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

[Joel Fernandes (Google)]

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

Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
 man2/memfd_create.2 | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/man2/memfd_create.2 b/man2/memfd_create.2
index 15b1362f5525..3b7f032407ed 100644
--- a/man2/memfd_create.2
+++ b/man2/memfd_create.2
@@ -280,7 +280,15 @@ in order to restrict further modifications on the file.
 (If placing the seal
 .BR F_SEAL_WRITE ,
 then it will be necessary to first unmap the shared writable mapping
-created in the previous step.)
+created in the previous step. Otherwise, behavior similar to
+.BR F_SEAL_WRITE
+can be achieved, by using
+.BR F_SEAL_FUTURE_WRITE
+which will prevent future writes via
+.BR mmap (2)
+and
+.BR write (2)
+from succeeding, while keeping existing shared writable mappings).
 .IP 4.
 A second process obtains a file descriptor for the
 .BR tmpfs (5)
@@ -425,6 +433,7 @@ main(int argc, char *argv[])
         fprintf(stderr, "\et\etg \- F_SEAL_GROW\en");
         fprintf(stderr, "\et\ets \- F_SEAL_SHRINK\en");
         fprintf(stderr, "\et\etw \- F_SEAL_WRITE\en");
+        fprintf(stderr, "\et\etW \- F_SEAL_FUTURE_WRITE\en");
         fprintf(stderr, "\et\etS \- F_SEAL_SEAL\en");
         exit(EXIT_FAILURE);
     }
@@ -463,6 +472,8 @@ main(int argc, char *argv[])
             seals |= F_SEAL_SHRINK;
         if (strchr(seals_arg, \(aqw\(aq) != NULL)
             seals |= F_SEAL_WRITE;
+        if (strchr(seals_arg, \(aqW\(aq) != NULL)
+            seals |= F_SEAL_FUTURE_WRITE;
         if (strchr(seals_arg, \(aqS\(aq) != NULL)
             seals |= F_SEAL_SEAL;
 
@@ -518,6 +529,8 @@ main(int argc, char *argv[])
         printf(" GROW");
     if (seals & F_SEAL_WRITE)
         printf(" WRITE");
+    if (seals & F_SEAL_FUTURE_WRITE)
+        printf(" FUTURE_WRITE");
     if (seals & F_SEAL_SHRINK)
         printf(" SHRINK");
     printf("\en");
-- 
2.21.0.360.g471c308f928-goog