devpts: Make each mount of devpts an independent filesystem.

2016-06-05T17:36:01Z

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" Cc: Greg KH Cc: Peter Hurley Cc: Peter Anvin Cc: Andy Lutomirski Cc: Al Viro Cc: Serge Hallyn Cc: Willy Tarreau Cc: Aurelien Jarno Cc: One Thousand Gnomes Cc: Jann Horn Cc: Jiri Slaby Cc: Florian Weimer Cc: Konstantin Khlebnikov Signed-off-by: Linus Torvalds

TTY, devpts: document pty count limiting

2016-03-08T00:11:14Z

Logic has been changed in kernel 3.4 by commit e9aba5158a80 ("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 Signed-off-by: Greg Kroah-Hartman

Document usage of multiple-instances of devpts

2009-01-02T18:19:36Z

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 Signed-off-by: Alan Cox Signed-off-by: Linus Torvalds

kernel/Documentation/filesystems/devpts.txt, branch linux-5.1.y

devpts: Make each mount of devpts an independent filesystem.

TTY, devpts: document pty count limiting

Document usage of multiple-instances of devpts