mm: memfd_luo: preserve file seals

mm: memfd_luo: preserve file seals

File seals are used on memfd for making shared memory communication with
untrusted peers safer and simpler.  Seals provide a guarantee that certain
operations won't be allowed on the file such as writes or truncations.
Maintaining these guarantees across a live update will help keeping such
use cases secure.

These guarantees will also be needed for IOMMUFD preservation with LUO.
Normally when IOMMUFD maps a memfd, it pins all its pages to make sure any
truncation operations on the memfd don't lead to IOMMUFD using freed
memory.  This doesn't work with LUO since the preserved memfd might have
completely different pages after a live update, and mapping them back to
the IOMMUFD will cause all sorts of problems.  Using and preserving the
seals allows IOMMUFD preservation logic to trust the memfd.

Since the uABI defines seals as an int, preserve them by introducing a new
u32 field.  There are currently only 6 possible seals, so the extra bits
are unused and provide room for future expansion.  Since the seals are
uABI, it is safe to use them directly in the ABI.  While at it, also add a
u32 flags field.  It makes sure the struct is nicely aligned, and can be
used later to support things like MFD_CLOEXEC.

Since the serialization structure is changed, bump the version number to
"memfd-v2".

It is important to note that the memfd-v2 version only supports seals that
existed when this version was defined.  This set is defined by
MEMFD_LUO_ALL_SEALS.  Any new seal might bring a completely different
semantic with it and the parser for memfd-v2 cannot be expected to deal
with that.  If there are any future seals added, they will need another
version bump.

Link: https://lkml.kernel.org/r/20260216185946.1215770-3-pratyush@kernel.org
Signed-off-by: Pratyush Yadav (Google) <pratyush@kernel.org>
Tested-by: Samiullah Khawaja <skhawaja@google.com>
Cc: Alexander Graf <graf@amazon.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>