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

Commit Message

Joel Fernandes Jan. 13, 2019, 12:14 a.m. UTC

From: "Joel Fernandes (Google)" <[e-mail]>

More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/[e-mail]/T/#t

Signed-off-by: Joel Fernandes (Google) <[e-mail]>
---
 man2/fcntl.2 | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

Patch

diff --git a/man2/fcntl.2 b/man2/fcntl.2
index 03533d65b..54772f949 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