What is the memory region ?

[Prabhu nath]

Looks like they have added a new section GNU_RELRO in the later versions.
The one you are referring is read-only section. It would be nice if you
share the section header table.
Plz see inline

Regards,
Prabhunath G
Linux Trainer
Bangalore


On Thu, Mar 7, 2013 at 3:31 PM, Jacky <jackyclivia at 163.com> wrote:

> Dear all,
>
> This is the Program Header for "cat" info:
>
> ================================
> readelf -l /bin/cat
> ...
> Program Headers:
>   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
>   PHDR           0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4
>   INTERP         0x000154 0x08048154 0x08048154 0x00013 0x00013 R   0x1
>       [Requesting program interpreter: /lib/ld-linux.so.2]
>   LOAD           0x000000 0x08048000 0x08048000 0x0a00c 0x0a00c R E 0x1000
>   LOAD           0x00af04 0x08053f04 0x08053f04 0x00258 0x00820 RW  0x1000
>   DYNAMIC        0x00af10 0x08053f10 0x08053f10 0x000e8 0x000e8 RW  0x4
>   NOTE           0x000168 0x08048168 0x08048168 0x00044 0x00044 R   0x4
>   GNU_EH_FRAME   0x009008 0x08051008 0x08051008 0x002d4 0x002d4 R   0x4
>   GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
>   GNU_RELRO      0x00af04 0x08053f04 0x08053f04 0x000fc 0x000fc R   0x1
> ==============================
>
> So there are just 2 PT_LOAD segments. But why kernel maps 3 memory regions
> ? The following is the maps output:
>
> Though the second PT_LOAD starts at the file offset  0xaf04, The first fc
bytes belong to GNU_RELRO segment (The last entry in the Program Header).
If you add af04 +fc you get afff. Looks like they have placed this section
advertently in such a way that the actual DATA segment will start at the
next virtual address page boundary 0x08054000. Thus the GNU_RELRO section
with read-only permissions is placed in the separate virtual address
region.
    This is the result of the maps file you see below.

> ============================
> cat /proc/self/maps
>
> 08048000-08053000 r-xp 00000000 08:01 261656     /bin/cat
> 08053000-08054000 r--p 0000a000 08:01 261656     /bin/cat
> 08054000-08055000 rw-p 0000b000 08:01 261656     /bin/cat
> 09b58000-09b79000 rw-p 00000000 00:00 0          [heap]
> b75bd000-b75be000 rw-p 00000000 00:00 0
> b75be000-b7761000 r-xp 00000000 08:01 523958     /lib/i386-linux-gnu/
> libc-2.15.so
> ...
> ==================
>
> The above output, there are 3 memory regions for "/bin/cat", and what is
> the following segment:
>
> 08053000-08054000 r--p 0000a000 08:01 261656     /bin/cat
>
> According the 'cat' program header, there is no "r" segment.
>
>
> Regards,
> Jacky
>
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