CODEWITHSUNDEEP
c++elfabirelocation
K.D.Dilshan
K.D.Dilshan

Reputation: 467

Managing Relocation Order Dependencies in ELF Shared Libraries

When you link an application let's call it main_program with a shared library message_library on a Linux device, there's a scenario to consider. This shared library contains a global variable message that both the library and the program need to use. Now, during the linking process, relocations happen. These relocations are important because they ensure everything works correctly. It's crucial to understand how the order of these relocations is managed to make sure everything behaves as expected.

Here's a simplified breakdown of the process:

main_program.cpp

#include <iostream>
extern const char* message;
void print_message();

int main() {
    std::cout << message << '\n';
    print_message();
}

message_lib.cpp

#include <iostream>
const char* message = "Meow!";
void print_message() {
    std::cout << message << '\n';
}

Compilation and linking:

$ g++ main_app.cpp -c -pie -o main_app.o
$ g++ message_lib.cpp -fPIC -shared -o libmessage.so
$ g++ main_app.o libmessage.so -o main_app

Relevant relocations as reported by readelf:

main_app:

Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000004150  000c00000005 R_X86_64_COPY     0000000000004150 message + 0

For libmessage.so:

Offset          Info           Type           Sym. Value    Sym. Name + Addend
000000004040  000000000008 R_X86_64_RELATIVE                    2000
000000003fd8  000b00000006 R_X86_64_GLOB_DAT 0000000000004040 message + 0

My question is: How is the order for applying relocations specified in ELF shared libraries? Is there a predefined mechanism within the ELF format or the ABI that dictates this order? Alternatively, does the dynamic linker autonomously resolve relocation dependencies to ensure correct behavior?

Furthermore, is there an implicit assumption that the static linker arranges relocations in such a way that those within the executable can be processed after those within shared libraries?

Upvotes: 0

Views: 14

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