Re: Question about memory mappings with inaccessible permissions

[Randolph Langley <langley at cs dot fsu dot edu>]

Does anyone have any ideas about this? Is this behavior intentional, or 
is it unintended?

If intentional, then why is it desirable (is it some sort of guard?)

Thanks,

Randolph




On 09/30/2015 01:01 PM, Randolph Langley wrote:
> I am curious about some of the memory mappings that I am seeing.
>
> Here's an example from a bash process:
>
>         $ cat /proc/4012/maps
>         00400000-004ef000 r-xp 00000000 08:11
>         3670022                            /bin/bash
>         006ef000-006f0000 r--p 000ef000 08:11
>         3670022                            /bin/bash
>         006f0000-006f9000 rw-p 000f0000 08:11
>         3670022                            /bin/bash
>         006f9000-006ff000 rw-p 00000000 00:00 0
>         01de9000-02038000 rw-p 00000000 00:00
>         0                                  [heap]
>         7fbd5cde6000-7fbd5cdf1000 r-xp 00000000 08:11 2363469
>         /lib/x86_64-linux-gnu/libnss_files-2.19.so
>         7fbd5cdf1000-7fbd5cff0000 ---p 0000b000 08:11 2363469
>         /lib/x86_64-linux-gnu/libnss_files-2.19.so
>         7fbd5cff0000-7fbd5cff1000 r--p 0000a000 08:11 2363469
>         /lib/x86_64-linux-gnu/libnss_files-2.19.so
>         7fbd5cff1000-7fbd5cff2000 rw-p 0000b000 08:11 2363469
>         /lib/x86_64-linux-gnu/libnss_files-2.19.so
>         7fbd5cff2000-7fbd5cffd000 r-xp 00000000 08:11 2363596
>         /lib/x86_64-linux-gnu/libnss_nis-2.19.so
>         7fbd5cffd000-7fbd5d1fc000 ---p 0000b000 08:11 2363596
>         /lib/x86_64-linux-gnu/libnss_nis-2.19.so
>         7fbd5d1fc000-7fbd5d1fd000 r--p 0000a000 08:11 2363596
>         /lib/x86_64-linux-gnu/libnss_nis-2.19.so
>         7fbd5d1fd000-7fbd5d1fe000 rw-p 0000b000 08:11 2363596
>         /lib/x86_64-linux-gnu/libnss_nis-2.19.so
>         7fbd5d1fe000-7fbd5d215000 r-xp 00000000 08:11
>         2363567 /lib/x86_64-linux-gnu/libnsl-2.19.so
>         7fbd5d215000-7fbd5d414000 ---p 00017000 08:11
>         2363567 /lib/x86_64-linux-gnu/libnsl-2.19.so
>         7fbd5d414000-7fbd5d415000 r--p 00016000 08:11
>         2363567 /lib/x86_64-linux-gnu/libnsl-2.19.so
>         7fbd5d415000-7fbd5d416000 rw-p 00017000 08:11
>         2363567 /lib/x86_64-linux-gnu/libnsl-2.19.so
>         7fbd5d416000-7fbd5d418000 rw-p 00000000 00:00 0
>         7fbd5d418000-7fbd5d421000 r-xp 00000000 08:11 2363566
>         /lib/x86_64-linux-gnu/libnss_compat-2.19.so
>         7fbd5d421000-7fbd5d620000 ---p 00009000 08:11 2363566
>         /lib/x86_64-linux-gnu/libnss_compat-2.19.so
>         7fbd5d620000-7fbd5d621000 r--p 00008000 08:11 2363566
>         /lib/x86_64-linux-gnu/libnss_compat-2.19.so
>         7fbd5d621000-7fbd5d622000 rw-p 00009000 08:11 2363566
>         /lib/x86_64-linux-gnu/libnss_compat-2.19.so
>         7fbd5d622000-7fbd5dabc000 r--p 00000000 08:11
>         661812                     /usr/lib/locale/locale-archive
>         7fbd5dabc000-7fbd5dc77000 r-xp 00000000 08:11
>         2363579 /lib/x86_64-linux-gnu/libc-2.19.so
>         7fbd5dc77000-7fbd5de76000 ---p 001bb000 08:11
>         2363579 /lib/x86_64-linux-gnu/libc-2.19.so
>         7fbd5de76000-7fbd5de7a000 r--p 001ba000 08:11
>         2363579 /lib/x86_64-linux-gnu/libc-2.19.so
>         7fbd5de7a000-7fbd5de7c000 rw-p 001be000 08:11
>         2363579 /lib/x86_64-linux-gnu/libc-2.19.so
>         7fbd5de7c000-7fbd5de81000 rw-p 00000000 00:00 0
>         7fbd5de81000-7fbd5de84000 r-xp 00000000 08:11
>         2363545 /lib/x86_64-linux-gnu/libdl-2.19.so
>         7fbd5de84000-7fbd5e083000 ---p 00003000 08:11
>         2363545 /lib/x86_64-linux-gnu/libdl-2.19.so
>         [ .... ]
>
>
> Several of these have permissions of "---p"; that is, there is no 
> permission to read, write, or execute these mappings.
>
> Can anyone explain why these inaccessible mappings exist? If so, are 
> these useful in some way? I have casually experimented with 
> munmap(2)-ing these, and so far I haven't seen any dire effects.
>
> Thanks,
>
> Randolph