Re: How to use a different ld-linux.so?

[John Reiser]

> I have a need to use a different runtime linker (ld-linux.so) for some
> of my applications.  I _don't_ want to change the executables themselves
> (changing the path to the runtime linker in the ELF image for example).

Why not?  http://www.BitWagon.com/rtldi/rtldi.html

> I know that I can just run my application as an argument to ld-linux.so,
> and that actually does work very well.  But, I am left with two
> problems:
>
>   * Some of my applications fork/exec other programs, and those other
>     programs also need to use the other runtime linker.  Just as I don't
>     want to change the ELF image, I certainly don't want to have to
>     recompile the programs to exec the runtime linker!

fork() is never a problem, but it is necessary to intercept execve()
if you wish to invoke a different executable or to change its environment.

>   * Debugging: I can't debug because I can't find a way to convince GDB
>     to invoke the program-to-be-debugged using an alternative runtime
>     linker.

        $ gdb /lib/ld-linux.so.2
        (gdb) run --library-path /lib /bin/date
If you want to set breakpoints then put one near the end of _dl_start_user
where it invokes _dl_init_internal.  Inspect /proc/<pid>/maps to find out
what has been mapped, then use the add-symbol-file command of gdb.
Find the offset from "objdump --section-headers a.elf  |  grep text"
then add the base from the map of the address space.

> ... another idea is to use LD_PRELOAD
> to install a private version of the execve() system call, which would
> set the alternative ld-linux.so as the first argument.

By using LD_PRELOAD one can supersede the visible uses of execve() [the ones
that use global symbol linkage to invoke execve()], but there can be "bare"
system calls that do not use symbols.