• Joel Fernandes (Google)'s avatar
    mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd · ab3948f5
    Joel Fernandes (Google) authored
    Android uses ashmem for sharing memory regions.  We are looking forward
    to migrating all usecases of ashmem to memfd so that we can possibly
    remove the ashmem driver in the future from staging while also
    benefiting from using memfd and contributing to it.  Note staging
    drivers are also not ABI and generally can be removed at anytime.
    
    One of the main usecases Android has is the ability to create a region
    and mmap it as writeable, then add protection against making any
    "future" writes while keeping the existing already mmap'ed
    writeable-region active.  This allows us to implement a usecase where
    receivers of the shared memory buffer can get a read-only view, while
    the sender continues to write to the buffer.  See CursorWindow
    documentation in Android for more details:
    
      https://developer.android.com/reference/android/database/CursorWindow
    
    This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
    To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
    which prevents any future mmap and write syscalls from succeeding while
    keeping the existing mmap active.
    
    A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
    where we don't need to modify core VFS structures to get the same
    behavior of the seal.  This solves several side-effects pointed by Andy.
    self-tests are provided in later patch to verify the expected semantics.
    
    [1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/
    
    Thanks a lot to Andy for suggestions to improve code.
    
    Link: http://lkml.kernel.org/r/20190112203816.85534-2-joel@joelfernandes.orgSigned-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
    Acked-by: default avatarJohn Stultz <john.stultz@linaro.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Jann Horn <jannh@google.com>
    Cc: Al Viro <viro@zeniv.linux.org.uk>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: J. Bruce Fields <bfields@fieldses.org>
    Cc: Jeff Layton <jlayton@kernel.org>
    Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
    Cc: Matthew Wilcox <willy@infradead.org>
    Cc: Mike Kravetz <mike.kravetz@oracle.com>
    Cc: Shuah Khan <shuah@kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    ab3948f5
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