Re: [PATCH] devpts: Make each mount of devpts an independent filesystem.

[Konstantin Khlebnikov]

On Wed, Apr 20, 2016 at 6:04 AM, Eric W. Biederman
<ebiederm@xxxxxxxxxxxx> wrote:
>
> 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.
>
> The kernel.pty.reserve sysctl is neutered with no way currently
> implemented to be able to use the reserved ptys.

I think we could convert this into reserve for init user namespace,
ssh in host will work even if containers eaten all ptys.

>
> 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@xxxxxxxxxxxx>
> ---
>  Documentation/filesystems/devpts.txt | 145 +++----------------------
>  drivers/tty/Kconfig                  |  11 --
>  drivers/tty/pty.c                    |  41 ++++---
>  fs/devpts/inode.c                    | 205 +++++++++--------------------------
>  fs/namei.c                           |  58 ++++++++--
>  include/linux/devpts_fs.h            |  31 ++----
>  include/linux/namei.h                |   2 +
>  7 files changed, 148 insertions(+), 345 deletions(-)