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From:   Joel Fernandes <joel@joelfernandes.org>
To:     linux-kernel@vger.kernel.org, mtk.manpages@gmail.com
Cc:     "Joel Fernandes (Google)" <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>, dancol@google.com,
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        linux-api@vger.kernel.org, linux-man@vger.kernel.org,
        linux-mm@kvack.org, marcandre.lureau@redhat.com,
        Matthew Wilcox <willy@infradead.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Shuah Khan <shuah@kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: [PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal
Date:   Sat, 12 Jan 2019 19:14:45 -0500
Message-Id: <20190113001446.158789-2-joel@joelfernandes.org>
In-Reply-To: <20190113001446.158789-1-joel@joelfernandes.org>
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From: "Joel Fernandes (Google)" <joel@joelfernandes.org>

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

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
 man2/fcntl.2 | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

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
-- 
2.20.1.97.g81188d93c3-goog