The /dev/ptmx device node is changed to lookup the directory entry "pts"
in the same directory as the /dev/ptmx device node was opened in. If
there is a "pts" entry and that entry is a devpts filesystem /dev/ptmx
uses that filesystem. Otherwise the open of /dev/ptmx fails.
The DEVPTS_MULTIPLE_INSTANCES configuration option is removed, so that
userspace can now safely depend on each mount of devpts creating a new
instance of the filesystem.
Each mount of devpts is now a separate and equal filesystem.
Reserved ttys are now available to all instances of devpts where the
mounter is in the initial mount namespace.
A new vfs helper path_pts is introduced that finds a directory entry
named "pts" in the directory of the passed in path, and changes the
passed in path to point to it. The helper path_pts uses a function
path_parent_directory that was factored out of follow_dotdot.
In the implementation of devpts:
- devpts_mnt is killed as it is no longer meaningful if all mounts of
devpts are equal.
- pts_sb_from_inode is replaced by just inode->i_sb as all cached
inodes in the tty layer are now from the devpts filesystem.
- devpts_add_ref is rolled into the new function devpts_ptmx. And the
unnecessary inode hold is removed.
- devpts_del_ref is renamed devpts_release and reduced to just a
deacrivate_super.
- The newinstance mount option continues to be accepted but is now
ignored.
In devpts_fs.h definitions for when !CONFIG_UNIX98_PTYS are removed as
they are never used.
Documentation/filesystems/devices.txt is updated to describe the current
situation.
This has been verified to work properly on openwrt-15.05, centos5,
centos6, centos7, debian-6.0.2, debian-7.9, debian-8.2, ubuntu-14.04.3,
ubuntu-15.10, fedora23, magia-5, mint-17.3, opensuse-42.1,
slackware-14.1, gentoo-20151225 (13.0?), archlinux-2015-12-01. With the
caveat that on centos6 and on slackware-14.1 that there wind up being
two instances of the devpts filesystem mounted on /dev/pts, the lower
copy does not end up getting used.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Greg KH <greg@kroah.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Peter Anvin <hpa@zytor.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Jann Horn <jann@thejh.net>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Florian Weimer <fw@deneb.enyo.de>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Logic has been changed in kernel 3.4 by commit e9aba5158a
("tty: rework pty count limiting") but still not documented.
Sysctl kernel.pty.max works as global limit, kernel.pty.reserve ptys
are reserved for initial devpts instance (mounted without "newinstance").
Per-instance limit also could be set by mount option "max=%d".
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Changelog [v2]:
- Add note indicating strict isolation is not possible unless all
mounts of devpts use the 'newinstance' mount option.
Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Eric W. Biederman
Konstantin Khlebnikov
Sukadev Bhattiprolu