What do the "---p" permissions in /proc/self/maps mean?

Question

I understand the meaning of rwxps bits. r-xp is for .text. rw-p is for .data/.bss/heap/stack. What is the use of just ---p pages?

For example see this output of cat /proc/self/maps

00400000-0040b000 r-xp 00000000 08:03 827490                             /bin/cat
0060b000-0060c000 rw-p 0000b000 08:03 827490                             /bin/cat
0060c000-0062d000 rw-p 00000000 00:00 0                                  [heap]
3819a00000-3819a1e000 r-xp 00000000 08:03 532487                         /lib64  ld-2.11.2.so
3819c1d000-3819c1e000 r--p 0001d000 08:03 532487                         /lib64/ld-2.11.2.so
3819c1e000-3819c1f000 rw-p 0001e000 08:03 532487                         /lib64/ld-2.11.2.so
3819c1f000-3819c20000 rw-p 00000000 00:00 0 
3819e00000-3819f70000 r-xp 00000000 08:03 532490                         /lib64/libc-2.11.2.so
3819f70000-381a16f000 ---p 00170000 08:03 532490                         /lib64/libc-2.11.2.so
381a16f000-381a173000 r--p 0016f000 08:03 532490                         /lib64/libc-2.11.2.so
381a173000-381a174000 rw-p 00173000 08:03 532490                         /lib64/libc-2.11.2.so
381a174000-381a179000 rw-p 00000000 00:00 0 
7fb859c49000-7fb85fa7a000 r--p 00000000 08:03 192261                     /usr/lib/locale/locale-archive
7fb85fa7a000-7fb85fa7d000 rw-p 00000000 00:00 0 
7fb85fa95000-7fb85fa96000 rw-p 00000000 00:00 0
7fff64894000-7fff648a9000 rw-p 00000000 00:00 0                          [stack]
7fff649ff000-7fff64a00000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]

Answer 1

According to the man page, it means private (copy on write). No idea what the usefulness of such a mapping is without being able to read/write/execute anything in it, though.

Possibly it is private to libc, allowing it to modify the permissions to access it without a user program accidentally mucking it up.