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07. Symbols, Resolution, and Relocation Mechanics (Symbols & Relocation)

Analyzing how compilers and linkers bind variable and function names to memory addresses through Symbol Resolution and patch instruction operands via Relocation.

1. Learning Objectives & Overview

  • Understand the layout of the ELF symbol table (Elf64_Sym) and the distinctions between Local, Global, and Weak symbols.
  • Master the Three Rules of Strong vs Weak Symbol Resolution when multiple definitions collide.
  • Compare symbol name mangling between C and C++.
  • Compute relocation offsets using the standard formula: \(\text{Offset} = S + A - P\).

2. Interactive Symbol Resolution & Relocation Diagram

Step through the 4 stages from unresolved symbols to patched opcodes below:

3. Symbol Table Architecture (Elf64_Sym)

Names declared in source code are represented within .symtab:

typedef struct {
    Elf64_Word    st_name;  /* Offset into .strtab string table */
    unsigned char st_info;  /* Type (FUNC, OBJECT) and Binding (LOCAL, GLOBAL, WEAK) */
    unsigned char st_other; /* Visibility (DEFAULT, HIDDEN) */
    Elf64_Section st_shndx; /* Section index (SHN_UNDEF if unresolved) */
    Elf64_Addr    st_value; /* Section offset or final virtual address */
    Elf64_Xword   st_size;  /* Size of the symbol object */
} Elf64_Sym;

4. Three Rules of Strong vs. Weak Symbols

Functions and initialized global variables are Strong symbols, while uninitialized globals and __attribute__((weak)) symbols are Weak symbols:

  1. Rule 1 (No Multiple Strong Symbols):
  2. Multiple strong symbols with the same name trigger a fatal link error (multiple definition of ...).
  3. Rule 2 (Strong Overrides Weak):
  4. Given one strong symbol and multiple weak symbols sharing a name, the linker selects the strong definition (Weak Symbol Overriding).
  5. Rule 3 (Arbitrary Weak Selection):
  6. Given only weak definitions, the linker arbitrarily chooses one.

[!TIP] Library Function Interception: Core C library functions (e.g., malloc, free) are provided as weak symbols, allowing performance profilers or security hooks to override standard routines cleanly.

5. Relocation Calculation Formula

Compilers emit placeholder bytes (e8 00 00 00 00) for unresolved calls, leaving relocation entries in .rela.text:

5.1 Common x86_64 Relocation Types

  • R_X86_64_PC32 (32-bit PC-Relative Offset): $\(\text{Offset} = S + A - P\)$
  • \(S\) (Symbol): Final virtual address of target function.
  • \(A\) (Addend): Fixed adjustment constant (typically -4).
  • \(P\) (Place): Address of the instruction being patched.
  • R_X86_64_64 (64-bit Absolute Address): $\(\text{Address} = S + A\)$

6. Lab Source Code & Weak Symbol Override Lab

6.1 Default Build (Weak Symbol Active)

cd labs/principles/07-symbols-linking
make run

=== [1] Running Default Build (Weak Symbol in effect) ===
./sym_demo_default
============================================================
 Symbol Resolution & Relocation Demonstration
============================================================
[+] calculate_add(10, 20) = 30
[+] g_operation_count address: 0x404024 (val=1)
[+] Calling custom_hook() at 0x4011e0:
    [calc.c] Default WEAK hook executed (no override provided).
============================================================

6.2 Overridden Build (Strong Symbol Takes Precedence)

make run-override

=== [2] Running Override Build (Strong Symbol overrides Weak) ===
./sym_demo_override
============================================================
 Symbol Resolution & Relocation Demonstration
============================================================
[+] calculate_add(10, 20) = 30
[+] g_operation_count address: 0x404024 (val=1)
[+] Calling custom_hook() at 0x4011e0:
    [weak_override.c] ★ STRONG hook successfully overrode the weak symbol!
============================================================

7. Summary & Next Chapter

  • Linkers resolve symbols through binding precedence and patch binary instruction targets via relocation formulas.
  • In the next chapter, we combine modules into a standalone binary: 08. Static Linking and Binary Loading.