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

[Chao Peng]

On Thu, Jul 21, 2022 at 11:44:11AM +0200, David Hildenbrand wrote:
> 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@xxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >  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.

Right, it also prevents 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().

Userfaultfd sounds interesting, will further investigate it. But a rough
look sounds it only faults to usrspace for write/read fault, not
write()? Also sounds it operates on vma and userfaultfd_register() takes
mmap_lock which is what we want to avoid for frequent
register/unregister during private/shared memory conversion.

Chao
> 
> -- 
> Thanks,
> 
> David / dhildenb