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fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal 

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>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Joel Fernandes (Google) 2019-03-14 17:48:43 -04:00 committed by Michael Kerrisk
parent f3491e47ba
commit 7b7d3b200a
1 changed files with 15 additions and 0 deletions
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15
man2/fcntl.2
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@ -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