RFC Providing mount in memfd_restricted() syscall

Hello,

This patchset builds upon the memfd_restricted() system call that was discussed in the 'KVM: mm: fd-based approach for supporting KVM' patch series, at
https://lore.kernel.org/lkml/[email protected]/T/

The tree can be found at:
https://github.com/googleprodkernel/linux-cc/tree/restrictedmem-provide-mount-fd-rfc-v4

In this patchset, a modification to the memfd_restricted() syscall is proposed, which allows userspace to provide a mount, on which the restrictedmem file will be created and returned from the memfd_restricted().

Allowing userspace to provide a mount allows userspace to control various memory binding policies via tmpfs mount options, such as Transparent HugePage memory allocation policy through 'huge=always/never' and NUMA memory allocation policy through 'mpol=local/bind:*'.

Changes since RFCv3:
+ Added check to ensure that bind mounts must be bind mounts of the whole filesystem
+ Removed inappropriate check on fd’s permissions as Christian suggested
+ Renamed RMFD_USERMNT to MEMFD_RSTD_USERMNT as David suggested
+ Added selftest to check that bind mounts must be bind mounts of the whole filesystem

Changes since RFCv2:
+ Tightened semantics to accept only fds of the root of a tmpfs mount, as Christian suggested
+ Added permissions check on the inode represented by the fd to guard against creation of restrictedmem files on read-only tmpfs filesystems or mounts
+ Renamed RMFD_TMPFILE to RMFD_USERMNT to better represent providing a userspace mount to create a restrictedmem file on
+ Updated selftests for tighter semantics and added selftests to check for permissions

Changes since RFCv1:
+ Use fd to represent mount instead of path string, as Kirill suggested. I believe using fds makes this syscall interface more aligned with the other syscalls like fsopen(), fsconfig(), and fsmount() in terms of using and passing around fds
+ Remove unused variable char *orig_shmem_enabled from selftests

Dependencies:
+ Chao’s work on UPM, at
https://github.com/chao-p/linux/commits/privmem-v11.5

Links to earlier patch series:
+ RFC v3: https://lore.kernel.org/lkml/[email protected]/T/
+ RFC v2: https://lore.kernel.org/lkml/[email protected]/T/
+ RFC v1: https://lore.kernel.org/lkml/[email protected]/T/

Ackerley Tng (2):
mm: restrictedmem: Allow userspace to specify mount for
memfd_restricted
selftests: restrictedmem: Check memfd_restricted()'s handling of
provided userspace mount

 include/linux/syscalls.h                      |   2 +-
 include/uapi/linux/restrictedmem.h            |   8 +
 mm/restrictedmem.c                            |  73 ++-
 tools/testing/selftests/mm/.gitignore         |   1 +
 tools/testing/selftests/mm/Makefile           |   1 +
 .../selftests/mm/memfd_restricted_usermnt.c   | 529 ++++++++++++++++++
 tools/testing/selftests/mm/run_vmtests.sh     |   3 +
 7 files changed, 611 insertions(+), 6 deletions(-)
 create mode 100644 include/uapi/linux/restrictedmem.h
 create mode 100644 tools/testing/selftests/mm/memfd_restricted_usermnt.c

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2.40.0.577.gac1e443424-goog