Re: [PATCH 5.10.y 0/4] Backport series: "permit write-sealed memfd read-only shared mappings"

[Isaac Manjarres <isaacmanjarres@google.com>]

On Wed, Jul 30, 2025 at 03:21:48PM +0100, Lorenzo Stoakes wrote:
> Hi Isaac,
> 
> Thanks very much for all your hard work on this!
> 
> I'll respond to this one, but this is a general comment for all the
> backports.
> 
> I just wonder if this is what backporting is for - really this is a new
> feature, yes the documentation is incorrect, which is why I made the
> change, but it's sort of debatable if that's a bug or a new feature.

Hi Lorenzo,

Thanks for your feedback on this. That's a good question. The rationale
that I had when backporting these fixes was: The original intent of
F_SEAL_WRITE was to just prevent any writes to region after it had
been write-sealed, and that the existing behavior on older kernels
may have been a result of oversight or just an accident, making it a
bug. So fixing it would be fixing a bug that has been around for a
while. I hadn't really thought of it as a new feature.

I also learned recently that, at least for Android, there were attempts
in the past to map write-sealed memfds as read-only and shared, which
failed. This was surprising to developers, and they ended up working
around it. I'm not sure why it wasn't reported then, but this being
a surprise to multiple developers makes it feel like more of a bug
to me on older kernels.

>
> Having said that, I'm not against you doing this, just wondering about
> that.
> 
> Also - what kind of testing have you do on these series?
I did the following tests:

1. I have a unit test that tries to map write-sealed memfds as
read-only and shared. I verified that this works for each kernel version
that this series is being applied to.

2. Android devices do use memfds as well, so I did try these patches out
on a device running each kernel version, and tried boot testing, using
several apps/games. I was looking for functional failures in these
scenarios but didn't encounter any.

Do you have any other recommendations of what I should test?

Thanks,
Isaac

> Cheers, Lorenzo
> 
> On Tue, Jul 29, 2025 at 06:53:58PM -0700, Isaac J. Manjarres wrote:
> > Hello,
> >
> > Until kernel version 6.7, a write-sealed memfd could not be mapped as
> > shared and read-only. This was clearly a bug, and was not inline with
> > the description of F_SEAL_WRITE in the man page for fcntl()[1].
> >
> > Lorenzo's series [2] fixed that issue and was merged in kernel version
> > 6.7, but was not backported to older kernels. So, this issue is still
> > present on kernels 5.4, 5.10, 5.15, 6.1, and 6.6.
> >
> > This series consists of backports of two of Lorenzo's series [2] and
> > [3].
> >
> > Note: for [2], I dropped the last patch in that series, since it
> > wouldn't make sense to apply it due to [4] being part of this tree. In
> > lieu of that, I backported [3] to ultimately allow write-sealed memfds
> > to be mapped as read-only.
> >
> > [1] https://man7.org/linux/man-pages/man2/fcntl.2.html
> > [2] https://lore.kernel.org/all/913628168ce6cce77df7d13a63970bae06a526e0.1697116581.git.lstoakes@gmail.com/T/#m28fbfb0d5727e5693e54a7fb2e0c9ac30e95eca5
> > [3] https://lkml.kernel.org/r/99fc35d2c62bd2e05571cf60d9f8b843c56069e0.1732804776.git.lorenzo.stoakes@oracle.com
> > [4] https://lore.kernel.org/all/6e0becb36d2f5472053ac5d544c0edfe9b899e25.1730224667.git.lorenzo.stoakes@oracle.com/T/#u
> >
> > Lorenzo Stoakes (4):
> >   mm: drop the assumption that VM_SHARED always implies writable
> >   mm: update memfd seal write check to include F_SEAL_WRITE
> >   mm: reinstate ability to map write-sealed memfd mappings read-only
> >   selftests/memfd: add test for mapping write-sealed memfd read-only
> >
> >  fs/hugetlbfs/inode.c                       |  2 +-
> >  include/linux/fs.h                         |  4 +-
> >  include/linux/memfd.h                      | 14 ++++
> >  include/linux/mm.h                         | 80 +++++++++++++++-------
> >  kernel/fork.c                              |  2 +-
> >  mm/filemap.c                               |  2 +-
> >  mm/madvise.c                               |  2 +-
> >  mm/memfd.c                                 |  2 +-
> >  mm/mmap.c                                  | 10 ++-
> >  mm/shmem.c                                 |  2 +-
> >  tools/testing/selftests/memfd/memfd_test.c | 43 ++++++++++++
> >  11 files changed, 129 insertions(+), 34 deletions(-)
> >
> > --
> > 2.50.1.552.g942d659e1b-goog
> >