[Bug 1669578] Re: Get ttyname() to work properly in containers

[Christian Brauner]

So, Stéphane brought to my attention that we're not including the patch
because it switches screen to use sockets instead of fifos and thus
makes it impossible to switch to existing fifo-based screen sessions.
I've written a patch that adds a compatibility layer to support both
fifo-based and socket-based screen sessions. The idea is to only re-use
existing fifo-sessions but never create new ones. All new screen
sessions will be socket-based.

** Patch added: "0001-add-compat-layer-to-handle-both-fifos-and-sockets.patch"
   https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1669578/+attachment/4857205/+files/0001-add-compat-layer-to-handle-both-fifos-and-sockets.patch

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Title:
  Get ttyname() to work properly in containers

Status in glibc package in Ubuntu:
  Fix Committed
Status in screen package in Ubuntu:
  Fix Committed
Status in tmux package in Ubuntu:
  Fix Released

Bug description:
  For the past year or so, the LXD team has been trying to resolve an
  issue affecting screen, tmux and a bunch of other software (including
  the "tty" command).

  The problem comes from the fact that when attaching to a container,
  your terminal's pts device comes from the host and therefore can't be
  found in /dev/pts/.

  glibc makes the assumption that it can readlink /proc/self/fd/0 and
  that the target path will exist. This simply isn't true as the symlink
  target returned by the kernel, is confusingly relative to the host's
  root and not the container's.

  Which means that if the target happens to exist, it's actually going
  to be an entirely different pts device from the one that you're
  actually attached to.

  
  You therefore need to do something along the lines of:
   - Resolve the symlink. If the target doesn't exist, return the symlink as the ttyname.
   - If the target does exist, check that its major and minor matches that of the symlink itself, if it doesn't, then return the symlink rather than the target.

  
  That's the ideal approach which makes existing software keep working properly without the need for any added code. After about a year of bikeshedding, the proposed glibc upstream fix has now evolved to instead returning ENODEV in the is_pty function. This allows downstream glibc users to detect this case and then use /proc/self/fd/0 rather than the return value of ttyname() but means every software using ttyname() now needs fixing.

  As we very much care about Ubuntu running properly inside LXD
  containers, our suggested patchset includes both the ENODEV patch as
  is still being considered by upstream (stuck on legal validation) AND
  another patch which has ttyname() return the symlink when it receives
  the ENODEV.

  
  We feel this is the best way to fix the problem entirely right now. Once glibc upstream merges the ENODEV side of this and all affected software get fixed upstream to deal with it, we'll then be able to drop that patch without causing any regressions.

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