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@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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