[PATCH -manpage 1/2] fcntl.2: Update manpage with new memfd F_SEAL_FUTURE_WRITE seal

@@ -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

@@ -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, "\\t\\tg \- F_SEAL_GROW\\n");
         fprintf(stderr, "\\t\\ts \- F_SEAL_SHRINK\\n");
         fprintf(stderr, "\\t\\tw \- F_SEAL_WRITE\\n");
+        fprintf(stderr, "\\t\\tW \- F_SEAL_FUTURE_WRITE\\n");
         fprintf(stderr, "\\t\\tS \- F_SEAL_SEAL\\n");
         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("\\n");