15 213 The course that gives CMU its Zip Linking Oct 9 2003 Topics class14 ppt Static linking Dynamic linking Case study Library interpositioning Example C Program main c swap c int buf 2 1 2 extern int buf int main swap return 0 int bufp0 buf 0 int bufp1 void swap int temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp 2 15 213 F 03 Static Linking Programs are translated and linked using a compiler driver unix gcc O2 g o p main c swap c unix p main c swap c Translators cpp cc1 as Source files Translators cpp cc1 as main o swap o Separately compiled relocatable object files Linker ld p 3 Fully linked executable object file contains code and data for all functions defined in main c and swap c 15 213 F 03 Why Linkers Modularity Program can be written as a collection of smaller source files rather than one monolithic mass Can build libraries of common functions more on this later e g Math library standard C library Efficiency Time Change one source file compile and then relink No need to recompile other source files Space Libraries of common functions can be aggregated into a single file Yet executable files and running memory images contain only code for the functions they actually use 4 15 213 F 03 What Does a Linker Do Symbol resolution Programs define and reference symbols variables and functions code a data int xp x reference to symbol a reference to symbol x Symbol definitions are stored by compiler in symbol table Each entry includes name size and location of object Linker associates each symbol reference with exactly one symbol definition Relocation 5 Merges separate code and data sections into single sections Relocates symbols from their relative locations in the o files to new absolute positions in the executable Updates all references to these symbols to reflect their new positions 15 213 F 03 Object Files Modules Relocatable object file o file Contains code and data in a form that can be combined with other relocatable object files at compile time to form an executable Executable object file Contains code and data in a form that can be copied directly into memory and then executed Shared object file so file 6 Special type of relocatable object file that can be loaded into memory and linked dynamically at either load time or runtime Called Dynamic Link Libraries DLLs by Windows 15 213 F 03 Executable and Linkable Format ELF Standard binary format for object files Derives from AT T System V Unix Later adopted by BSD Unix variants and Linux One unified format for Relocatable object files o Executable object files Shared object files so Generic name ELF binaries 7 15 213 F 03 ELF Object File Format Elf header Magic number type o exec so machine byte ordering etc Program header table Page size virtual addresses memory segments sections segment sizes text section Code data section Initialized static data bss section 8 Uninitialized static data Block Started by Symbol Better Save Space Has section header but occupies no space ELF header Program header table required for executables text section data section bss section symtab rel txt rel data debug Section header table required for relocatables 15 213 F 03 0 ELF Object File Format cont symtab section Symbol table Procedure and static variable names Section names and locations rel text section Relocation info for text section Addresses of instructions that will need to be modified in the executable Instructions for modifying rel data section Relocation info for data section Addresses of pointer data that will need to be modified in the merged executable debug section 9 ELF header Program header table required for executables text section data section bss section symtab rel text rel data debug Section header table required for relocatables Info for symbolic debugging gcc g 15 213 F 03 0 Linker Symbols Global symbols Symbols defined by module m that can be referenced by other modules Ex non static C functions and non static global variables External symbols Global symbols that are referenced by module m but defined by some other module Local symbols Symbols that are defined and referenced exclusively by module m Ex C functions and variables that are defined with the static attribute Key Point Local linker symbols are not local program variables 10 15 213 F 03 Resolving Symbols Ref to external symbol buf Def of global symbol buf swap c main c int buf 2 1 2 static int bufp0 buf 0 static int bufp1 int main swap return 0 Ref to external symbol swap extern int buf void swap int temp Def of local symbol bufp0 Linker knows nothing of temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp 11 15 213 F 03 Relocating Symbol Definitions and References Relocatable Object Files system code text system data data Executable Object File 0 headers system code main m o a o 12 main text int e 7 data a text int ep e int x 15 int y data bss text a more system code system data int e 7 int ep e int x 15 uninitialized data symtab debug data bss 15 213 F 03 main o Relocation Info int buf 2 1 2 int main swap return 0 0000000 main 0 55 1 89 e5 3 83 ec 08 6 e8 fc ff ff ff b d f 10 31 c0 89 ec 5d c3 push ebp mov esp ebp sub 0x8 esp call 7 main 0x7 7 R 386 PC32 swap xor eax eax mov ebp esp pop ebp ret Disassembly of section data 00000000 buf 0 01 00 00 00 02 00 00 00 Source objdump 13 15 213 F 03 swap o Relocation Info text extern int buf static int bufp0 buf 0 static int bufp1 void swap int temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp Disassembly of section text 00000000 swap 0 55 1 8b 15 00 00 00 00 7 a1 c 89 e c7 15 00 18 1a 1c 1e 89 8b 89 a1 23 89 25 5d 26 c3 14 push ebp mov 0x0 edx 3 R 386 32 bufp0 0 00 00 00 mov 0x4 eax 8 R 386 32 buf e5 mov esp ebp 05 00 00 00 00 04movl 0x4 0x0 00 00 10 R 386 32 bufp1 14 R 386 32 buf ec mov ebp esp 0a mov edx ecx 02 mov eax edx 00 00 00 00 mov 0x0 eax 1f R 386 32 bufp1 08 mov ecx eax pop ebp ret 15 213 F 03 a o Relocation Info data extern int buf Disassembly of section data static int bufp0 buf 0 static int bufp1 00000000 bufp0 0 00 00 00 00 0 R 386 32 buf void swap int temp bufp1 buf 1 temp bufp0 bufp0 bufp1 bufp1 temp 15 15 213 F 03 Executable After Relocation text 080483b4 main 80483b4 55 80483b5 89 80483b7 83 80483ba e8 80483bf 31 80483c1 89 80483c3 5d 80483c4 c3 080483c8 swap 80483c8 55 80483c9 8b 80483cf a1 80483d4 89 80483d6 c7 80483dd …
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