The first step in implementing the new file-descriptor based mount API in
gVisor. A "filesystem creation context" is created using fsopen(). The context
is then configured using fsconfig(), and the FSCONFIG_CMD_CREATE command is
used to create a filesystem. A "mount object" can be obtained using fsmount(),
which can be operated on directly using the *at() functions or mounted onto the
real directory tree using move_mount().
For compatibility reasons, the new mount API is gated behind the runtime flag
`--new-mount-api`, which is disabled by default.
The current implementation has the following limitations:
- Linux passes configuration options to filesystems on-the-fly as fsopen() and
fsconfig() are called, allowing per-parameter error handling. gVisor
currently queues up all options and passes them to the filesystem all-at-once
when FSCONFIG_CMD_CREATE occurs. Lifting this will require refactoring the
option parsing for all existing filesystems.
- Only flag and string arguments to fsconfig() are supported.
- FSCONFIG_CMD_RECONFIGURE is not supported.
- move_mount(): flags MOVE_MOUNT_SET_GROUP and MOVE_MOUNT_BENEATH are not
supported.
- The filesystem context message retrieval interface is not supported.
In the interest of review size, there is no implementation yet for
mount_setattr(2), open_tree(2), or fspick(2).
PiperOrigin-RevId: 923640824
