Dynamic Linking

Dynamic-linked executable: at runtime, the dynamic linker (ld-linux.so) resolves shared library symbols.

Pros:

• Smaller executables.
• Memory shared across processes (libc loaded once).
• Easier to update libraries (security patches without rebuild).

Cons:

• Runtime dependency.
• Slight startup cost.
• Versioning headaches ("DLL hell").

ELF interpreter:

• PT_INTERP segment names the dynamic linker (typically /lib64/ld-linux-x86-64.so.2).
• Kernel loads BOTH the executable AND the interpreter.
• Jumps to interpreter's entry point (NOT the executable's).
• Interpreter resolves symbols + then calls executable's entry.

Steps the dynamic linker performs:

1. Load all needed libraries (DT_NEEDED entries in .dynamic).
2. Recurse: each library's deps too.
3. Build symbol lookup table.
4. Resolve relocations: for each unresolved address in code/data, look up actual addr.
5. Initialize libraries (run constructors / DT_INIT_ARRAY).
6. Jump to executable's entry.

ldd /bin/cat:

linux-vdso.so.1
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6
/lib64/ld-linux-x86-64.so.2

Library search:

• DT_RPATH / DT_RUNPATH (built into binary).
• LD_LIBRARY_PATH env.
• /etc/ld.so.cache (cached results of /etc/ld.so.conf paths).
• Default: /lib, /usr/lib.

LD_PRELOAD: forces loading of a library FIRST, overriding symbols. Used for debugging, instrumentation, malicious purposes.

LD_DEBUG=all: trace dynamic linker actions.

ld.so versioning:

• glibc tied to specific ld.so.
• Major version change = ABI break.
• Multiple ld.so coexist on same system rare.