[PATCH 0/4 v5 RESEND] devpts: handle bind-mounts correctly

[Christian Brauner]

Resending to CC grekh.

Hey everyone,

This is the fith iteration of this patch. Per-patch changes are
summarized in the individual patches:

ChangeLog v4->v5:
* added non-functional patch to document devpts_mntget().
  Reason for putting this in a separate patch is that it allows you,
  Linus and Eric, to simply drop it if judged useless.
* reverse error handling logic to further simplify
* dput() dentry in the non-function patch. This was not really a problem
  since the following patch included a fix for it. But better to get it
  right in all individual patches.
* I did another rewrite of the problem analysis for
  posterity in the patch "Subject: [PATCH 2/3 v3] devpts: resolve devpts
  bind-mounts" and in this cover letter.

ChangeLog v3->v4:
* small logical simplifications
* add test that bind-mounts of /dev/pts/ptmx to locations that do not
  resolve to a valid slave pty path under the originating devpts mount
  fail

ChangeLog v2->v3:
* rewritten commit message to thoroughly analyse the problem for
  posterity in the patch "Subject: [PATCH 2/3 v3] devpts: resolve devpts
  bind-mounts" and in this cover letter.
* extended selftests to test for correct handling of /dev/pts/ptmx
  bind-mounts to /dev/ptmx and non-standard devpts mounts such as
  mount -t devpts devpts /mnt

ChangeLog v1->v2:
* see individual patches
ChangeLog v0->v1:
* see individual patches

Christian Brauner (4):
  devpts: hoist out check for DEVPTS_SUPER_MAGIC
  devpts: resolve devpts bind-mounts
  devpts: comment devpts_mntget()
  selftests: add devpts selftests

 fs/devpts/inode.c                                |  66 +++--
 tools/testing/selftests/Makefile                 |   1 +
 tools/testing/selftests/filesystems/.gitignore   |   1 +
 tools/testing/selftests/filesystems/Makefile     |   2 +-
 tools/testing/selftests/filesystems/devpts_pts.c | 313 +++++++++++++++++++++++
 5 files changed, 363 insertions(+), 20 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/devpts_pts.c

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2.15.1