config: improve bind mount and propagation doc

Current documentation talks about bind/rbind and various propagation
flags (shared, slave, private, unbindable etc.) as being filesystem-
specific options, and refers to mount(8) section describing
filesystem-specific options.

That is wrong, because bind mounts are a separate class of mounts,
not related to file systems.

Fix the confusion, and link to sharedsubtree.txt kernel document
as it happens to be the best source of information about the topic
(not counting the kernel sources).

Signed-off-by: Kir Kolyshkin <[email protected]>

config.md

@@ -94,7 +94,7 @@ Runtimes MUST/SHOULD/MAY implement the following option strings for Linux:
 ------------------|-------------|-----------------------------------------------------
  `async` | MUST | [^1]
  `atime` | MUST | [^1]
- `bind` | MUST | [^2] (bind mounts)
+ `bind` | MUST | Bind mount [^2]
  `defaults` | MUST | [^1]
  `dev` | MUST | [^1]
  `diratime` | MUST | [^1]
@@ -115,9 +115,9 @@ Runtimes MUST/SHOULD/MAY implement the following option strings for Linux:
  `nostrictatime` | MUST | [^1]
  `nosuid` | MUST | [^1]
  `nosymfollow` | SHOULD | [^1] (Introduced in kernel 5.10, util-linux 2.38)
- `private` | MUST | [^2] (bind mounts)
+ `private` | MUST | Bind mount propagation [^2]
  `ratime` | SHOULD | Recursive `atime` [^3]
- `rbind` | MUST | [^2] (bind mounts)
+ `rbind` | MUST | Recursive bind mount [^2]
  `rdev` | SHOULD | Recursive `dev` [^3]
  `rdiratime` | SHOULD | Recursive `diratime` [^3]
  `relatime` | MUST | [^1]
@@ -131,39 +131,40 @@ Runtimes MUST/SHOULD/MAY implement the following option strings for Linux:
  `rnosuid` | SHOULD | Recursive `nosuid` [^3]
  `rnosymfollow` | SHOULD | Recursive `nosymfollow` [^3]
  `ro` | MUST | [^1]
- `rprivate` | MUST | [^2] (bind mounts)
+ `rprivate` | MUST | Bind mount propagation [^2]
  `rrelatime ` | SHOULD | Recursive `relatime` [^3]
  `rro` | SHOULD | Recursive `ro` [^3]
  `rrw` | SHOULD | Recursive `rw` [^3]
- `rshared` | MUST | [^2] (bind mounts)
- `rslave` | MUST | [^2] (bind mounts)
+ `rshared` | MUST | Bind mount propagation [^2]
+ `rslave` | MUST | Bind mount propagation [^2]
  `rstrictatime` | SHOULD | Recursive `strictatime` [^3]
  `rsuid` | SHOULD | Recursive `suid` [^3]
  `rsymfollow` | SHOULD | Recursive `symfollow` [^3]
- `runbindable` | MUST | [^2] (bind mounts)
+ `runbindable` | MUST | Bind mount propagation [^2]
  `rw` | MUST | [^1]
  `shared` | MUST | [^1]
  `silent` | MUST | [^1]
- `slave` | MUST | [^2] (bind mounts)
+ `slave` | MUST | Bind mount propagation [^2]
  `strictatime` | MUST | [^1]
  `suid` | MUST | [^1]
  `symfollow` | SHOULD | Opposite of `nosymfollow`
  `sync` | MUST | [^1]
  `tmpcopyup` | MAY | copy up the contents to a tmpfs
- `unbindable` | MUST | [^2] (bind mounts)
+ `unbindable` | MUST | Bind mount propagation [^2]
  `idmap` | SHOULD | Indicates that the mount has `uidMappings` and `gidMappings` specified. This option SHOULD NOT be passed to the underlying [`mount(2)`][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)`][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].
+[^2]: Corresponds to [bind mounts and shared subtrees][mount-bind].
 [^3]: These `AT_RECURSIVE` options need kernel 5.12 or later. See [`mount_setattr(2)`][mount_setattr.2]
 
 The "MUST" options correspond to [`mount(8)`][mount.8].
 
 Runtimes MAY also implement custom option strings that are not listed in the table above.
 If a custom option string is already recognized by [`mount(8)`][mount.8], the runtime SHOULD follow the behavior of [`mount(8)`][mount.8].
 
-Runtimes SHOULD pass unknown options to [`mount(2)`][mount.2] via the fifth argument (`const void *data`).
+Runtimes SHOULD treat unknown options as [filesystem-specific ones][mount.8-filesystem-specific])
+and pass those as a comma-separated string to the fifth (`const void *data`) argument of [`mount(2)`][mount.2].
 
 ### Example (Windows)
 
@@ -1135,6 +1136,7 @@ Here is a full example `config.json` for reference.
 [mount.8-filesystem-independent]: http://man7.org/linux/man-pages/man8/mount.8.html#FILESYSTEM-INDEPENDENT_MOUNT_OPTIONS
 [mount.8-filesystem-specific]: http://man7.org/linux/man-pages/man8/mount.8.html#FILESYSTEM-SPECIFIC_MOUNT_OPTIONS
 [mount_setattr.2]: http://man7.org/linux/man-pages/man2/mount_setattr.2.html
+[mount-bind]: https://docs.kernel.org/filesystems/sharedsubtree.html
 [getrlimit.2]: http://man7.org/linux/man-pages/man2/getrlimit.2.html
 [getrlimit.3]: http://pubs.opengroup.org/onlinepubs/9699919799/functions/getrlimit.html
 [stdin.3]: http://man7.org/linux/man-pages/man3/stdin.3.html