dlopen with two shared libraries, exporting symbols
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.
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.
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