Carnegie Mellon Introduction to Computer Systems 15 21318

Carnegie Mellon Introduction to Computer Systems 15 -213/18 -243, fall 2009 9 th Lecture, Sep 29 th Instructors: Roger B. Dannenberg and Greg Ganger 1

Carnegie Mellon First Exam <20 21 -30 31 -40 41 -50 51 -60 61 -70 71 -80 81 -90 91 -100 Exam 1 Grade as Percent 2

Carnegie Mellon Today ¢ ¢ Address Space at Run Time Linking 3

Carnegie Mellon IA 32 Linux Memory Layout ¢ ¢ not drawn to scale C 0000000 Stack § Runtime stack (8 MB limit) Heap § Dynamically allocated storage § When call malloc(), calloc(), new() Data § Statically allocated data § E. g. , arrays & strings declared in code Text § Executable machine instructions § Read-only Stack 8 MB 08000000 Heap Data Text 4

Carnegie Mellon Memory Allocation Example C 0000000 not drawn to scale Stack char big_array[1<<24]; /* 16 MB */ char huge_array[1<<28]; /* 256 MB */ int beyond; char *p 1, *p 2, *p 3, *p 4; int useless() { int { p 1 p 2 p 3 p 4 /* } return 0; } main() = malloc(1 Some print <<28); /* << 8); /* statements Where does everything go? 256 256. . . MB B */ */ */ 08000000 Heap Data Text 5

Carnegie Mellon not drawn to scale IA 32 Example Addresses address range $esp p 3 p 1 p 4 p 2 &p 2 beyond big_array huge_array main() useless() final malloc() C 0000000 ~232 0 xffffbcd 0 0 x 65586008 0 x 55585008 0 x 1904 a 110 0 x 1904 a 008 0 x 18049760 0 x 08049744 0 x 18049780 0 x 08049760 0 x 080483 c 6 0 x 08049744 0 x 006 be 166 malloc() is dynamically linked address determined at runtime Stack 80000000 Heap 08000000 00 Data Text 6

Carnegie Mellon x 86 -64 Example Addresses not drawn to scale 00007 F Stack address range ~247 $rsp p 1 p 3 p 4 p 2 beyond big_array &p 2 huge_array main() useless() final malloc() 0 x 7 fff, 7576, 0050 0 x 2 b 7 f, e 3 f 7, 1010 0 x 2 b 7 f, f 3 f 7, 2010 0 x 0000, 1150, 1120 0 x 0000, 1150, 1010 0 x 0000, 1150, 0 a 28 0 x 0000, 1050, 0 a 20 0 x 0000, 1050, 0 a 00 0 x 0000, 0040, 0510 0 x 0000, 0040, 0500 0 x 0038, 6 ae 6, a 170 malloc() is dynamically linked address determined at runtime 000030 Heap Data Text 000000 7

Carnegie Mellon Example C Program main. c swap. c int buf[2] = {1, 2}; extern int buf[]; int main() { swap(); return 0; } static int *bufp 0 = &buf[0]; static int *bufp 1; void swap() { int temp; } bufp 1 = &buf[1]; temp = *bufp 0; *bufp 0 = *bufp 1; *bufp 1 = temp; 8

Carnegie Mellon Static Linking ¢ Programs are translated and linked using a compiler driver: unix> gcc -O 2 -g -o p main. c swap. c unix>. /p main. c swap. c Translators (cpp, cc 1, as) Source files Translators (cpp, cc 1, as) main. o swap. o Separately compiled relocatable object files Linker (ld) p Fully linked executable object file (contains code and data for all functions defined in main. c and swap. c 9

Carnegie Mellon 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 10

Carnegie Mellon Why Linkers? Efficiency! ¢ Time: Separate Compilation § Change one source file, compile, and then relink. § No need to recompile other source files. ¢ Space: Libraries § 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. 11

Carnegie Mellon What Do Linkers Do? ¢ Step 1: Symbol resolution § Programs define and reference symbols (variables and functions): void swap() {…} § swap(); § int *xp = &x; § /* define symbol swap */ /* reference symbol swap */ /* define xp, reference x */ § Symbol definitions are stored (by compiler) in symbol table. Symbol table is an array of structs § Each entry includes name, type, size, and location of symbol. § § Linker associates each symbol reference with exactly one symbol definition. 12

Carnegie Mellon What Do Linkers Do? (cont. ) ¢ Step 2: Relocation § Merges separate code and data sections into single sections § Relocates symbols from their relative locations in the. o files to their final absolute memory locations in the executable. § Updates all references to these symbols to reflect their new positions. 13

Carnegie Mellon Three Kinds of Object Files (Modules) ¢ Relocatable object file (. o file) § Contains code and data in a form that can be combined with other relocatable object files to form executable object file. § Each. o file is produced from exactly one source (. c) file ¢ 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) § Special type of relocatable object file that can be loaded into memory and linked dynamically, at either load time or run-time. § Called Dynamic Link Libraries (DLLs) by Windows 14

Carnegie Mellon Executable and Linkable Format (ELF) ¢ ¢ Standard binary format for object files Originally proposed by 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 15

Carnegie Mellon ELF Object File Format ¢ Elf header § Word size, byte ordering, file type (. o, exec, . so), machine type, etc. ¢ Segment header table § For executables: virtual address, segment size, alignments ¢ . text section § Code ¢ . rodata section ¢ § Read only data: jump tables, . . data section § Initialized global variables ¢ . bss section § Uninitialized global variables § “Block Started by Symbol” § “Better Save Space” § Has section header but occupies no space ELF header 0 Segment header table (required for executables). text section. rodata section. bss section. symtab section. rel. txt section. rel. data section. debug section Section header table 16

Carnegie Mellon 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 § Info for symbolic debugging (gcc -g) Section header table § Offsets and sizes of each section ELF header 0 Segment header table (required for executables). text section. rodata section. bss section. symtab section. rel. txt section. rel. data section. debug section Section header table 17

Carnegie Mellon Linker Symbols ¢ Global symbols § Symbols defined by module m that can be referenced by other modules. § E. g. : 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. § E. g. : C functions and variables defined with the static attribute. § Local linker symbols are not local program variables 18

Carnegie Mellon Resolving Symbols Global External Local int buf[2] = {1, 2}; extern int buf[]; int main() { swap(); return 0; } static int *bufp 0 = &buf[0]; static int *bufp 1; External main. c void swap() { int temp; Linker knows nothing of temp } Global bufp 1 = &buf[1]; temp = *bufp 0; *bufp 0 = *bufp 1; *bufp 1 = temp; swap. c 19

Carnegie Mellon Relocating Code and Data Relocatable Object Files System code . text System data Executable Object File 0 Headers System code main() main. o swap() main() . text int buf[2]={1, 2} . data More system code . text System data int buf[2]={1, 2} int *bufp 0=&buf[0] Uninitialized data. symtab. debug swap. o swap() int *bufp 0=&buf[0]. data int *bufp 1. bss . text . data. bss 20

Carnegie Mellon Relocation Info (main) main. c main. o int buf[2] = {1, 2}; 0000000 <main>: int main() { swap(); return 0; } 0: 1: 3: 6: b: d: f: 10: 55 89 e 5 83 ec 08 e 8 fc ff ff ff 31 c 0 89 ec 5 d c 3 push %ebp mov %esp, %ebp sub $0 x 8, %esp call 7 <main+0 x 7> 7: R_386_PC 32 swap xor %eax, %eax mov %ebp, %esp pop %ebp ret Disassembly of section. data: 0000 <buf>: 0: 01 00 00 00 02 00 00 00 Source: objdump 21

Carnegie Mellon Relocation Info (swap, . text) swap. c swap. o extern int buf[]; Disassembly of section. text: static int *bufp 0 = &buf[0]; static int *bufp 1; 0000 <swap>: 0: 55 1: 8 b 15 00 00 void swap() { int temp; } bufp 1 = &buf[1]; temp = *bufp 0; *bufp 0 = *bufp 1; *bufp 1 = temp; 7: a 1 c: 89 e: c 7 15: 00 18: 1 a: 1 c: 1 e: 89 8 b 89 a 1 23: 89 25: 5 d 26: c 3 push %ebp mov 0 x 0, %edx 3: R_386_32 bufp 0 0 00 00 00 mov 0 x 4, %eax 8: R_386_32 buf e 5 mov %esp, %ebp 05 00 00 04 movl $0 x 4, 0 x 0 00 00 10: R_386_32 bufp 1 14: R_386_32 buf ec mov %ebp, %esp 0 a mov (%edx), %ecx 02 mov %eax, (%edx) 00 00 mov 0 x 0, %eax 1 f: R_386_32 bufp 1 08 mov %ecx, (%eax) pop %ebp ret 22

Carnegie Mellon Relocation Info (swap, . data) swap. c extern int buf[]; static int *bufp 0 = &buf[0]; static int *bufp 1; Disassembly of section. data: 0000 <bufp 0>: 0: 00 00 0: R_386_32 buf void swap() { int temp; } bufp 1 = &buf[1]; temp = *bufp 0; *bufp 0 = *bufp 1; *bufp 1 = temp; 23

Carnegie Mellon Executable After Relocation (. text) 080483 b 4 <main>: 80483 b 4: 55 80483 b 5: 89 80483 b 7: 83 80483 ba: e 8 80483 bf: 31 80483 c 1: 89 80483 c 3: 5 d 80483 c 4: c 3 080483 c 8 <swap>: 80483 c 8: 55 80483 c 9: 8 b 80483 cf: a 1 80483 d 4: 89 80483 d 6: c 7 80483 dd: 94 80483 e 0: 89 80483 e 2: 8 b 80483 e 4: 89 80483 e 6: a 1 80483 eb: 89 80483 ed: 5 d 80483 ee: c 3 e 5 ec 08 09 00 00 00 c 0 ec 15 58 e 5 05 04 ec 0 a 02 48 08 5 c 94 04 08 48 95 04 08 58 08 95 04 08 push mov sub call xor mov pop ret %ebp %esp, %ebp $0 x 8, %esp 80483 c 8 <swap> %eax, %eax %ebp, %esp %ebp push mov movl %ebp 0 x 804945 c, %edx 0 x 8049458, %eax %esp, %ebp $0 x 8049458, 0 x 8049548 mov mov mov pop ret %ebp, %esp (%edx), %ecx %eax, (%edx) 0 x 8049548, %eax %ecx, (%eax) %ebp 24

Carnegie Mellon Executable After Relocation (. data) Disassembly of section. data: 08049454 <buf>: 8049454: 01 00 00 00 02 00 00 00 0804945 c <bufp 0>: 804945 c: 54 94 04 08 25

Carnegie Mellon Strong and Weak Symbols ¢ Program symbols are either strong or weak § Strong: procedures and initialized globals § Weak: uninitialized globals p 1. c p 2. c strong int foo=5; int foo; weak strong p 1() { } p 2() { } strong 26

Carnegie Mellon Linker’s Symbol Rules ¢ Rule 1: Multiple strong symbols are not allowed § Each item can be defined only once § Otherwise: Linker error ¢ Rule 2: Given a strong symbol and multiple weak symbol, choose the strong symbol § References to the weak symbol resolve to the strong symbol ¢ Rule 3: If there are multiple weak symbols, pick an arbitrary one § Can override this with gcc –fno-common 27

Carnegie Mellon Linker Puzzles int x; p 1() {} Link time error: two strong symbols (p 1) int x; p 1() {} int x; p 2() {} References to x will refer to the same uninitialized int. Is this what you really want? int x; int y; p 1() {} double x; p 2() {} Writes to x in p 2 might overwrite y! Evil! int x=7; int y=5; p 1() {} double x; p 2() {} Writes to x in p 2 will overwrite y! Nasty! int x=7; p 1() {} int x; p 2() {} References to x will refer to the same initialized variable. Nightmare scenario: two identical weak structs, compiled by different compilers with different alignment rules. 28

Carnegie Mellon Global Variables ¢ Avoid if you can ¢ Otherwise § Use static if you can § Initialize if you define a global variable § Use extern if you use external global variable 29

Carnegie Mellon Packaging Commonly Used Functions ¢ How to package functions commonly used by programmers? § Math, I/O, memory management, string manipulation, etc. ¢ Awkward, given the linker framework so far: § Option 1: Put all functions into a single source file Programmers link big object file into their programs § Space and time inefficient § Option 2: Put each function in a separate source file § Programmers explicitly link appropriate binaries into their programs § More efficient, but burdensome on the programmer § 30

Carnegie Mellon Solution: Static Libraries ¢ Static libraries (. a archive files) § Concatenate related relocatable object files into a single file with an index (called an archive). § Enhance linker so that it tries to resolve unresolved external references by looking for the symbols in one or more archives. § If an archive member file resolves reference, link into executable. 31

Carnegie Mellon Creating Static Libraries atoi. c printf. c Translator atoi. o printf. o Archiver (ar) libc. a ¢ ¢ random. c . . . Translator random. o unix> ar rs libc. a atoi. o printf. o … random. o C standard library Archiver allows incremental updates Recompile function that changes and replace. o file in archive. 32

Carnegie Mellon Commonly Used Libraries libc. a (the C standard library) § 8 MB archive of 900 object files. § I/O, memory allocation, signal handling, string handling, data and time, random numbers, integer math libm. a (the C math library) § 1 MB archive of 226 object files. § floating point math (sin, cos, tan, log, exp, sqrt, …) % ar -t /usr/libc. a | sort … fork. o … fprintf. o fpu_control. o fputc. o freopen. o fscanf. o fseek. o fstab. o … % ar -t /usr/libm. a | sort … e_acos. o e_acosf. o e_acoshf. o e_acoshl. o e_acosl. o e_asinf. o e_asinl. o … 33

Carnegie Mellon Linking with Static Libraries addvec. o multvec. o main 2. c vector. h Translators (cpp, cc 1, as) Relocatable object files main 2. o Archiver (ar) libvector. a addvec. o libc. a Static libraries printf. o and any other modules called by printf. o Linker (ld) p 2 Fully linked executable object file 34

Carnegie Mellon Using Static Libraries ¢ Linker’s algorithm for resolving external references: § Scan. o files and. a files in the command line order. § During the scan, keep a list of the current unresolved references. § As each new. o or. a file, obj, is encountered, try to resolve each unresolved reference in the list against the symbols defined in obj. § If any entries in the unresolved list at end of scan, then error. ¢ Problem: § Command line order matters! § Moral: put libraries at the end of the command line. unix> gcc -L. libtest. o -lmine unix> gcc -L. -lmine libtest. o: In function `main': libtest. o(. text+0 x 4): undefined reference to `libfun' 35

Carnegie Mellon Loading Executable Object Files Executable Object File ELF header 0 Kernel virtual memory 0 xc 0000000 Program header table (required for executables) User stack (created at runtime) . init section. text section. rodata section Memory invisible to user code %esp (stack pointer) Memory-mapped region for shared libraries 0 x 40000000 . data section. bss section Run-time heap (created by malloc) . symtab. debug Read/write segment (. data, . bss) . line. strtab Section header table (required for relocatables) Read-only segment (. init, . text, . rodata) 0 x 08048000 0 brk Loaded from the executable file Unused 36

Carnegie Mellon Shared Libraries ¢ Static libraries have the following disadvantages: § Duplication in the stored executables (every function need std libc) § Duplication in the running executables § Minor bug fixes of system libraries require each application to explicitly relink ¢ Modern Solution: Shared Libraries § Object files that contain code and data that are loaded and linked into an application dynamically, at either load-time or run-time § Also called: dynamic link libraries, DLLs, . so files 37

Carnegie Mellon Shared Libraries (cont. ) ¢ Dynamic linking can occur when executable is first loaded and run (load-time linking). § Common case for Linux, handled automatically by the dynamic linker (ld-linux. so). § Standard C library (libc. so) usually dynamically linked. ¢ Dynamic linking can also occur after program has begun (run-time linking). § In Unix, this is done by calls to the dlopen() interface. High-performance web servers. § Runtime library interpositioning § ¢ Shared library routines can be shared by multiple processes. § More on this when we learn about virtual memory 38

Carnegie Mellon Dynamic Linking at Load-time main 2. c vector. h Translators (cpp, cc 1, as) Relocatable object file main 2. o unix> gcc -shared -o libvector. so addvec. c multvec. c libc. so libvector. so Relocation and symbol table info Linker (ld) Partially linked executable object file p 2 Loader (execve) libc. so libvector. so Code and data Fully linked executable in memory Dynamic linker (ld-linux. so) 39

Carnegie Mellon Dynamic Linking at Runtime #include <stdio. h> #include <dlfcn. h> int x[2] = {1, 2}; int y[2] = {3, 4}; int z[2]; int main() { void *handle; void (*addvec)(int *, int); char *error; /* dynamically load the shared lib that contains addvec() */ handle = dlopen(". /libvector. so", RTLD_LAZY); if (!handle) { fprintf(stderr, "%sn", dlerror()); exit(1); } 40

Carnegie Mellon Dynamic Linking at Run-time. . . /* get a pointer to the addvec() function we just loaded */ addvec = dlsym(handle, "addvec"); if ((error = dlerror()) != NULL) { fprintf(stderr, "%sn", error); exit(1); } /* Now we can call addvec() it just like any other function */ addvec(x, y, z, 2); printf("z = [%d %d]n", z[0], z[1]); } /* unload the shared library */ if (dlclose(handle) < 0) { fprintf(stderr, "%sn", dlerror()); exit(1); } return 0; 41

Carnegie Mellon Summary ¢ ELF files contain § Object files § Libraries § Executables ¢ ¢ Linking Loading Dynamic Linking Details: § How are globals, externals, static symbols handled? § How are names searched and resolved by linkers? § How can you interpose your own library implementation? 45