Re: Library mapping

[Muzaffer Ozakca <muzaffer dot ozakca at bilten dot metu dot edu dot tr>]

Hello,

Regarding the things you've said below, I wonder who decides where these 
segments get mapped. I've been checking the source code for elf binary 
handler functions, load_elf_interp and load_elf_binary, they seem to be 
the functions making do_mmap() calls, reading from the elf header of an 
elf executable.

I am really confused, where and how the addresses where libraries' text, 
data, bss are decided. What algorithm is used in doing it and where in 
source tree is it? Is it do_mmap() who decides it?

( note: I know that this is not even close to be related with glibc, sorry)


Martin v. Loewis wrote:

>>I'm watching through /proc/<pid>/maps on my Linux 2.4.3 computers that 
>>the same program gets its libraries (and their data segments) mapped to 
>>different locations on virtual memory. The same OS configuration and 
>>libraries are installed on a (1) Pentium 225 and a (2) Pentium III.
>>
> 
> When you say "the same", do you mean "the very same", i.e. with no
> differences whatsoever in md5sums of all shared libraries? Including
> ld-linux.so.2?
> 
> 
>>I'm a compiling the program on (1) and executing on both of them. I'm 
>>expecting to see their /proc/<pid>/maps be the same for the same point 
>>in the program. But they are not. Might this be because of their 
>>hardware differences? 
>>
> 
> No, it is rather because differences in the environment (in a general
> sense). Things that may contribute:
> 
> - exact setting of the enviroment variables (in the strict sense of
>   execve)
> - size, contents, and layout of /etc/ld.so.cache
> - path name of each file that is being loaded
> - contents of each directory that is searched when loading shared libraries,
>   and exact order of entries returned by getdents(2)
> - value of cwd string
> 
> 
>>When a program compiled (non-statically), are the locations where
>>shared objects are mapped known at compile time or are they resolved
>>at load time by ld.so?
>>
> 
> They are resolved by ld.so, on a first-come-first-served basis.
> 
> 
>>Note: I'm doing some low level stuff to overwrite the data segments
>>for libraries after loading.
>>
> 
> Then you better use the source, Luke.
> 
> Regards,
> Martin
> 
>