[Bug 1732550] Re: linux ttyname{_r}: Don't bail prematurely [BZ #22145]

[Christian Brauner]

** Patch added: "0001-manual-Update-to-mention-ENODEV-for-ttyname-and-ttyn.patch"
   https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1732550/+attachment/5009700/+files/0001-manual-Update-to-mention-ENODEV-for-ttyname-and-ttyn.patch

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Title:
  linux ttyname{_r}: Don't bail prematurely [BZ #22145]

Status in glibc package in Ubuntu:
  New

Bug description:
  Hey guys,

  I just pushed a patch series by Luke to glibc master that fixes a bug
  with pty file descriptors from devpts mounts in a different mount
  namespace. Here's the description:

  linux ttyname{_r}: Don't bail prematurely [BZ #22145]

  Commit 15e9a4f378c8607c2ae1aa465436af4321db0e23 introduced logic for ttyname()
  sending back ENODEV to signal that we can't get a name for the TTY because we
  inherited it from a different mount namespace.

  However, just because we inherited it from a different mount namespace and it
  isn't available at its original path, doesn't mean that its name is unknowable;
  we can still try to find it by allowing the normal fall back on iterating
  through devices.

  An example scenario where this happens is with "/dev/console" in containers.
  It's a common practice among container managers to allocate a PTY master/slave
  pair in the host's mount namespace (the slave having a path like "/dev/pty/$X"),
  bind mount the slave to "/dev/console" in the container's mount namespace, and
  send the slave FD to a process in the container. Inside of the
  container, the slave-end isn't available at its original path ("/dev/pts/$X"),
  since the container mount namespace has a separate devpts instance from the host
  (that path may or may not exist in the container; if it does exist, it's not the
   same PTY slave device). Currently ttyname{_r} sees that the file at the
  original "/dev/pts/$X" path doesn't match the FD passed to it, and fails early
  and gives up, even though if it kept searching it would find the TTY at
  "/dev/console". Fix that; don't have the ENODEV path force an early return
  inhibiting the fall-back search.

  This change is based on the previous patch that adds use of is_mytty in
  getttyname and getttyname_r. Without that change, this effectively reverts
  15e9a4f, which made us disregard the false similarity of file pointed to by
  "/proc/self/fd/$Y", because if it doesn't bail prematurely then that file
  ("/dev/pts/$X") will just come up again anyway in the fall-back search.

  Reviewed-by: Christian Brauner <christian.brauner at ubuntu.com>

  The relevant patches from glibc master are:

  495a56fdeb05d20a88304ff5da577d23a8e81ae1
  9b5a87502d048905c383b65c51768f4a1db8c685
  d10d6cab168ffa26ef6a506655ee5dc8537c8ed7
  2fbce9c2031e70b6bd67876accfc34b0ec492878
  a09dfc19edcbac3f96d5410529b724db0a583879
  d9611e308592355718b36fe085b7b61aa52911e5

  The patches will be in glibc 2.27 but this should be backported to
  glibc 2.26.

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