[PATCH 0/2] fcntl: add fcntl(F_CHECK_ORIGINAL_MEMFD)

[Michal Clapinski <mclapinski@google.com>]

This change introduces a new fcntl to check if an fd points to a memfd's
original open fd (the one created by memfd_create).

We encountered an issue with migrating memfds in CRIU (checkpoint
restore in userspace - it migrates running processes between
machines). Imagine a scenario:
1. Create a memfd. By default it's open with O_RDWR and yet one can
exec() to it (unlike with regular files, where one would get ETXTBSY).
2. Reopen that memfd with O_RDWR via /proc/self/fd/<fd>.

Now those 2 fds are indistinguishable from userspace. You can't exec()
to either of them (since the reopen incremented inode->i_writecount)
and their /proc/self/fdinfo/ are exactly the same. Unfortunately they
are not the same. If you close the second one, the first one becomes
exec()able again. If you close the first one, the other doesn't become
exec()able. Therefore during migration it does matter which is recreated
first and which is reopened but there is no way for CRIU to tell which
was first.

Michal Clapinski (2):
  fcntl: add fcntl(F_CHECK_ORIGINAL_MEMFD)
  selftests: test fcntl(F_CHECK_ORIGINAL_MEMFD)

 fs/fcntl.c                                 |  3 ++
 include/uapi/linux/fcntl.h                 |  9 ++++++
 tools/testing/selftests/memfd/memfd_test.c | 32 ++++++++++++++++++++++
 3 files changed, 44 insertions(+)

-- 
2.42.0.283.g2d96d420d3-goog