symbol.c revision 1.33
1 /* $NetBSD: symbol.c,v 1.33 2003/08/12 09:18:50 skrll Exp $ */ 2 3 /* 4 * Copyright 1996 John D. Polstra. 5 * Copyright 1996 Matt Thomas <matt (at) 3am-software.com> 6 * Copyright 2002 Charles M. Hannum <root (at) ihack.net> 7 * All rights reserved. 8 * 9 * Redistribution and use in source and binary forms, with or without 10 * modification, are permitted provided that the following conditions 11 * are met: 12 * 1. Redistributions of source code must retain the above copyright 13 * notice, this list of conditions and the following disclaimer. 14 * 2. Redistributions in binary form must reproduce the above copyright 15 * notice, this list of conditions and the following disclaimer in the 16 * documentation and/or other materials provided with the distribution. 17 * 3. All advertising materials mentioning features or use of this software 18 * must display the following acknowledgement: 19 * This product includes software developed by John Polstra. 20 * 4. The name of the author may not be used to endorse or promote products 21 * derived from this software without specific prior written permission. 22 * 23 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR 24 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 25 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 26 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, 27 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 28 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 29 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 30 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 31 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 32 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 33 */ 34 35 /* 36 * Dynamic linker for ELF. 37 * 38 * John Polstra <jdp (at) polstra.com>. 39 */ 40 41 #include <err.h> 42 #include <errno.h> 43 #include <fcntl.h> 44 #include <stdarg.h> 45 #include <stdio.h> 46 #include <stdlib.h> 47 #include <string.h> 48 #include <unistd.h> 49 #include <sys/types.h> 50 #include <sys/mman.h> 51 #include <dirent.h> 52 53 #include "debug.h" 54 #include "rtld.h" 55 56 static bool 57 _rtld_is_exported(const Elf_Sym *def) 58 { 59 static Elf_Addr _rtld_exports[] = { 60 (Elf_Addr)dlopen, 61 (Elf_Addr)dlclose, 62 (Elf_Addr)dlsym, 63 (Elf_Addr)dlerror, 64 (Elf_Addr)dladdr, 65 NULL 66 }; 67 int i; 68 69 Elf_Addr value; 70 value = (Elf_Addr)(_rtld_objself.relocbase + def->st_value); 71 72 for (i = 0; _rtld_exports[i] != NULL; i++) { 73 if (value == _rtld_exports[i]) 74 return true; 75 } 76 return false; 77 } 78 79 /* 80 * Hash function for symbol table lookup. Don't even think about changing 81 * this. It is specified by the System V ABI. 82 */ 83 unsigned long 84 _rtld_elf_hash(const char *name) 85 { 86 const unsigned char *p = (const unsigned char *) name; 87 unsigned long h = 0; 88 unsigned long g; 89 unsigned long c; 90 91 for (; __predict_true((c = *p) != '\0'); p++) { 92 h <<= 4; 93 h += c; 94 if ((g = h & 0xf0000000) != 0) { 95 h ^= g; 96 h ^= g >> 24; 97 } 98 } 99 return (h); 100 } 101 102 const Elf_Sym * 103 _rtld_symlook_list(const char *name, unsigned long hash, const Objlist *objlist, 104 const Obj_Entry **defobj_out, bool in_plt) 105 { 106 const Elf_Sym *symp; 107 const Elf_Sym *def; 108 const Obj_Entry *defobj; 109 const Objlist_Entry *elm; 110 111 def = NULL; 112 defobj = NULL; 113 SIMPLEQ_FOREACH(elm, objlist, link) { 114 rdbg(("search object %p (%s)", elm->obj, elm->obj->path)); 115 if ((symp = _rtld_symlook_obj(name, hash, elm->obj, in_plt)) 116 != NULL) { 117 if ((def == NULL) || 118 (ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 119 def = symp; 120 defobj = elm->obj; 121 if (ELF_ST_BIND(def->st_info) != STB_WEAK) 122 break; 123 } 124 } 125 } 126 if (def != NULL) 127 *defobj_out = defobj; 128 return def; 129 } 130 131 /* 132 * Search the symbol table of a single shared object for a symbol of 133 * the given name. Returns a pointer to the symbol, or NULL if no 134 * definition was found. 135 * 136 * The symbol's hash value is passed in for efficiency reasons; that 137 * eliminates many recomputations of the hash value. 138 */ 139 const Elf_Sym * 140 _rtld_symlook_obj(const char *name, unsigned long hash, 141 const Obj_Entry *obj, bool in_plt) 142 { 143 unsigned long symnum; 144 145 for (symnum = obj->buckets[hash % obj->nbuckets]; 146 symnum != ELF_SYM_UNDEFINED; 147 symnum = obj->chains[symnum]) { 148 const Elf_Sym *symp; 149 const char *strp; 150 151 assert(symnum < obj->nchains); 152 symp = obj->symtab + symnum; 153 strp = obj->strtab + symp->st_name; 154 rdbg(("check %s vs %s in %p", name, strp, obj)); 155 if (name[1] == strp[1] && !strcmp(name, strp)) { 156 if (symp->st_shndx != SHN_UNDEF) 157 return symp; 158 #ifndef __mips__ 159 /* 160 * XXX DANGER WILL ROBINSON! 161 * If we have a function pointer in the executable's 162 * data section, it points to the executable's PLT 163 * slot, and there is NO relocation emitted. To make 164 * the function pointer comparable to function pointers 165 * in shared libraries, we must resolve data references 166 * in the libraries to point to PLT slots in the 167 * executable, if they exist. 168 */ 169 else if (!in_plt && symp->st_value != 0 && 170 ELF_ST_TYPE(symp->st_info) == STT_FUNC) 171 return symp; 172 #endif 173 else 174 return NULL; 175 } 176 } 177 178 return NULL; 179 } 180 181 /* 182 * Given a symbol number in a referencing object, find the corresponding 183 * definition of the symbol. Returns a pointer to the symbol, or NULL if 184 * no definition was found. Returns a pointer to the Obj_Entry of the 185 * defining object via the reference parameter DEFOBJ_OUT. 186 */ 187 const Elf_Sym * 188 _rtld_find_symdef(unsigned long symnum, const Obj_Entry *refobj, 189 const Obj_Entry **defobj_out, bool in_plt) 190 { 191 const Elf_Sym *ref; 192 const Elf_Sym *def; 193 const Elf_Sym *symp; 194 const Obj_Entry *obj; 195 const Obj_Entry *defobj; 196 const Objlist_Entry *elm; 197 const char *name; 198 unsigned long hash; 199 200 ref = refobj->symtab + symnum; 201 name = refobj->strtab + ref->st_name; 202 203 hash = _rtld_elf_hash(name); 204 def = NULL; 205 defobj = NULL; 206 207 /* Look first in the referencing object if linked symbolically */ 208 if (refobj->symbolic) { 209 symp = _rtld_symlook_obj(name, hash, refobj, in_plt); 210 if (symp != NULL) { 211 def = symp; 212 defobj = refobj; 213 } 214 } 215 216 /* Search all objects loaded at program start up. */ 217 if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) { 218 rdbg(("search _rtld_list_main")); 219 symp = _rtld_symlook_list(name, hash, &_rtld_list_main, &obj, in_plt); 220 if (symp != NULL && 221 (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 222 def = symp; 223 defobj = obj; 224 } 225 } 226 227 /* Search all dlopened DAGs containing the referencing object. */ 228 SIMPLEQ_FOREACH(elm, &refobj->dldags, link) { 229 if (def != NULL && ELF_ST_BIND(def->st_info) != STB_WEAK) 230 break; 231 rdbg(("search DAG with root %p (%s)", elm->obj, elm->obj->path)); 232 symp = _rtld_symlook_list(name, hash, &elm->obj->dagmembers, &obj, in_plt); 233 if (symp != NULL && 234 (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 235 def = symp; 236 defobj = obj; 237 } 238 } 239 240 /* Search all RTLD_GLOBAL objects. */ 241 if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) { 242 rdbg(("search _rtld_list_global")); 243 symp = _rtld_symlook_list(name, hash, &_rtld_list_global, &obj, in_plt); 244 if (symp != NULL && 245 (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 246 def = symp; 247 defobj = obj; 248 } 249 } 250 251 /* 252 * Search the dynamic linker itself, and possibly resolve the 253 * symbol from there. This is how the application links to 254 * dynamic linker services such as dlopen. Only the values listed 255 * in the "_rtld_exports" array can be resolved from the dynamic linker. 256 */ 257 if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) { 258 symp = _rtld_symlook_obj(name, hash, &_rtld_objself, in_plt); 259 if (symp != NULL && _rtld_is_exported(symp)) { 260 def = symp; 261 defobj = &_rtld_objself; 262 } 263 } 264 265 /* 266 * If we found no definition and the reference is weak, treat the 267 * symbol as having the value zero. 268 */ 269 if (def == NULL && ELF_ST_BIND(ref->st_info) == STB_WEAK) { 270 rdbg((" returning _rtld_sym_zero@_rtld_objmain")); 271 def = &_rtld_sym_zero; 272 defobj = _rtld_objmain; 273 } 274 275 if (def != NULL) 276 *defobj_out = defobj; 277 else { 278 rdbg(("lookup failed")); 279 _rtld_error("%s: Undefined %ssymbol \"%s\" (symnum = %ld)", 280 refobj->path, in_plt ? "PLT " : "", name, symnum); 281 } 282 return def; 283 } 284 285 /* 286 * Given a symbol name in a referencing object, find the corresponding 287 * definition of the symbol. Returns a pointer to the symbol, or NULL if 288 * no definition was found. Returns a pointer to the Obj_Entry of the 289 * defining object via the reference parameter DEFOBJ_OUT. 290 */ 291 const Elf_Sym * 292 _rtld_symlook_default(const char *name, unsigned long hash, 293 const Obj_Entry *refobj, const Obj_Entry **defobj_out, bool in_plt) 294 { 295 const Elf_Sym *def; 296 const Elf_Sym *symp; 297 const Obj_Entry *obj; 298 const Obj_Entry *defobj; 299 const Objlist_Entry *elm; 300 def = NULL; 301 defobj = NULL; 302 303 /* Look first in the referencing object if linked symbolically. */ 304 if (refobj->symbolic) { 305 symp = _rtld_symlook_obj(name, hash, refobj, in_plt); 306 if (symp != NULL) { 307 def = symp; 308 defobj = refobj; 309 } 310 } 311 312 /* Search all objects loaded at program start up. */ 313 if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) { 314 symp = _rtld_symlook_list(name, hash, &_rtld_list_main, &obj, in_plt); 315 if (symp != NULL && 316 (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 317 def = symp; 318 defobj = obj; 319 } 320 } 321 322 /* Search all dlopened DAGs containing the referencing object. */ 323 SIMPLEQ_FOREACH(elm, &refobj->dldags, link) { 324 if (def != NULL && ELF_ST_BIND(def->st_info) != STB_WEAK) 325 break; 326 symp = _rtld_symlook_list(name, hash, &elm->obj->dagmembers, &obj, 327 in_plt); 328 if (symp != NULL && 329 (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 330 def = symp; 331 defobj = obj; 332 } 333 } 334 335 /* Search all DAGs whose roots are RTLD_GLOBAL objects. */ 336 SIMPLEQ_FOREACH(elm, &_rtld_list_global, link) { 337 if (def != NULL && ELF_ST_BIND(def->st_info) != STB_WEAK) 338 break; 339 symp = _rtld_symlook_list(name, hash, &elm->obj->dagmembers, &obj, 340 in_plt); 341 if (symp != NULL && 342 (def == NULL || ELF_ST_BIND(symp->st_info) != STB_WEAK)) { 343 def = symp; 344 defobj = obj; 345 } 346 } 347 348 #ifdef notyet 349 /* 350 * Search the dynamic linker itself, and possibly resolve the 351 * symbol from there. This is how the application links to 352 * dynamic linker services such as dlopen. Only the values listed 353 * in the "exports" array can be resolved from the dynamic linker. 354 */ 355 if (def == NULL || ELF_ST_BIND(def->st_info) == STB_WEAK) { 356 symp = _rtld_symlook_obj(name, hash, &_rtld_objself, in_plt); 357 if (symp != NULL && is_exported(symp)) { 358 def = symp; 359 defobj = &_rtld_objself; 360 } 361 } 362 #endif 363 364 if (def != NULL) 365 *defobj_out = defobj; 366 return def; 367 } 368
Indexes created Tue Sep 30 11:09:46 GMT 2025