When / How does Linux load shared libraries into address space?

When is the address of shared objects specified in programs? During linking? Loading? If I wanted to find the memory address of the system command inside of libc inside of my program I could find it easily in gdb, but what i开发者_如何学运维f I don't want to bring the program into a debugger?

Could this address change from run to run? Are there any other static analysis tool that will allow be to view where libraries or functions will be loaded into this program's memory space when run?

I want this information outside of the program (ie. using utilities like objdump to gather information)

Libraries are loaded by ld.so (dynamic linker or run-time linker aka rtld, ld-linux.so.2 or ld-linux.so.* in case of Linux; part of glibc). It is declared as "interpreter" (INTERP; .interp section) of all dynamic linked ELF binaries. So, when you start program, Linux will start an ld.so (load into memory and jump to its entry point), then ld.so will load your program into memory, prepare it and then run it. You can also start dynamic program with

 /lib/ld-linux.so.2 ./your_program your_prog_params

ld.so does an actual open and mmap of all needed ELF files, both ELF file of your program and ELF files of all neeeded libraries. Also, it fills GOT and PLT tables and does relocations resolving (it writes addresses of functions from libraries to call sites, in many cases with indirect calls).

The typical load address of some library you can get with ldd utility. It is actually a bash script, which sets a debug environment variable of ld.so (actually LD_TRACE_LOADED_OBJECTS=1 in case of glibc's rtld) and starts a program. You even can also do it yourself without needs of the script, e.g. with using bash easy changing of environment variables for single run:

 LD_TRACE_LOADED_OBJECTS=1 /bin/echo

The ld.so will see this variable and will resolve all needed libraries and print load addresses of them. But with this variable set, ld.so will not actually start a program (not sure about static constructors of program or libraries). If the ASLR feature is disabled, load address will be the same most times. Modern Linuxes often has ASLR enabled, so to disable it, use echo 0 | sudo tee /proc/sys/kernel/randomize_va_space.

You can find offset of system function inside the libc.so with nm utility from binutils. I think, you should use nm -D /lib/libc.so or objdump -T /lib/libc.so and grep output.

"Go right to the source and ask the horse..."

Drepper - How To Write Shared Libraries

Must-read documentation for Linux library writers. Explains the mechanics of loading in some detail.

If you just want the address of a function while not hardcoding the name, you could dlopen() the main program:

void *self = dlopen(NULL, RTLD_NOW);
dlsym(self, "system"); // returns the pointer to the system() function

If you just want the address of a function of which you know the name at compile-time, simply use void *addr = &system;

The nm command, used on libc.so, will show you the location of the system symbol in libc.so. However, if ASLR is enabled, the address libc.so is loaded at, and thus the final address of system will vary randomly each time your program is run. Even without ASLR, you'll need to determine the address libc.so gets loaded at and offset the address of system by that amount.

Id recommend that your environment have the LD_LIBRARY_PATH path. This defines where the shared libraries are to be found. You may also have to look into /etc/ld.so.conf Look at this posting http://www.google.com/url?sa=t&source=web&cd=3&ved=0CCQQFjAC&url=http%3A%2F%2Fubuntuforums.org%2Fshowthread.php%3Ft%3D324660&ei=KqJpTey7JofEsAPE9_imBA&usg=AFQjCNEIbGGrTHp4fufRuj4Yfc58RTHcag&sig2=_9tdlyadMbPc-FcOdCko-w