Re: why the DSO was loaded at address 0

[OndÅej BÃlka <neleai at seznam dot cz>]

On Tue, May 21, 2013 at 08:06:47PM +0800, Abin Xu wrote:
> There is a simple test program,it dlopen()s a DSO "/lib/libc.so.6" and
> then prints the address of symbol "printf".
> 
> #include <dlfcn.h>
> #include <stdio.h>
> 
> int main() {
>     void* handle;
>     void (*func)();
>     char* error_test;
>     if (handle = dlopen("/lib/libc.so.6", RTLD_NOW)) {
>         func = dlsym(handle, "printf");
>         (*func)("address:%p\n",func);
>         sleep(-1);
>         dlclose(handle);
>         return 0;
>     }
>     return -1;
> }
> 
> 
> 
> When I debug it with "LD_DEBUG=all",it shows that:
> 
> [root@localhost glibc_test]# LD_DEBUG=all LD_DEBUG_OUTPUT=error ./test &
> [18] 14690
> [root@localhost glibc_test]# address:0x2f87b0
> 
> 
> 
> But the file "error" shows that "/lib/libc.so.6" was loaded to address
> 0! That`s why?

Where did you found that /lib/libc.so.6 is loaded to address 0?
everything here looks like working as expected.

>      14690:    initialize program: ./test
>      14690:
>      14690:
>      14690:    transferring control: ./test
>      14690:
>      14690:    symbol=dlopen;  lookup in file=./test [0]
>      14690:    symbol=dlopen;  lookup in file=/changed_glibc/lib/libdl.so.2 [0]
>      14690:    binding file ./test [0] to
> /changed_glibc/lib/libdl.so.2 [0]: normal symbol `dlopen' [GLIBC_2.1]
>      14690:
>      14690:    file=/lib/libc.so.6 [0];  needed by ./test [0]
>      14690:    file=/lib/libc.so.6 [0];  generating link map
>      14690:      dynamic: 0x0043cd7c  base: 0x00000000   size: 0x00191988
>      14690:        entry: 0x002c5e40  phdr: 0x002af034  phnum:         10
> 
> 
> 
> What`s more,the "/proc/14270/maps" file perform normal,and the address
> of symbol "func"(0x2f87b0) in the test program lies in right
> range(002af000-0043b000).
> 
> [root@localhost ~]# cat /proc/14690/maps
> 002af000-0043b000 r-xp 00000000 fd:00 1714117    /lib/libc-2.12.so
> 0043b000-0043d000 r--p 0018c000 fd:00 1714117    /lib/libc-2.12.so
> 0043d000-0043e000 rw-p 0018e000 fd:00 1714117    /lib/libc-2.12.so
> 0043e000-00441000 rw-p 00000000 00:00 0