Hi

On Fri, Jun 13, 2014 at 12:36 PM, David Herrmann [off-list ref] wrote:

Hi

This is v3 of the File-Sealing and memfd_create() patches. You can find v1 with
a longer introduction at gmane:
  http://thread.gmane.org/gmane.comp.video.dri.devel/102241
An LWN article about memfd+sealing is available, too:
  https://lwn.net/Articles/593918/
v2 with some more discussions can be found here:
  http://thread.gmane.org/gmane.linux.kernel.mm/115713

This series introduces two new APIs:
  memfd_create(): Think of this syscall as malloc() but it returns a
                  file-descriptor instead of a pointer. That file-descriptor is
                  backed by anon-memory and can be memory-mapped for access.
  sealing: The sealing API can be used to prevent a specific set of operations
           on a file-descriptor. You 'seal' the file and give thus the
           guarantee, that it cannot be modified in the specific ways.

A short high-level introduction is also available here:
  http://dvdhrm.wordpress.com/2014/06/10/memfd_create2/


Changed in v3:
 - fcntl() now returns EINVAL if the FD does not support sealing. We used to
   return EBADF like pipe_fcntl() does, but that is really weird and I don't
   like repeating that.
 - seals are now saved as "unsigned int" instead of "u32".
 - i_mmap_writable is now an atomic so we can deny writable mappings just like
   i_writecount does.
 - SHMEM_ALLOW_SEALING is dropped. We initialize all objects with F_SEAL_SEAL
   and only unset it for memfds that shall support sealing.
 - memfd_create() no longer has a size argument. It was redundant, use
   ftruncate() or fallocate().
 - memfd_create() flags are "unsigned int" now, instead of "u64".
 - NAME_MAX off-by-one fix
 - several cosmetic changes
 - Added AIO/Direct-IO page-pinning protection

The last point is the most important change in this version: We now bail out if
any page-refcount is elevated while setting SEAL_WRITE. This prevents parallel
GUP users from writing to sealed files _after_ they were sealed. There is also a
new FUSE-based test-case to trigger such situations.

The last 2 patches try to improve the page-pinning handling. I included both in
this series, but obviously only one of them is needed (or we could stack them):
 - 6/7: This waits for up to 150ms for pages to be unpinned
 - 7/7: This isolates pinned pages and replaces them with a fresh copy

Hugh, patch 6 is basically your code. In case that gets merged, can I put your
Signed-off-by on it?

Hugh, any comments on patch 5, 6 and 7? Those are the last outstanding
issues with memfd+sealing. Patch 7 (isolating pages) is still my
favorite and has been running just fine on my machine for the last
months. I think it'd be nice if we could give it a try in -next. We
can always fall back to Patch 5 or Patch 5+6. Those will detect any
racing AIO and just fail or wait for the IO to finish for a short
period.

Are there any other blockers for this?

Thanks
David

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