[CRIU] [PATCH v4 0/7] support restoring ghost files on readonly mounts

[Pavel Tikhomirov]

We have a problem restoring ghost-files on readonly mounts:

Failed to resume the CT: PRL_ERR_VZCTL_OPERATION_FAILED (Details: (04.800253)      1: Error (criu/files-reg.c:381): Can't create ghost regfile: Read-only file system
(04.916178) Error (criu/files-reg.c:868): Couldn't unlink remap 17 lib/x86_64-linux-gnu/libm-2.24.so.dpkg-new.cr.22.ghost: Read-only file system
(04.916190) Error (criu/files-reg.c:868): Couldn't unlink remap 17 lib/x86_64-linux-gnu/libm-2.24.so.dpkg-new.cr.22.ghost: No such file or directory
(04.945551) Error (criu/cr-restore.c:2340): Restoring FAILED.

First prepare do_restore_task_mnt_ns to be used in next patches.

Second improve mnt_is_overmounted helper to use it safely after
mounts remap.

Third actually fix the problem by making mounts writable just before
restore wants to do any operations on them while restoring files, and
later remount them back where needed.

Just to note: I thought/tried two other possible ways of making these
work: 1) Find a writable bind-mount - but there can be no such a thing, see
the (test) patch 4. 2) Remount all mounts writable initially, these way
we can do a lot of extra remounts while having no ghost-files, that's
bad. 3) Make all ghost operations in service mntns, but that looks too
complicated.

While on it add a check for unsupported open files on overmounted mounts
early on dump, which also implies ghost-files.

Also add a check that mount flags restore fine in zdtm, so that we can
be sure these patchset does not break threm.

Pavel Tikhomirov (7):
  mount: remove unused argument of do_restore_task_mnt_ns
  mount: save the result of mnt_is_overmounted
  mount: remount ro mounts writable before ghost-file restore
  zdtm: ghost on readonly fs
  files: open files on overmounted mounts are not supported
  zdtm: test dumping file on overmounted mount fails
  zdtm.py: also check that mount flags are restored correct

 criu/cr-restore.c                      |   3 +
 criu/files-reg.c                       |  38 +++++-
 criu/include/mount.h                   |  19 +++
 criu/mount.c                           | 175 ++++++++++++++++++++++--
 test/zdtm.py                           |   2 +-
 test/zdtm/static/Makefile              |   2 +
 test/zdtm/static/ghost_on_rofs.c       | 179 +++++++++++++++++++++++++
 test/zdtm/static/ghost_on_rofs.desc    |   1 +
 test/zdtm/static/overmounted_file.c    | 109 +++++++++++++++
 test/zdtm/static/overmounted_file.desc |   1 +
 10 files changed, 513 insertions(+), 16 deletions(-)
 create mode 100644 test/zdtm/static/ghost_on_rofs.c
 create mode 100644 test/zdtm/static/ghost_on_rofs.desc
 create mode 100644 test/zdtm/static/overmounted_file.c
 create mode 100644 test/zdtm/static/overmounted_file.desc

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2.17.1