/proc/pid/maps containg anonymous maps that have PROT_NONE

["Török Edwin" <edwintorok@gmail.com>]

Hi,

I noticed that there are (quite large) entries in /proc/pid/maps that
have PROT_NONE, right after an existing mapping:
7fffe4000000-7fffe406a000 rw-p 7fffe4000000 00:00 0
7fffe406a000-7fffe8000000 ---p 7fffe406a000 00:00 0
7ffff76d1000-7ffff76e0000 r-xp 00000000 09:03 260750                    
/lib/libbz2.so.1.0.4
7ffff76e0000-7ffff78df000 ---p 0000f000 09:03 260750                    
/lib/libbz2.so.1.0.4

I don't mind that 2Mb map, but what is 7fffe406a000-7fffe8000000 ---p ?
(63M)

Is it coming from glibc mapping memory as PROT_NONE, and using
mprotect/madvise to make it writable, and then
caching the mappings for future use, rather than freeing them?

/proc/pid/smaps shows:
7fffe406a000-7fffe8000000 ---p 7fffe406a000 00:00 0
Size:              65112 kB
Rss:                   0 kB
Pss:                   0 kB
Shared_Clean:          0 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:            0 kB
Swap:                  0 kB

I straced the program creating these, and I couldn't find anything with
7fffe406a000, but only before that address:

[pid 31928] mprotect(0x7fffe4000000, 135168, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4021000, 131072, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4041000, 32768, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4049000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe404a000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe404b000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe404c000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe404d000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe404e000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe404f000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4050000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4051000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4052000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4053000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4054000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4055000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4056000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4057000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4058000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe4059000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe405a000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe405b000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe405c000, 4096, PROT_READ|PROT_WRITE) = 0
[pid 31928] mprotect(0x7fffe405d000, 53248, PROT_READ|PROT_WRITE) = 0
[pid 31928] madvise(0x7fffe4069000, 4096, 0x4 /* MADV_??? */) = 0
[pid 31928] madvise(0x7fffe4021000, 294912, 0x4 /* MADV_??? */) = 0
[pid 31928] madvise(0x7fffe4021000, 4096, 0x4 /* MADV_??? */) = 0

There is an mprotect and madvise that end at 0x7fffe406a000.
Those mprotects and madvise are coming from glibc. Its strange that I
don't see the mmap only the mprotect, but I used strace -f.

This happens on:
Linux debian 2.6.26-1-amd64 #1 SMP Thu Oct 9 14:16:53 UTC 2008 x86_64
GNU/Linux

strace is here:
http://edwintorok.googlepages.com/log2.bz2

Best regards,
--Edwin