[PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

From: Joel Fernandes (Google)
Date: Wed Dec 12 2018 - 17:05:33 EST

More details of the seal can be found in the LKML patch:

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 03533d65b49d..54772f94964c 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.
+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
+Also any attempts to write to the memfd via
+.BR write (2)
+will fail with
+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