Dynamically linked libraries..

vishalbutte · 04-16-2006, 02:42 PM

Hey anyone knows which function loads the dynamically linked libraries required by executables?
Or is there really such function in linux kernel?
I want to k @ runtime which shared library is required by the executable..?

your replies are always welcome!!

futhark · 04-16-2006, 02:46 PM

If you want to know what shared libraries a program uses, try "ldd".

If you wonder what C functions do that, have a look at "man dlopen". Can't help you with those though.

vishalbutte · 04-16-2006, 02:51 PM

thanks for the reply..
I want the function in the kernel code which loads the dynamically linked libraries..?

Mara · 04-16-2006, 03:40 PM

The kernel doesn't load libraries (exception: kernel modules). What you probably mean is dlopen or its internals. Is it correct?

vishalbutte · 04-17-2006, 07:26 AM

yes you got the point..

If kernel comes to know that the currently executing executable requires some dynamically linked library the what it does?

aluser · 04-17-2006, 08:40 PM

I did a fair amount of grepping and munging around to get a partial answer to this question. It seems part of the answer is in fs/binfmt_elf.c; load_elf_binary() calls load_elf_interp(), which in turn sets up /lib/ld-linux.so.2 to do the actual work. I suspect that the code for ld-linux is in glibc-2.4.tar.gz which you can get here: http://directory.fsf.org/GNU/glibc.html , but I haven't pinned down where yet.

The reason I know that the interpreter is /lib/ld-linux.so.2 is this:

Code:

$ objdump -p hw | grep -A1 INTERP
  INTERP off    0x00000114 vaddr 0x08048114 paddr 0x08048114 align 2**0
         filesz 0x00000013 memsz 0x00000013 flags r--
$ dd if=hw bs=1 count=19 skip=276 2>/dev/null; echo
/lib/ld-linux.so.2

(0x114 is 276 and 0x13 is 19) You can see load_elf_binary() reading this string from the file analogously to my dd command:

Code:

		if (elf_ppnt->p_type == PT_INTERP) {
/* skip a bit */
			retval = kernel_read(bprm->file, elf_ppnt->p_offset,
					   elf_interpreter,
					   elf_ppnt->p_filesz);

If you strace the executable (mine is just 'hello world'), you'll see the loader mmapping shared libraries:

Code:

$ strace ./hw
execve("./hw", ["./hw"], [/* 40 vars */]) = 0
/* skip a bit... */
open("/lib/tls/libc.so.6", O_RDONLY)    = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0\320O\1"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1266800, ...}) = 0
old_mmap(NULL, 1272764, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0xb7e6a000
/* more... */

So ultimately, the code to load a shared library is just an mmap(), after read()ing the elf header from the library. See 'man 5 elf' for the elf format.

I would feel a lot more confident of everything I just wrote if I had disected ld-linux. I haven't, so take it all with several grains of salt...

aluser · 04-17-2006, 08:54 PM

of course all that assumes that you're concerned with normal shared libraries which are loaded by the loader at runtime but before your program starts executing. If you mean to load a library later, use dlopen() or such