why the DSO was loaded at address 0

[Florian Weimer]

On 05/21/2013 04:24 PM, Paul Pluzhnikov wrote:
> On Tue, May 21, 2013 at 5:06 AM, Abin Xu <abin1525@gmail.com> wrote:
>
>> But the file "error" shows that "/lib/libc.so.6" was loaded to address 0!
>
> No, it does not.
>
>>       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
>
> Presumably you are mis-interpreting base:0x00000000 is the load
> address. It is not.
>
> What it is is the relocation (difference) between linked-at address
> and loaded-at address.
>
> Usually, shared libraries are linked at address 0, and when they are,
> the relocation is indeed the same as loaded-at address.  But your
> libc.so.6 has been prelinked to address 0x002af000, and was loaded at
> that address, giving you 0 relocation.

By the way, do you know the correct way to obtain an address from 
dladdr() which can be passed to addr2line?  So far, I haven't managed to 
do this when prelinking is involved.

-- 
Florian Weimer / Red Hat Product Security Team