nfs", "iso9660"). For bind mounts (when `options` include either `bind` or `rbind`), the type is a dummy, often "none" (not listed in */proc/filesystems*).
  * Solaris: corresponds to "type" of the fs resource in [zonecfg(1M)][zonecfg.1m].
-* **`uidMappings`** (array of type LinuxIDMapping, OPTIONAL) The mapping to convert UIDs from the source file system to the destination mount point.\
-The format is the same as [user namespace mappings](config-linux.md#user-namespace-mappings).
+* **`uidMappings`** (array of type LinuxIDMapping, OPTIONAL) The mapping to convert UIDs from the source file system to the destination mount point.
+ If specified, the `options` field of the `mounts` structure SHOULD contain either `idmap` or `ridmap` to specify whether the mapping should be applied recursively for `rbind` mounts, as well as to ensure that older runtimes will not silently ignore this field.
+ The format is the same as [user namespace mappings](config-linux.md#user-namespace-mappings).
+ If specified, it MUST be specified along with `gidMappings`.
 * **`gidMappings`** (array of type LinuxIDMapping, OPTIONAL) The mapping to convert GIDs from the source file system to the destination mount point.
-For more details see `uidMappings`.
+ If specified, the `options` field of the `mounts` structure SHOULD contain either `idmap` or `ridmap` to specify whether the mapping should be applied recursively for `rbind` mounts, as well as to ensure that older runtimes will not silently ignore this field.
+ For more details see `uidMappings`.
+ If specified, it MUST be specified along with `uidMappings`.
 
 
 ### Example (Linux)