dlopen + dlsym

From: William Ahern (william_at_wilbur.25thandClement.com)
Date: 07/30/04

  • Next message: Ulrich Eckhardt: "Re: dlopen + dlsym" 
    Date: Thu, 29 Jul 2004 15:34:49 -0700

    Problem:

    Load a library using dlopen(3) which provides an alternative malloc(3) (and
    eventually connect(2) and other calls).

    Solution:

    At first I thought that after loading the library w/ dlopen(3) everything
    would just magically work; libdl would sequentially interpose each and every
    malloc() symbol found.

    Not so.

    So, I accepted that I would have to provide my own malloc(3) *within* the
    main binary, which itself would call dlsym(RTLD_NEXT,"malloc"). I had hoped
    not, because I didn't want to have to predetermine all the library calls a
    loadable module could interpose, but w'ever.

    However, this doesn't work, either! The only way I can get such libary
    iterposition to work is to 1) LD_PRELOAD the modules or 2) manually pull out
    the symbol using dlsym(my_module_handle,"malloc").

    The problem w/ LD_PRELOAD is that I want to load modules according to
    directives in a configuration file. The problem w/ manually pulling out the
    symbols from each dlopen handle using dlsym is that I don't know what the
    interposition ordering is, so that when the module uses
    dlsym(RTLD_NEXT,"malloc") itself if may or may not get a symbol to the
    malloc definition in the next module, or from libc itself.

    Any ideas on what I'm doing wrong, or if this is possible at all?
    I'm using a recent glibc (trying this on Mandrake 9.1 and Debian/Testing).

    TIA,

    Bill

  • Next message: Ulrich Eckhardt: "Re: dlopen + dlsym"