More details of the seal can be found in the LKML patch: https://lore.kernel.org/lkml/20181120052137.74317-1-joel@xxxxxxxxxxxxxxxxx/T/#t Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> --- man2/fcntl.2 | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/man2/fcntl.2 b/man2/fcntl.2 index fce4f4c2b3bd..e01e2c075b5b 100644 --- a/man2/fcntl.2 +++ b/man2/fcntl.2 @@ -1525,6 +1525,21 @@ Furthermore, if there are any asynchronous I/O operations .RB ( io_submit (2)) pending on the file, all outstanding writes will be discarded. +.TP +.BR F_SEAL_FUTURE_WRITE +If this seal is set, the contents of the file can be modified only from +existing writeable mappings that were created prior to the seal being set. +Any attempt to create a new writeable mapping on the memfd via +.BR mmap (2) +will fail with +.BR EPERM. +Also any attempts to write to the memfd via +.BR write (2) +will fail with +.BR EPERM. +This is useful in situations where existing writable mapped regions need to be +kept intact while preventing any future writes. For example, to share a +read-only memory buffer to other processes that only the sender can write to. .\" .SS File read/write hints Write lifetime hints can be used to inform the kernel about the relative -- 2.21.0.360.g471c308f928-goog
