[PATCH 0/2] mm/memfd: add F_SEAL_PUNCH_HOLE

[Samiullah Khawaja]

This series introduces F_SEAL_PUNCH_HOLE, a new memfd seal that prevents
holes from being punched into a shmem or hugetlbfs memfd while leaving
the file fully writable.

Background
==========

Drivers such as udmabuf and iommufd (via IOMMU_IOAS_MAP_FILE) use
memfd_pin_folios() to pin the folios backing a memfd and hand them to a
device for DMA. Pinning keeps those folios allocated, but it does not
keep them in the file. If userspace later calls
fallocate(FALLOC_FL_PUNCH_HOLE) or madvise(MADV_REMOVE) on the range,
the folios are dropped from the page cache while the device continues
to use them. The pin holder is never notified, and from that point on
the device and the file silently diverge.

The existing seals only partially address this. F_SEAL_SHRINK prevents
pages from being dropped by truncation, which is why udmabuf requires
it. Hole punching, however, is only blocked by F_SEAL_WRITE and
F_SEAL_FUTURE_WRITE, and both of those also forbid writes. That makes
them unusable for memory that must stay writable, such as guest RAM or
DMA buffers, and udmabuf explicitly rejects memfds that carry them.

Proposal
========

F_SEAL_PUNCH_HOLE fills this gap. Once it is set,
fallocate(FALLOC_FL_PUNCH_HOLE) and madvise(MADV_REMOVE) fail with
-EPERM, while reads, writes and resizing are unaffected. Combined with
F_SEAL_SHRINK, it guarantees that userspace cannot remove pages from
the memfd while the memfd remains writable.

Motivation
==========

The immediate user is IOMMU live update. Before iommufd preserves a
memfd-backed mapping across kexec, it must confirm that every pinned
page still belongs to the file. Today this requires walking each mapped
page at preserve time, which is costly for large mappings. With
F_SEAL_PUNCH_HOLE and F_SEAL_SHRINK in place, iommufd can verify the
seals once at IOMMU_IOAS_MAP_FILE time and skip the walk entirely.
udmabuf can benefit in the same way. Neither user is changed in this
series.

The iommufd change that would use the seal is discussed in the IOMMU
live update series:
https://lore.kernel.org/all/arVYa_90j0oN1hnu@xxxxxxxxxx/

Testing
=======

Patch 2 extends the memfd selftests to cover the new seal on both shmem
and hugetlbfs. Both "memfd_test" and "memfd_test hugetlbfs" pass in
QEMU.

The hugetlbfs run depends on a separately posted fix. Without it, the
run aborts in the existing SEAL-EXEC test before it reaches the new
tests:
https://lore.kernel.org/all/20261026-fixes_shmem-v1-5-703206d5fa0c@xxxxxxxxxx/

Samiullah Khawaja (2):
  mm/memfd: add F_SEAL_PUNCH_HOLE
  selftests/memfd: add F_SEAL_PUNCH_HOLE tests

 fs/hugetlbfs/inode.c                       |   3 +-
 include/uapi/linux/fcntl.h                 |   1 +
 mm/memfd.c                                 |   4 +-
 mm/shmem.c                                 |   3 +-
 tools/testing/selftests/memfd/memfd_test.c | 111 ++++++++++++++++++++-
 5 files changed, 118 insertions(+), 4 deletions(-)


base-commit: 7bf01f16a4d05028fbeff4d90bab40df25142a24
-- 
2.56.0.307.g5fe7bd680c-goog