Re: [PATCH v7 01/14] mm: Add F_SEAL_AUTO_ALLOCATE seal to memfd

David Hildenbrand
2022-07-21T09:44:21Z

On 06.07.22 10:20, Chao Peng wrote:
> Normally, a write to unallocated space of a file or the hole of a sparse
> file automatically causes space allocation, for memfd, this equals to
> memory allocation. This new seal prevents such automatically allocating,
> either this is from a direct write() or a write on the previously
> mmap-ed area. The seal does not prevent fallocate() so an explicit
> fallocate() can still cause allocating and can be used to reserve
> memory.
>
> This is used to prevent unintentional allocation from userspace on a
> stray or careless write and any intentional allocation should use an
> explicit fallocate(). One of the main usecases is to avoid memory double
> allocation for confidential computing usage where we use two memfds to
> back guest memory and at a single point only one memfd is alive and we
> want to prevent memory allocation for the other memfd which may have
> been mmap-ed previously. More discussion can be found at:
>
> https://lkml.org/lkml/2022/6/14/1255
>
> Suggested-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
> include/uapi/linux/fcntl.h | 1 +
> mm/memfd.c | 3 ++-
> mm/shmem.c | 16 ++++++++++++++--
> 3 files changed, 17 insertions(+), 3 deletions(-)
>
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 2f86b2ad6d7e..98bdabc8e309 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -43,6 +43,7 @@
> #define F_SEAL_GROW 0x0004 /* prevent file from growing */
> #define F_SEAL_WRITE 0x0008 /* prevent writes */
> #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */
> +#define F_SEAL_AUTO_ALLOCATE 0x0020 /* prevent allocation for writes */

Why only "on writes" and not "on reads". IIRC, shmem doesn't support the
shared zeropage, so you'll simply allocate a new page via read() or on
read faults.

Also, I *think* you can place pages via userfaultfd into shmem. Not sure
if that would count "auto alloc", but it would certainly bypass fallocate().

--
Thanks,

David / dhildenb