Question

The commands

gcc main.c -o main
ldd main

yield

linux-gate.so.1 => (0x00f67000)
libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0x00b7d000)
/lib/ld-linux.so.2 (0x00ae5000)

Is it possible to change the address where libc.so.6 will be mapped in memory? For example, have libc.so.6 instead be mapped at, say, 0xb0000000.

I’m running Xubuntu 32bit: Linux 3.2.0-23-generic i686 i686 i386 GNU/Linux

Answer 1

There is an prelink utility (wiki page) which is capable of changing loading address (it is called "base address") of so libs.

There is some info about how prelink works: http://www.acsu.buffalo.edu/~charngda/elf.html

What does this prelink do? It changes the base address of a dynamic library to the actual address in the user program’s address space when it is loaded into memory. Of course, ld.so recognizes GNU_PRELINKED tag and will load a dynamic library to its this base address (recall the first argument of mmap is the preferred address; of course, this is subject to the operating system.)

Normally, a dynamic library is built as position independent code, i.e. the -fPIC compiler command-line option, and thus the base address is 0. For example, a normal libc.so has ELF program header as follows (readelf -l command)…

According to man prelink there is an option of prelink utility to rebase (relocate) given library to specified address:

-r --reloc-only=ADDRESS
Instead of prelinking, just relink given shared libraries to the specified base address.

The prelink --reloc-only=0x7896000 libc.so.6 should be enough to achieve the change you want.

PS: you can do this relocation on your local copy of libc and then give the path to it via export LD_LIBRARY_PATH=/path/to/your/rebased/copy:$LD_LIBRARY_PATH