dlopen with two shared libraries, exporting symbols

Question

I have a linux shared library, foo.so, which is loaded from an executable using dlopen("foo.so", RTLD_NOW | RTLD_LOCAL). From foo.so I'd like to dlopen another library, bar.so, which references symbols defined in foo.so, but the linker fails to find them. I can't change RTLD_LOCAL to RTLD_GLOBAL, because I don't have the source to the executable doing the loading. I thought -Wl,--export-dynamic when linking foo.so might help but it doesn't override the local flag to dlopen. GCC's new attribute visibility feature doesn't look like it offers the answer either.

Is there a way I can instruct the linker to resolve references to undefined symbols in bar.so to those definitions in foo.so, without linking bar with -lfoo or similarity moving the symbols into a 3rd library and linking both foo and bar against it? The only thing that occurs to me is to dlopen foo.so with RTLD_GLOBAL from within foo.so itself, then dlopen bar.so, but that strikes me as a bit of a mess. Thanks.

Answer 1

Link foo.so against bar.so.

When the executable dlopen()s foo.so, bar.so will also be loaded.

Alternatively, binary-patch the executable to add RTLD_GLOBAL to the flags for dlopen() call. The code will look something like

    movl    $2, 4(%esp)       # $2 == RTLD_NOW; RTLD_LOCAL is 0
    movl    $0xNNNNN, (%esp)  # $0xNNNNN == &"foo.so"
    call    dlopen

Patch it to movl $0x102, 4(%esp) instead (RTLD_GLOBAL == 0x100), and voilà.

EDIT:
If you know the name of bar.so, then you can link foo.so against a "stub" bar.so. It doesn't matter that you don't have "real" bar.so; all that matters is that foo.so has a dependency on it. At runtime that dependency will cause bar.so to be loaded whenever foo.so is loaded.