From mboxrd@z Thu Jan  1 00:00:00 1970
From: joel@joelfernandes.org (Joel Fernandes (Google))
Date: Mon, 19 Nov 2018 21:25:44 -0800
Subject: [PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd
 F_SEAL_FUTURE_WRITE seal
Message-ID: <20181120052545.76560-1-joel@joelfernandes.org>
Content-Type: text/plain; charset="UTF-8"
Message-ID: <20181120052544.WxEz3Eg-ZlAaXbZ80iY861rWM0fnKHEmQarb5hELYQ0@z>

More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/20181120052137.74317-1-joel at joelfernandes.org/T/#t

Signed-off-by: Joel Fernandes (Google) <joel at joelfernandes.org>
---
 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.
+.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
-- 
2.19.1.1215.g8438c0b245-goog