Introduction to Computer Systems 15-213/18-243, spring 2009

Introduction to Computer Systems 15-213/18-243, spring 2009

Carnegie Mellon 14-513 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 18-613 1 Carnegie Mellon Linking 15-213/18-213/14-513/15-513/18-613: Introduction to Computer Systems 14th Lecture, October 10th, 2019 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 2 Carnegie Mellon Today Linking

Motivation What it does How it works Dynamic linking Case study: Library interpositioning Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 3 Carnegie Mellon Example C Program int sum(int *a, int n); int array[2] = {1, 2}; int main(int argc, char** argv) { int val = sum(array, 2); return val; } main.c Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition int sum(int *a, int n) { int i, s = 0; for (i = 0; i < n; i++) { s += a[i]; } return s;

} sum.c 4 Carnegie Mellon Linking Programs are translated and linked using a compiler driver: linux> gcc -Og -o prog main.c sum.c linux> ./prog main.c sum.c Translators (cpp, cc1, as) Translators (cpp, cc1, as) main.o sum.o Source files Separately compiled relocatable object files Linker (ld)

prog Fully linked executable object file (contains code and data for all functions defined in main.c and sum.c) Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 5 Carnegie Mellon Why Linkers? Reason 1: 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 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 6 Carnegie Mellon Why Linkers? (cont)

Reason 2: Efficiency Time: Separate compilation Change one source file, compile, and then relink. No need to recompile other source files. Can compile multiple files concurrently. Space: Libraries Common functions can be aggregated into a single file... Option 1: Static Linking Executable files and running memory images contain only the library code they actually use Option 2: Dynamic linking Executable files contain no library code During execution, single copy of library code can be shared across all executing processes Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 7 Carnegie Mellon What Do Linkers Do? Step 1: Symbol resolution Programs define and reference symbols (global variables and functions): void swap() {} swap(); int *xp = &x; /* define symbol swap */

/* reference symbol swap */ /* define symbol xp, reference x */ Symbol definitions are stored in object file (by assembler) in symbol table. Symbol table is an array of entries Each entry includes name, size, and location of symbol. During symbol resolution step, the linker associates each symbol reference with exactly one symbol definition. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 8 Carnegie Mellon Symbols in Example C Program Definitions int sum(int *a, int n); int sum(int *a, int n) { int i, s = 0; int array[2] = {1, 2}; int main(int argc, char** argv) { int val = sum(array, 2); return val; } main.c

for (i = 0; i < n; i++) { s += a[i]; } return s; } sum.c Reference Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 9 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. Lets look at these two steps in more detail. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 10

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 (a.out 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 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 11 Carnegie Mellon Executable and Linkable Format (ELF) Standard binary format for object files

One unified format for Relocatable object files (.o), Executable object files (a.out) Shared object files (.so) Generic name: ELF binaries Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 12 Carnegie Mellon ELF Object File Format Elf header Word size, byte ordering, file type (.o, exec, .so), machine type, etc. Segment header table Page size, virtual address memory segments (sections), segment sizes.

.text section Code .rodata section Read only data: jump tables, string constants, ... .data section Initialized global variables .bss section Uninitialized global variables Block Started by Symbol Better Save Space Has section header but occupies no space Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 0 ELF header Segment header table (required for executables) .text section .rodata section .data section .bss section .symtab section .rel.txt section .rel.data section .debug section

Section header table 13 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 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 0 ELF header Segment header table (required for executables) .text section .rodata section .data section .bss section .symtab section .rel.txt section .rel.data section .debug section Section header table 14 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 global variables defined with the static attribute. Local linker symbols are not local program variables Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 15 Carnegie Mellon Step 1: Symbol Resolution Referencing a global thats defined here int sum(int *a, int n); int array[2] = {1, 2}; int main(int argc,char **argv) { int val = sum(array, 2); return val; } main.c Defining

a global int sum(int *a, int n) { int i, s = 0; } for (i = 0; i < n; i++) { s += a[i]; } return s; sum.c Referencing Linker knows a global nothing of val thats defined here Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition Linker knows nothing of i or s 16 Carnegie Mellon Symbol Identification Which of the following names will be in the symbol table of symbols.o? Names: symbols.c: int incr = 1;

static int foo(int a) { int b = a + incr; return b; } int main(int argc, char* argv[]) { printf("%d\n", foo(5)); return 0; } Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition incr foo a argc argv b main printf "%d\n" Others?

Can find this with readelf: linux> readelf s symbols.o 17 Carnegie Mellon Local Symbols Local non-static C variables vs. local static C variables local non-static C variables: stored on the stack local static C variables: stored in either .bss, or .data static int x = 15; int f() { static int x = 17; return x++; } int g() { static int x = 19; return x += 14; } Compiler allocates space in .data for each definition of x Creates local symbols in the symbol table with unique names, e.g., x, x.1721 and x.1724. int h() { return x += 27; }

static-local.c Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 18 Carnegie Mellon How Linker Resolves Duplicate Symbol Definitions Program symbols are either strong or weak Strong: procedures and initialized globals Weak: uninitialized globals Or ones declared with specifier extern p1.c p2.c strong int foo=5; int foo; weak strong p1() { }

p2() { } strong Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 19 Carnegie Mellon Linkers 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 symbols, 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

Puzzles on the next slide Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 20 Carnegie Mellon Linker Puzzles int x; p1() {} p1() {} Link time error: two strong symbols (p1) int x; p1() {} int x; p2() {} References to x will refer to the same uninitialized int. Is this what you really want? int x; int y; p1() {} double x; p2() {} int x=7;

int y=5; p1() {} double x; p2() {} Writes to x in p2 might overwrite y! Nasty! int x=7; p1() {} int x; p2() {} References to x will refer to the same initialized variable. Writes to x in p2 might overwrite y! Evil! Important: Linker does not do type checking. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 21 Carnegie Mellon Type Mismatch Example long int x; /* Weak symbol */

int main(int argc, char *argv[]) { printf("%ld\n", x); return 0; } /* Global strong symbol */ double x = 3.14; mismatch-main.c mismatch-variable.c Compiles without any errors or warnings What gets printed? Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 22 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 reference an external global variable Treated as weak symbol But also causes linker error if not defined in some file Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 23 Carnegie Mellon Use of extern in .h Files (#1) c1.c #include "global.h" int f() { return g+1; } global.h extern int g; int f(); c2.c #include #include "global.h int g = 0; int main(int argc, char argv[]) { int t = f(); printf("Calling f yields %d\n", t); return 0;

} Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 24 Carnegie Mellon Linking Example int sum(int *a, int n); int array[2] = {1, 2}; int main(int argc,char **argv) { int val = sum(array, 2); return val; } main.c Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition int sum(int *a, int n) { int i, s = 0; } for (i = 0; i < n; i++) { s += a[i]; } return s; sum.c 26

Carnegie Mellon Step 2: Relocation Relocatable Object Files System code .text System data .data Executable Object File 0 Headers System code main() main.o main() .text int array[2]={1,2} .data sum() More system code System data sum.o sum()

.text Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition .text int array[2]={1,2} .data .symtab .debug 27 Carnegie Mellon Relocation Entries int array[2] = {1, 2}; int main(int argc, char** argv) { int val = sum(array, 2); return val; } main.c 0000000000000000 0: 48 83 ec 4: be 02 00 9:

bf 00 00 e: 13: 17:

: 08 00 00 00 00 e8 00 00 00 00 48 83 c4 08 c3 sub mov mov a: R_X86_64_32 $0x8,%rsp $0x2,%esi $0x0,%edi array # %edi = &array # Relocation entry callq 13 # sum() f: R_X86_64_PC32 sum-0x4 # Relocation entry add $0x8,%rsp retq

Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition main.o Source: objdump r d main.o 28 Carnegie Mellon Relocated .text section 00000000004004d0

: 4004d0: 48 83 ec 4004d4: be 02 00 4004d9: bf 18 10 4004de: e8 05 00 4004e3: 48 83 c4 4004e7: c3 08 00 00 60 00 00 00 08 sub

mov mov callq add retq 00000000004004e8 : 4004e8: b8 00 00 00 00 4004ed: ba 00 00 00 00 4004f2: eb 09 4004f4: 48 63 ca 4004f7: 03 04 8f 4004fa: 83 c2 01 4004fd: 39 f2 4004ff: 7c f3 400501: f3 c3 $0x8,%rsp $0x2,%esi $0x601018,%edi 4004e8 $0x8,%rsp # %edi = &array

# sum() mov $0x0,%eax mov $0x0,%edx jmp 4004fd movslq %edx,%rcx add (%rdi,%rcx,4),%eax add $0x1,%edx cmp %esi,%edx jl 4004f4 repz retq callq instruction uses PC-relative addressing for sum(): 0x4004e8 = 0x4004e3 + 0x5 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition Source: objdump -d prog 29 Carnegie Mellon Loading Executable Object Files Executable Object File ELF header

Kernel virtual memory 0 User stack (created at runtime) Program header table (required for executables) .init section .text section Memory invisible to user code %rsp (stack pointer) Memory-mapped region for shared libraries .rodata section .data section .bss section brk Run-time heap (created by malloc) .symtab .debug

Read/write data segment (.data, .bss) .line .strtab Section header table (required for relocatables) 0x400000 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 0 Read-only code segment (.init, .text, .rodata) Loaded from the executable file Unused 30 Carnegie Mellon Quiz Time! Check out: https://canvas.cmu.edu/courses/10968

Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 31 Carnegie Mellon Libraries: Packaging a Set of 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 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 32 Carnegie Mellon Old-fashioned 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 it into the executable. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 33 Carnegie Mellon Creating Static Libraries atoi.c printf.c Translator Translator atoi.o random.c ... printf.o

random.o unix> ar rs libc.a \ atoi.o printf.o random.o Archiver (ar) libc.a Translator C standard library Archiver allows incremental updates Recompile function that changes and replace .o file in archive. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 34 Carnegie Mellon Commonly Used Libraries libc.a (the C standard library) 4.6 MB archive of 1496 object files. I/O, memory allocation, signal handling, string handling, data and time, random numbers, integer math libm.a (the C math library) 2 MB archive of 444 object files. floating point math (sin, cos, tan, log, exp, sqrt, )

% ar t /usr/lib/libc.a | sort fork.o fprintf.o fpu_control.o fputc.o freopen.o fscanf.o fseek.o fstab.o Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition % ar t /usr/lib/libm.a | sort e_acos.o e_acosf.o e_acosh.o e_acoshf.o e_acoshl.o e_acosl.o e_asin.o e_asinf.o e_asinl.o 35 Carnegie Mellon Linking with Static Libraries #include

#include "vector.h" int x[2] = {1, 2}; int y[2] = {3, 4}; int z[2]; int main(int argc, char** argv) { addvec(x, y, z, 2); printf("z = [%d %d]\n, z[0], z[1]); return 0; main2.c } libvector.a void addvec(int *x, int *y, int *z, int n) { int i; for (i = 0; i < n; i++) z[i] = x[i] + y[i]; } addvec.c void multvec(int *x, int *y, int *z, int n) { int i; for (i = 0; i < n; i++) z[i] = x[i] * y[i]; }

Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition multvec.c 36 Carnegie Mellon Linking with Static Libraries addvec.o multvec.o main2.c vector.h Translators (cpp, cc1, as) Relocatable object files main2.o Archiver (ar) libvector.a addvec.o Linker (ld) prog2c libc.a Static libraries printf.o and any other modules called by printf.o

unix> gcc static o prog2c \ main2.o -L. -lvect Fully linked executable object file (861,232 bytes) c for compile-time Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 37 Carnegie Mellon Using Static Libraries Linkers 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 -static -o prog2c -L. -lvector main2.o main2.o: In function `main': main2.c:(.text+0x19): undefined reference to `addvec' collect2: error: ld returned 1 exit status

Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 38 Carnegie Mellon Modern Solution: Shared Libraries Static libraries have the following disadvantages: Duplication in the stored executables (every function needs libc) Duplication in the running executables Minor bug fixes of system libraries require each application to explicitly relink Rebuild everything with glibc? https://security.googleblog.com/2016/02/cve-2015-7547-glibc-geta ddrinfo-stack.html 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 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 39 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 Linux, this is done by calls to the dlopen() interface. Distributing software. 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 Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 40 Carnegie Mellon What dynamic libraries are required?

.interp section Specifies the dynamic linker to use (i.e., ld-linux.so) .dynamic section Specifies the names, etc of the dynamic libraries to use Follow an example of prog (NEEDED) Shared library: [libm.so.6] Where are the libraries found? Use ldd to find out: unix> ldd prog linux-vdso.so.1 => (0x00007ffcf2998000) libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f99ad927000) /lib64/ld-linux-x86-64.so.2 (0x00007f99adcef000) Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 41 Carnegie Mellon Dynamic Library Example addvec.c

multvec.c unix> gcc Og c addvec.c multvec.c -fpic Translator Translator addvec.o multvec.o unix> gcc -shared -o libvector.so \ addvec.o multvec.o Loader (ld) libvector.so Dynamic vector library Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 42 Carnegie Mellon Dynamic Linking at Load-time main2.c vector.h unix> gcc -shared -o libvector.so \ addvec.c multvec.c -fpic

Translators (cpp, cc1, as) Relocatable object file libc.so libvector.so Relocation and symbol table info main2.o Linker (ld) Partially linked executable object file (8488 bytes) prog2l Loader (execve) unix> gcc o prog2l \ main2.o ./libvector.so libc.so libvector.so Code and data Fully linked executable in memory

Dynamic linker (ld-linux.so) Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 43 Carnegie Mellon Dynamic Linking at Run-time #include #include #include int x[2] = {1, 2}; int y[2] = {3, 4}; int z[2]; int main(int argc, char** argv) { void *handle; void (*addvec)(int *, int *, int *, int); char *error; /* Dynamically load the shared library that contains addvec() */ handle = dlopen("./libvector.so", RTLD_LAZY); if (!handle) { fprintf(stderr, "%s\n", dlerror()); exit(1); } . . . dll.c Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 44 Carnegie Mellon

Dynamic Linking at Run-time (cont) ... /* Get a pointer to the addvec() function we just loaded */ addvec = dlsym(handle, "addvec"); if ((error = dlerror()) != NULL) { fprintf(stderr, "%s\n", error); exit(1); } /* Now we can call addvec() 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, "%s\n", dlerror()); exit(1); } return 0; } Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition dll.c 45 Carnegie Mellon Dynamic Linking at Run-time dll.c vector.h unix> gcc -shared -o libvector.so \ addvec.c multvec.c -fpic

Translators (cpp, cc1, as) libc.so Relocatable object file dll.o libvector.so Relocation and symbol table info Linker (ld) Partially linked executable object file (8784 bytes) Fully linked executable in memory unix> gcc -rdynamic o prog2r \ dll.o -ldl prog2r libc.so Loader Code and data (execve) Dynamic linker (ld-linux.so) Call to dynamic linker via dlopen

Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 46 Carnegie Mellon Linking Summary Linking is a technique that allows programs to be constructed from multiple object files. Linking can happen at different times in a programs lifetime: Compile time (when a program is compiled) Load time (when a program is loaded into memory) Run time (while a program is executing) Understanding linking can help you avoid nasty errors and make you a better programmer. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 47 Carnegie Mellon Today

Linking Case study: Library interpositioning Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 48 Carnegie Mellon Case Study: Library Interpositioning Documented in Section 7.13 of book Library interpositioning : powerful linking technique that allows programmers to intercept calls to arbitrary functions Interpositioning can occur at: Compile time: When the source code is compiled Link time: When the relocatable object files are statically linked to form an executable object file Load/run time: When an executable object file is loaded into memory, dynamically linked, and then executed. Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 49 Carnegie Mellon

Some Interpositioning Applications Security Confinement (sandboxing) Behind the scenes encryption Debugging In 2014, two Facebook engineers debugged a treacherous 1-year old bug in their iPhone app using interpositioning Code in the SPDY networking stack was writing to the wrong location Solved by intercepting calls to Posix write functions (write, writev, pwrite) Source: Facebook engineering blog post at: https://code.facebook.com/posts/313033472212144/debugging-file-corruption-on-ios/ Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 50 Carnegie Mellon Some Interpositioning Applications Monitoring and Profiling Count number of calls to functions Characterize call sites and arguments to functions Malloc tracing

Detecting memory leaks Generating address traces Error Checking C Programming Lab used customized versions of malloc/free to do careful error checking Other labs (malloc, shell, proxy) also use interpositioning to enhance checking capabilities Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 51 Carnegie Mellon Example program #include #include #include int main(int argc, char *argv[]) { int i; for (i = 1; i < argc; i++) { void *p =

malloc(atoi(argv[i])); free(p); } return(0); } int.c Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition Goal: trace the addresses and sizes of the allocated and freed blocks, without breaking the program, and without modifying the source code. Three solutions: interpose on the library malloc and free functions at compile time, link time, and load/run time. 52 Carnegie Mellon Compile-time Interpositioning #ifdef COMPILETIME #include #include /* malloc wrapper function */ void *mymalloc(size_t size) { void *ptr = malloc(size);

printf("malloc(%d)=%p\n", (int)size, ptr); return ptr; } /* free wrapper function */ void myfree(void *ptr) { free(ptr); printf("free(%p)\n", ptr); } #endif Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition mymalloc.c 53 Carnegie Mellon Compile-time Interpositioning #define malloc(size) mymalloc(size) #define free(ptr) myfree(ptr) void *mymalloc(size_t size); void myfree(void *ptr); malloc.h linux> make intc gcc -Wall -DCOMPILETIME -c mymalloc.c gcc -Wall -I. -o intc int.c mymalloc.o linux> make runc ./intc 10 100 1000 Search for leads to malloc(10)=0x1ba7010 /usr/include/malloc.h free(0x1ba7010) malloc(100)=0x1ba7030

free(0x1ba7030) malloc(1000)=0x1ba70a0 free(0x1ba70a0) Search for leads to linux> Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 54 Carnegie Mellon Link-time Interpositioning #ifdef LINKTIME #include void *__real_malloc(size_t size); void __real_free(void *ptr); /* malloc wrapper function */ void *__wrap_malloc(size_t size) { void *ptr = __real_malloc(size); /* Call libc malloc */ printf("malloc(%d) = %p\n", (int)size, ptr); return ptr; } /* free wrapper function */ void __wrap_free(void *ptr) { __real_free(ptr); /* Call libc free */ printf("free(%p)\n", ptr); } #endif Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition mymalloc.c

55 Carnegie Mellon Link-time Interpositioning linux> make intl Search for leads to gcc -Wall -DLINKTIME -c mymalloc.c /usr/include/malloc.h gcc -Wall -c int.c gcc -Wall -Wl,--wrap,malloc -Wl,--wrap,free -o intl \ int.o mymalloc.o linux> make runl ./intl 10 100 1000 malloc(10) = 0x91a010 free(0x91a010) . . . The -Wl flag passes argument to linker, replacing each comma with a space. The --wrap,malloc arg instructs linker to resolve references in a special way: Refs to malloc should be resolved as __wrap_malloc Refs to __real_malloc should be resolved as malloc Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 56

Carnegie Mellon #ifdef RUNTIME #define _GNU_SOURCE #include #include #include Load/Run-time Interpositioning Observe that DONT have #include /* malloc wrapper function */ void *malloc(size_t size) { void *(*mallocp)(size_t size); char *error; mallocp = dlsym(RTLD_NEXT, "malloc"); /* Get addr of libc malloc */ if ((error = dlerror()) != NULL) { fputs(error, stderr); exit(1); } char *ptr = mallocp(size); /* Call libc malloc */ printf("malloc(%d) = %p\n", (int)size, ptr); return ptr; } Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition mymalloc.c 57

Carnegie Mellon Load/Run-time Interpositioning /* free wrapper function */ void free(void *ptr) { void (*freep)(void *) = NULL; char *error; if (!ptr) return; freep = dlsym(RTLD_NEXT, "free"); /* Get address of libc free */ if ((error = dlerror()) != NULL) { fputs(error, stderr); exit(1); } freep(ptr); /* Call libc free */ printf("free(%p)\n", ptr); } #endif mymalloc.c Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 58 Carnegie Mellon Load/Run-time Interpositioning linux> make intr gcc -Wall -DRUNTIME -shared -fpic -o mymalloc.so mymalloc.c -ldl gcc -Wall -o intr int.c linux> make runr (LD_PRELOAD="./mymalloc.so" ./intr 10 100 1000)

malloc(10) = 0x91a010 Search for leads to free(0x91a010) /usr/include/malloc.h . . . linux> The LD_PRELOAD environment variable tells the dynamic linker to resolve unresolved refs (e.g., to malloc)by looking in mymalloc.so first. Type into (some) shells as: env LD_PRELOAD=./mymalloc.so ./intr 10 100 1000) Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 59 Carnegie Mellon Interpositioning Recap Compile Time Apparent calls to malloc/free get macro-expanded into calls to mymalloc/myfree Simple approach. Must have access to source & recompile

Link Time Use linker trick to have special name resolutions malloc __wrap_malloc __real_malloc malloc Load/Run Time Implement custom version of malloc/free that use dynamic linking to load library malloc/free under different names Can use with ANY dynamically linked binary env LD_PRELOAD=./mymalloc.so gcc c int.c) Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 60 Carnegie Mellon Linking Recap Usually: Just happens, no big deal Sometimes: Strange errors Bad symbol resolution Ordering dependence of linked .o, .a, and .so files

For power users: Interpositioning to trace programs with & without source Bryant and OHallaron, Computer Systems: A Programmers Perspective, Third Edition 61

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  • An Approach for Developing Biological Reference Points for ...

    An Approach for Developing Biological Reference Points for ...

    These results are consistent with other analysis for yearling anadromous salmonids, that dome shape models (Ricker) do not fit this life history type well. Yellow Line- drainage area only Fitting a curve with no S/R data Pink Line - Individual...
  • Agenda Topic - ohio.edu

    Agenda Topic - ohio.edu

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  • apworlduhs.weebly.com

    apworlduhs.weebly.com

    Merchants established a second major gold-salt trade route northeast across the Sahara that passed through Tunis, and Cairo, and ended in Egypt's interior. This route complimented the traditional Western Sudan--Maghreb--Europe trade route. As the second trade route grew in popularity,...
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    Sample Selection Example - University of Notre Dame

    Sample Selection Example Bill Evans * * Draw 10,000 obs at random educ uniform over [0,16] age uniform over [18,64] wearnl=4.49 + 0.08*educ + 0.012*age + ε Generate missing data for wearnl * drawn from standard normal [0,1] d*=-1.5+0.15*educ+0.01*age+0.15*z+v wearnl...