Enable multiple instances of devpts filesystem so each container can allocate
ptys independently.
User interface:
Since supporting multiple mounts of devpts can break user-space, this
feature is enabled only if:
- CONFIG_DEVPTS_MULTIPLE_INSTANCES=y (new config token), and
- new mount option, -o newinstance is specified
If CONFIG_DEVPTS_MULTIPLE_INSTANCES=n, there should be no change in
behavior.
See [PATCH 10/10] - Documentation/filesystems/devpts.txt for detailed
usage/compatibility information.
[PATCH 01/10] Remove devpts_root global
[PATCH 02/10] Per-mount allocated_ptys
[PATCH 03/10] Per-mount 'config' object
[PATCH 04/10] Extract option parsing to new function
[PATCH 05/10] Add DEVPTS_MULTIPLE_INSTANCES config token
[PATCH 06/10] Define mknod_ptmx()
[PATCH 07/10] Update ptmx permissions during remount
[PATCH 08/10] Define get_sb_ref()
[PATCH 09/10] Enable multiple instances of devpts
[PATCH 10/10] Document usage of multiple-instances of devpts
Implementation notes:
1. To enable multiple mounts of /dev/pts, (most) devpts interfaces
need to know which instance of devpts is being accessed. This
patchset uses the 'struct inode' or 'struct tty_struct' of the
device being accessed to identify the appropriate devpts instance.
Hence the need for the /dev/pts/ptmx bind-mount.
2. See comments in get_sb_ref() in fs/super.c (could not find
existing interfaces that accomplish it).
3. Mount options must be parsed twice during mount (once to determine
the mode of mount (single/multi-instance) and once to actually
save the options. There does not seem to be an easy way to parse
once and reuse (See 'safe_process_mount_opts()' in [PATCH 9/10])
Changelog [v4]:
- Port to 2008-09-04 ttydev tree (and drop patches that were merged in)
- Add DEVPTS_MULTIPLE_INSTANCES config token (patch 5) and move new
behavior under #ifdef CONFIG_DEVPTS_MULTIPLE_INSTANCES.
- Create ptmx node even in initial mount
- Change default permissions of pts/ptmx node to 0000 (patch 6)
- Cache ptmx dentry and use to update permissions of ptmx node on
remount so legacy mode can use the node with a simpler change to
/etc/fstab (patch 7)
- Move get_sb_ref() helper code to separate patch (patch 8)
- Add Documentation/filesystems/devpts.txt and describe usage info
in that file.
Changelog [v3]:
- Port to 2008-08-28 ttydev tree
- Rename new mount options to 'ptmxmode' and 'newinstance'.
- [Alan Cox] Use tty driver data to identify devpts (this is used to
cleanup get_node() in devpts_pty_kill()).
- [H. Peter Anvin] get_node() cleanup in devpts (which was enabled by
the inode/tty parameter to devpts interfaces)
- Bugfix in multi-mount mode (see Patch 11/11).
- Executed pty tests in LTP (in both single-instance and multi-instance
mode)
- Should be bisect-safe :-)
Changelog [v2]:
- New mount option '-o newmnt' added (patch 8/8)
- Support both single-mount and multi-mount semantics (patch 8/8)
- Automatically create ptmx node when devpts is mounted (patch 7/8)
- Extract option parsing code to new function (patch 6/8)
- Make 'config' params per-mount variables (patch 5/8)
- Slightly re-ordered existing patches in set (patches 1/8, 2/8)
TODO:
- Do we need a '-o ptmxuid' and '-o ptmxgid' options as well ?
- Update mount(8) man page
- (Sometime in future) Remove even initial kernel mount of devpts
- Any other good test suites to test this (besides LTP, sshd).
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