Suppose I have an ELF binary that’s dynamic linked, and I want to override/redirect certain library calls. I know I can do this with LD_PRELOAD, but I want a solution that’s permanent in the binary, independent of the environment, and that works for setuid/setgid binaries, none of which LD_PRELOAD can achieve.
What I’d like to do is add code from additional object files (possibly in new sections, if necessary) and add the symbols from these object files to the binary’s symbol table so that the newly added version of the code gets used in place of the shared library code. I believe this should be possible without actually performing any relocations in the existing code; even though they’re in the same file, these should be able to be resolved at runtime in the usual PLT way (for what it’s worth I only care about functions, not data).
Please don’t give me answers along the line of “You don’t want to do this!” or “That’s not portable!” What I’m working on is a way of interfacing binaries with slightly-ABI-incompatible alternate shared-library implementations. The platform in question is i386-linux (i.e. 32-bit) if it matters. Unless I’m mistaken about what’s possible, I could write some tools to parse the ELF files and perform my hacks, but I suspect there’s a fancy way to use the GNU linker and other tools to accomplish this without writing new code.
I suggest the
elfshet al. tools from the ERESI (alternate) project, if you want to instrument the ELF files themselves. Compatibility with i386-linux is not a problem, as I’ve used it myself for the same purpose.The relevant how-tos are here.