More details of the seal can be found in the LKML patch:
https://lore.kernel.org/lkml/20181120052137.74317-1-j...@joelfernandes.org/T/#t
Signed-off-by: Joel Fernandes (Google) <j...@joelfernandes.org>
---
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
