  1#include <stdio.h>
  2#include <string.h>
  3#include <assert.h>
  4#include <stdint.h>
  5#include <stdlib.h>
  6#include <stdbool.h>
  7#include <unistd.h>
  8#include <sys/mman.h>
  9#include <fcntl.h>
 10#include "dl.h"
 11
 12#define SIZE 4096
 13#define LENGTH(arr) (sizeof(arr) / sizeof(arr[0]))
 14
 15struct dlib {
 16  struct dl_hdr hdr;
 17  struct symbol *symtab; // borrowed spaces from header
 18  const char *path;
 19};
 20
 21static struct dlib *dlopen(const char *path);
 22
 23struct dlib *dlopen_chk(const char *path) {
 24  struct dlib *lib = dlopen(path);
 25  if (!lib) {
 26    fprintf(stderr, "Not a valid dlib file: %s.\n", path);
 27    exit(1);
 28  }
 29  return lib;
 30}
 31
 32// Implementation of binutils
 33
 34void dl_gcc(const char *path) {
 35  char buf[256], *dot = strrchr(path, '.');
 36  if (dot) {
 37    *dot = '\0';
 38    sprintf(buf, "gcc -m64 -fPIC -c %s.S && "
 39      "objcopy -S -j .text -O binary %s.o %s.dl", path, path, path);
 40    system(buf);
 41  }
 42}
 43
 44
 45void dl_readdl(const char *path) {
 46  struct dlib *h = dlopen_chk(path);
 47  printf("DLIB file %s:\n\n", h->path);
 48  for (struct symbol *sym = h->symtab; sym->type; sym++) {
 49    switch (sym->type) {
 50      case '+': printf("    LOAD  %s\n", sym->name); break;
 51      case '?': printf("  EXTERN  %s\n", sym->name); break;
 52      case '#': printf(   "%08lx  %s\n", sym->offset, sym->name); break;
 53    }
 54  }
 55}
 56
 57void dl_objdump(const char *path) {
 58  struct dlib *h = dlopen_chk(path);
 59  char *hc = (char *)h, cmd[64];
 60  FILE *fp = NULL;
 61
 62  printf("Disassembly of binary %s:\n", h->path);
 63
 64  for (char *code = hc + h->hdr.code_off; code < hc + h->hdr.file_sz; code++) {
 65    for (struct symbol *sym = h->symtab; sym->type; sym++) {
 66      if (hc + sym->offset == code) {
 67        int off = code - hc - h->hdr.code_off;
 68        if (fp) pclose(fp);
 69        sprintf(cmd, "ndisasm - -b 64 -o 0x%08x\n", off);
 70        fp = popen(cmd, "w");
 71        printf("\n%016x <%s>:\n", off, sym->name);
 72        fflush(stdout);
 73      }
 74    }
 75    if (fp) fputc(*code, fp);
 76  }
 77  if (fp) pclose(fp);
 78}
 79
 80// binutils: interpreter
 81void dl_interp(const char *path) {
 82  struct dlib *h = dlopen_chk(path);
 83  int (*entry)() = NULL;
 84  for (struct symbol *sym = h->symtab; sym->type; sym++)
 85    if (strcmp(sym->name, "main") == 0)
 86      entry = (void *)((char *)h + sym->offset);
 87  if (entry) {
 88    exit(entry());
 89  }
 90}
 91
 92struct cmd {
 93  const char *cmd;
 94  void (*handler)(const char *path);
 95} commands[] = {
 96  { "gcc",     dl_gcc },
 97  { "readdl",  dl_readdl },
 98  { "objdump", dl_objdump },
 99  { "interp",  dl_interp },
100  { "",        NULL },
101};
102
103int main(int argc, char *argv[]) {
104  if (argc < 3) {
105    fprintf(stderr, "Usage: %s {gcc|readdl|objdump|interp} FILE...\n", argv[0]);
106    return 1;
107  }
108
109  for (struct cmd *cmd = &commands[0]; cmd->handler; cmd++) {
110    for (char **path = &argv[2]; *path && strcmp(argv[1], cmd->cmd) == 0; path++) {
111      if (path != argv + 2) printf("\n");
112      cmd->handler(*path);
113    }
114  }
115}
116
117// Implementation of dlopen()
118
119static struct symbol *libs[16], syms[128];
120
121static void *dlsym(const char *name);
122static void dlexport(const char *name, void *addr);
123static void dlload(struct symbol *sym);
124
125static struct dlib *dlopen(const char *path) {
126  struct dl_hdr hdr;
127  struct dlib *h;
128
129  int fd = open(path, O_RDONLY);
130  if (fd < 0) goto bad;
131  if (read(fd, &hdr, sizeof(hdr)) < sizeof(hdr)) goto bad;
132  if (strncmp(hdr.magic, DL_MAGIC, strlen(DL_MAGIC)) != 0) goto bad;
133
134  h = mmap(NULL, hdr.file_sz, PROT_READ | PROT_WRITE | PROT_EXEC, MAP_PRIVATE, fd, 0);
135  if (h == (void *)-1) goto bad;
136
137  h->symtab = (struct symbol *)((char *)h + REC_SZ);
138  h->path = path;
139
140  for (struct symbol *sym = h->symtab; sym->type; sym++) {
141    switch (sym->type) {
142      case '+': dlload(sym); break; // (recursively) load
143      case '?': sym->offset = (uintptr_t)dlsym(sym->name); break; // resolve
144      case '#': dlexport(sym->name, (char *)h + sym->offset); break; // export
145    }
146  }
147
148  return h;
149
150bad:
151  if (fd > 0) close(fd);
152  return NULL;
153}
154
155static void *dlsym(const char *name) {
156  for (int i = 0; i < LENGTH(syms); i++)
157    if (strcmp(syms[i].name, name) == 0)
158      return (void *)syms[i].offset;
159  assert(0);
160}
161
162static void dlexport(const char *name, void *addr) {
163  for (int i = 0; i < LENGTH(syms); i++)
164    if (!syms[i].name[0]) {
165      syms[i].offset = (uintptr_t)addr; // load-time offset
166      strcpy(syms[i].name, name);
167      return;
168    }
169  assert(0);
170}
171
172static void dlload(struct symbol *sym) {
173  for (int i = 0; i < LENGTH(libs); i++) {
174    if (libs[i] && strcmp(libs[i]->name, sym->name) == 0) return; // already loaded
175    if (!libs[i]) {
176      libs[i] = sym;
177      dlopen(sym->name); // load recursively
178      return;
179    }
180  }
181  assert(0);
182}
21. 可执行文件和加载¶

Take-away Messages¶

课后习题/编程作业¶

1. 编程实践¶
