libbacktrace/elf.c

   1 /* elf.c -- Get debug data from an ELF file for backtraces.
   2    Copyright (C) 2012-2015 Free Software Foundation, Inc.
   3    Written by Ian Lance Taylor, Google.
   4
   5 Redistribution and use in source and binary forms, with or without
   6 modification, are permitted provided that the following conditions are
   7 met:
   8
   9     (1) Redistributions of source code must retain the above copyright
  10     notice, this list of conditions and the following disclaimer.
  11
  12     (2) Redistributions in binary form must reproduce the above copyright
  13     notice, this list of conditions and the following disclaimer in
  14     the documentation and/or other materials provided with the
  15     distribution.
  16
  17     (3) The name of the author may not be used to
  18     endorse or promote products derived from this software without
  19     specific prior written permission.
  20
  21 THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  22 IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  23 WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  24 DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
  25 INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  26 (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  27 SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
  29 STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
  30 IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  31 POSSIBILITY OF SUCH DAMAGE.  */
  32
  33 #include "config.h"
  34
  35 #include <stdlib.h>
  36 #include <string.h>
  37 #include <sys/types.h>
  38
  39 #ifdef HAVE_DL_ITERATE_PHDR
  40 #include <link.h>
  41 #endif
  42
  43 #include "backtrace.h"
  44 #include "internal.h"
  45
  46 #ifndef HAVE_DL_ITERATE_PHDR
  47
  48 /* Dummy version of dl_iterate_phdr for systems that don't have it.  */
  49
  50 #define dl_phdr_info x_dl_phdr_info
  51 #define dl_iterate_phdr x_dl_iterate_phdr
  52
  53 struct dl_phdr_info
  54 {
  55   uintptr_t dlpi_addr;
  56   const char *dlpi_name;
  57 };
  58
  59 static int
  60 dl_iterate_phdr (int (*callback) (struct dl_phdr_info *,
  61                                   size_t, void *) ATTRIBUTE_UNUSED,
  62                  void *data ATTRIBUTE_UNUSED)
  63 {
  64   return 0;
  65 }
  66
  67 #endif /* ! defined (HAVE_DL_ITERATE_PHDR) */
  68
  69 /* The configure script must tell us whether we are 32-bit or 64-bit
  70    ELF.  We could make this code test and support either possibility,
  71    but there is no point.  This code only works for the currently
  72    running executable, which means that we know the ELF mode at
  73    configure mode.  */
  74
  75 #if BACKTRACE_ELF_SIZE != 32 && BACKTRACE_ELF_SIZE != 64
  76 #error "Unknown BACKTRACE_ELF_SIZE"
  77 #endif
  78
  79 /* <link.h> might #include <elf.h> which might define our constants
  80    with slightly different values.  Undefine them to be safe.  */
  81
  82 #undef EI_NIDENT
  83 #undef EI_MAG0
  84 #undef EI_MAG1
  85 #undef EI_MAG2
  86 #undef EI_MAG3
  87 #undef EI_CLASS
  88 #undef EI_DATA
  89 #undef EI_VERSION
  90 #undef ELF_MAG0
  91 #undef ELF_MAG1
  92 #undef ELF_MAG2
  93 #undef ELF_MAG3
  94 #undef ELFCLASS32
  95 #undef ELFCLASS64
  96 #undef ELFDATA2LSB
  97 #undef ELFDATA2MSB
  98 #undef EV_CURRENT
  99 #undef ET_DYN
 100 #undef SHN_LORESERVE
 101 #undef SHN_XINDEX
 102 #undef SHN_UNDEF
 103 #undef SHT_SYMTAB
 104 #undef SHT_STRTAB
 105 #undef SHT_DYNSYM
 106 #undef STT_OBJECT
 107 #undef STT_FUNC
 108
 109 /* Basic types.  */
 110
 111 typedef uint16_t b_elf_half;    /* Elf_Half.  */
 112 typedef uint32_t b_elf_word;    /* Elf_Word.  */
 113 typedef int32_t  b_elf_sword;   /* Elf_Sword.  */
 114
 115 #if BACKTRACE_ELF_SIZE == 32
 116
 117 typedef uint32_t b_elf_addr;    /* Elf_Addr.  */
 118 typedef uint32_t b_elf_off;     /* Elf_Off.  */
 119
 120 typedef uint32_t b_elf_wxword;  /* 32-bit Elf_Word, 64-bit ELF_Xword.  */
 121
 122 #else
 123
 124 typedef uint64_t b_elf_addr;    /* Elf_Addr.  */
 125 typedef uint64_t b_elf_off;     /* Elf_Off.  */
 126 typedef uint64_t b_elf_xword;   /* Elf_Xword.  */
 127 typedef int64_t  b_elf_sxword;  /* Elf_Sxword.  */
 128
 129 typedef uint64_t b_elf_wxword;  /* 32-bit Elf_Word, 64-bit ELF_Xword.  */
 130
 131 #endif
 132
 133 /* Data structures and associated constants.  */
 134
 135 #define EI_NIDENT 16
 136
 137 typedef struct {
 138   unsigned char e_ident[EI_NIDENT];     /* ELF "magic number" */
 139   b_elf_half    e_type;                 /* Identifies object file type */
 140   b_elf_half    e_machine;              /* Specifies required architecture */
 141   b_elf_word    e_version;              /* Identifies object file version */
 142   b_elf_addr    e_entry;                /* Entry point virtual address */
 143   b_elf_off     e_phoff;                /* Program header table file offset */
 144   b_elf_off     e_shoff;                /* Section header table file offset */
 145   b_elf_word    e_flags;                /* Processor-specific flags */
 146   b_elf_half    e_ehsize;               /* ELF header size in bytes */
 147   b_elf_half    e_phentsize;            /* Program header table entry size */
 148   b_elf_half    e_phnum;                /* Program header table entry count */
 149   b_elf_half    e_shentsize;            /* Section header table entry size */
 150   b_elf_half    e_shnum;                /* Section header table entry count */
 151   b_elf_half    e_shstrndx;             /* Section header string table index */
 152 } b_elf_ehdr;  /* Elf_Ehdr.  */
 153
 154 #define EI_MAG0 0
 155 #define EI_MAG1 1
 156 #define EI_MAG2 2
 157 #define EI_MAG3 3
 158 #define EI_CLASS 4
 159 #define EI_DATA 5
 160 #define EI_VERSION 6
 161
 162 #define ELFMAG0 0x7f
 163 #define ELFMAG1 'E'
 164 #define ELFMAG2 'L'
 165 #define ELFMAG3 'F'
 166
 167 #define ELFCLASS32 1
 168 #define ELFCLASS64 2
 169
 170 #define ELFDATA2LSB 1
 171 #define ELFDATA2MSB 2
 172
 173 #define EV_CURRENT 1
 174
 175 #define ET_DYN 3
 176
 177 typedef struct {
 178   b_elf_word    sh_name;                /* Section name, index in string tbl */
 179   b_elf_word    sh_type;                /* Type of section */
 180   b_elf_wxword  sh_flags;               /* Miscellaneous section attributes */
 181   b_elf_addr    sh_addr;                /* Section virtual addr at execution */
 182   b_elf_off     sh_offset;              /* Section file offset */
 183   b_elf_wxword  sh_size;                /* Size of section in bytes */
 184   b_elf_word    sh_link;                /* Index of another section */
 185   b_elf_word    sh_info;                /* Additional section information */
 186   b_elf_wxword  sh_addralign;           /* Section alignment */
 187   b_elf_wxword  sh_entsize;             /* Entry size if section holds table */
 188 } b_elf_shdr;  /* Elf_Shdr.  */
 189
 190 #define SHN_UNDEF       0x0000          /* Undefined section */
 191 #define SHN_LORESERVE   0xFF00          /* Begin range of reserved indices */
 192 #define SHN_XINDEX      0xFFFF          /* Section index is held elsewhere */
 193
 194 #define SHT_SYMTAB 2
 195 #define SHT_STRTAB 3
 196 #define SHT_DYNSYM 11
 197
 198 #if BACKTRACE_ELF_SIZE == 32
 199
 200 typedef struct
 201 {
 202   b_elf_word    st_name;                /* Symbol name, index in string tbl */
 203   b_elf_addr    st_value;               /* Symbol value */
 204   b_elf_word    st_size;                /* Symbol size */
 205   unsigned char st_info;                /* Symbol binding and type */
 206   unsigned char st_other;               /* Visibility and other data */
 207   b_elf_half    st_shndx;               /* Symbol section index */
 208 } b_elf_sym;  /* Elf_Sym.  */
 209
 210 #else /* BACKTRACE_ELF_SIZE != 32 */
 211
 212 typedef struct
 213 {
 214   b_elf_word    st_name;                /* Symbol name, index in string tbl */
 215   unsigned char st_info;                /* Symbol binding and type */
 216   unsigned char st_other;               /* Visibility and other data */
 217   b_elf_half    st_shndx;               /* Symbol section index */
 218   b_elf_addr    st_value;               /* Symbol value */
 219   b_elf_xword   st_size;                /* Symbol size */
 220 } b_elf_sym;  /* Elf_Sym.  */
 221
 222 #endif /* BACKTRACE_ELF_SIZE != 32 */
 223
 224 #define STT_OBJECT 1
 225 #define STT_FUNC 2
 226
 227 /* An index of ELF sections we care about.  */
 228
 229 enum debug_section
 230 {
 231   DEBUG_INFO,
 232   DEBUG_LINE,
 233   DEBUG_ABBREV,
 234   DEBUG_RANGES,
 235   DEBUG_STR,
 236   DEBUG_MAX
 237 };
 238
 239 /* Names of sections, indexed by enum elf_section.  */
 240
 241 static const char * const debug_section_names[DEBUG_MAX] =
 242 {
 243   ".debug_info",
 244   ".debug_line",
 245   ".debug_abbrev",
 246   ".debug_ranges",
 247   ".debug_str"
 248 };
 249
 250 /* Information we gather for the sections we care about.  */
 251
 252 struct debug_section_info
 253 {
 254   /* Section file offset.  */
 255   off_t offset;
 256   /* Section size.  */
 257   size_t size;
 258   /* Section contents, after read from file.  */
 259   const unsigned char *data;
 260 };
 261
 262 /* Information we keep for an ELF symbol.  */
 263
 264 struct elf_symbol
 265 {
 266   /* The name of the symbol.  */
 267   const char *name;
 268   /* The address of the symbol.  */
 269   uintptr_t address;
 270   /* The size of the symbol.  */
 271   size_t size;
 272 };
 273
 274 /* Information to pass to elf_syminfo.  */
 275
 276 struct elf_syminfo_data
 277 {
 278   /* Symbols for the next module.  */
 279   struct elf_syminfo_data *next;
 280   /* The ELF symbols, sorted by address.  */
 281   struct elf_symbol *symbols;
 282   /* The number of symbols.  */
 283   size_t count;
 284 };
 285
 286 /* A dummy callback function used when we can't find any debug info.  */
 287
 288 static int
 289 elf_nodebug (struct backtrace_state *state ATTRIBUTE_UNUSED,
 290              uintptr_t pc ATTRIBUTE_UNUSED,
 291              backtrace_full_callback callback ATTRIBUTE_UNUSED,
 292              backtrace_error_callback error_callback, void *data)
 293 {
 294   error_callback (data, "no debug info in ELF executable", -1);
 295   return 0;
 296 }
 297
 298 /* A dummy callback function used when we can't find a symbol
 299    table.  */
 300
 301 static void
 302 elf_nosyms (struct backtrace_state *state ATTRIBUTE_UNUSED,
 303             uintptr_t addr ATTRIBUTE_UNUSED,
 304             backtrace_syminfo_callback callback ATTRIBUTE_UNUSED,
 305             backtrace_error_callback error_callback, void *data)
 306 {
 307   error_callback (data, "no symbol table in ELF executable", -1);
 308 }
 309
 310 /* Compare struct elf_symbol for qsort.  */
 311
 312 static int
 313 elf_symbol_compare (const void *v1, const void *v2)
 314 {
 315   const struct elf_symbol *e1 = (const struct elf_symbol *) v1;
 316   const struct elf_symbol *e2 = (const struct elf_symbol *) v2;
 317
 318   if (e1->address < e2->address)
 319     return -1;
 320   else if (e1->address > e2->address)
 321     return 1;
 322   else
 323     return 0;
 324 }
 325
 326 /* Compare an ADDR against an elf_symbol for bsearch.  We allocate one
 327    extra entry in the array so that this can look safely at the next
 328    entry.  */
 329
 330 static int
 331 elf_symbol_search (const void *vkey, const void *ventry)
 332 {
 333   const uintptr_t *key = (const uintptr_t *) vkey;
 334   const struct elf_symbol *entry = (const struct elf_symbol *) ventry;
 335   uintptr_t addr;
 336
 337   addr = *key;
 338   if (addr < entry->address)
 339     return -1;
 340   else if (addr >= entry->address + entry->size)
 341     return 1;
 342   else
 343     return 0;
 344 }
 345
 346 /* Initialize the symbol table info for elf_syminfo.  */
 347
 348 static int
 349 elf_initialize_syminfo (struct backtrace_state *state,
 350                         uintptr_t base_address,
 351                         const unsigned char *symtab_data, size_t symtab_size,
 352                         const unsigned char *strtab, size_t strtab_size,
 353                         backtrace_error_callback error_callback,
 354                         void *data, struct elf_syminfo_data *sdata)
 355 {
 356   size_t sym_count;
 357   const b_elf_sym *sym;
 358   size_t elf_symbol_count;
 359   size_t elf_symbol_size;
 360   struct elf_symbol *elf_symbols;
 361   size_t i;
 362   unsigned int j;
 363
 364   sym_count = symtab_size / sizeof (b_elf_sym);
 365
 366   /* We only care about function symbols.  Count them.  */
 367   sym = (const b_elf_sym *) symtab_data;
 368   elf_symbol_count = 0;
 369   for (i = 0; i < sym_count; ++i, ++sym)
 370     {
 371       int info;
 372
 373       info = sym->st_info & 0xf;
 374       if ((info == STT_FUNC || info == STT_OBJECT)
 375           && sym->st_shndx != SHN_UNDEF)
 376         ++elf_symbol_count;
 377     }
 378
 379   elf_symbol_size = elf_symbol_count * sizeof (struct elf_symbol);
 380   elf_symbols = ((struct elf_symbol *)
 381                  backtrace_alloc (state, elf_symbol_size, error_callback,
 382                                   data));
 383   if (elf_symbols == NULL)
 384     return 0;
 385
 386   sym = (const b_elf_sym *) symtab_data;
 387   j = 0;
 388   for (i = 0; i < sym_count; ++i, ++sym)
 389     {
 390       int info;
 391
 392       info = sym->st_info & 0xf;
 393       if (info != STT_FUNC && info != STT_OBJECT)
 394         continue;
 395       if (sym->st_shndx == SHN_UNDEF)
 396         continue;
 397       if (sym->st_name >= strtab_size)
 398         {
 399           error_callback (data, "symbol string index out of range", 0);
 400           backtrace_free (state, elf_symbols, elf_symbol_size, error_callback,
 401                           data);
 402           return 0;
 403         }
 404       elf_symbols[j].name = (const char *) strtab + sym->st_name;
 405       elf_symbols[j].address = sym->st_value + base_address;
 406       elf_symbols[j].size = sym->st_size;
 407       ++j;
 408     }
 409
 410   backtrace_qsort (elf_symbols, elf_symbol_count, sizeof (struct elf_symbol),
 411                    elf_symbol_compare);
 412
 413   sdata->next = NULL;
 414   sdata->symbols = elf_symbols;
 415   sdata->count = elf_symbol_count;
 416
 417   return 1;
 418 }
 419
 420 /* Add EDATA to the list in STATE.  */
 421
 422 static void
 423 elf_add_syminfo_data (struct backtrace_state *state,
 424                       struct elf_syminfo_data *edata)
 425 {
 426   if (!state->threaded)
 427     {
 428       struct elf_syminfo_data **pp;
 429
 430       for (pp = (struct elf_syminfo_data **) (void *) &state->syminfo_data;
 431            *pp != NULL;
 432            pp = &(*pp)->next)
 433         ;
 434       *pp = edata;
 435     }
 436   else
 437     {
 438       while (1)
 439         {
 440           struct elf_syminfo_data **pp;
 441
 442           pp = (struct elf_syminfo_data **) (void *) &state->syminfo_data;
 443
 444           while (1)
 445             {
 446               struct elf_syminfo_data *p;
 447
 448               p = backtrace_atomic_load_pointer (pp);
 449
 450               if (p == NULL)
 451                 break;
 452
 453               pp = &p->next;
 454             }
 455
 456           if (__sync_bool_compare_and_swap (pp, NULL, edata))
 457             break;
 458         }
 459     }
 460 }
 461
 462 /* Return the symbol name and value for an ADDR.  */
 463
 464 static void
 465 elf_syminfo (struct backtrace_state *state, uintptr_t addr,
 466              backtrace_syminfo_callback callback,
 467              backtrace_error_callback error_callback ATTRIBUTE_UNUSED,
 468              void *data)
 469 {
 470   struct elf_syminfo_data *edata;
 471   struct elf_symbol *sym = NULL;
 472
 473   if (!state->threaded)
 474     {
 475       for (edata = (struct elf_syminfo_data *) state->syminfo_data;
 476            edata != NULL;
 477            edata = edata->next)
 478         {
 479           sym = ((struct elf_symbol *)
 480                  bsearch (&addr, edata->symbols, edata->count,
 481                           sizeof (struct elf_symbol), elf_symbol_search));
 482           if (sym != NULL)
 483             break;
 484         }
 485     }
 486   else
 487     {
 488       struct elf_syminfo_data **pp;
 489
 490       pp = (struct elf_syminfo_data **) (void *) &state->syminfo_data;
 491       while (1)
 492         {
 493           edata = backtrace_atomic_load_pointer (pp);
 494           if (edata == NULL)
 495             break;
 496
 497           sym = ((struct elf_symbol *)
 498                  bsearch (&addr, edata->symbols, edata->count,
 499                           sizeof (struct elf_symbol), elf_symbol_search));
 500           if (sym != NULL)
 501             break;
 502
 503           pp = &edata->next;
 504         }
 505     }
 506
 507   if (sym == NULL)
 508     callback (data, addr, NULL, 0, 0);
 509   else
 510     callback (data, addr, sym->name, sym->address, sym->size);
 511 }
 512
 513 /* Add the backtrace data for one ELF file.  Returns 1 on success,
 514    0 on failure (in both cases descriptor is closed) or -1 if exe
 515    is non-zero and the ELF file is ET_DYN, which tells the caller that
 516    elf_add will need to be called on the descriptor again after
 517    base_address is determined.  */
 518
 519 static int
 520 elf_add (struct backtrace_state *state, int descriptor, uintptr_t base_address,
 521          backtrace_error_callback error_callback, void *data,
 522          fileline *fileline_fn, int *found_sym, int *found_dwarf, int exe)
 523 {
 524   struct backtrace_view ehdr_view;
 525   b_elf_ehdr ehdr;
 526   off_t shoff;
 527   unsigned int shnum;
 528   unsigned int shstrndx;
 529   struct backtrace_view shdrs_view;
 530   int shdrs_view_valid;
 531   const b_elf_shdr *shdrs;
 532   const b_elf_shdr *shstrhdr;
 533   size_t shstr_size;
 534   off_t shstr_off;
 535   struct backtrace_view names_view;
 536   int names_view_valid;
 537   const char *names;
 538   unsigned int symtab_shndx;
 539   unsigned int dynsym_shndx;
 540   unsigned int i;
 541   struct debug_section_info sections[DEBUG_MAX];
 542   struct backtrace_view symtab_view;
 543   int symtab_view_valid;
 544   struct backtrace_view strtab_view;
 545   int strtab_view_valid;
 546   off_t min_offset;
 547   off_t max_offset;
 548   struct backtrace_view debug_view;
 549   int debug_view_valid;
 550
 551   *found_sym = 0;
 552   *found_dwarf = 0;
 553
 554   shdrs_view_valid = 0;
 555   names_view_valid = 0;
 556   symtab_view_valid = 0;
 557   strtab_view_valid = 0;
 558   debug_view_valid = 0;
 559
 560   if (!backtrace_get_view (state, descriptor, 0, sizeof ehdr, error_callback,
 561                            data, &ehdr_view))
 562     goto fail;
 563
 564   memcpy (&ehdr, ehdr_view.data, sizeof ehdr);
 565
 566   backtrace_release_view (state, &ehdr_view, error_callback, data);
 567
 568   if (ehdr.e_ident[EI_MAG0] != ELFMAG0
 569       || ehdr.e_ident[EI_MAG1] != ELFMAG1
 570       || ehdr.e_ident[EI_MAG2] != ELFMAG2
 571       || ehdr.e_ident[EI_MAG3] != ELFMAG3)
 572     {
 573       error_callback (data, "executable file is not ELF", 0);
 574       goto fail;
 575     }
 576   if (ehdr.e_ident[EI_VERSION] != EV_CURRENT)
 577     {
 578       error_callback (data, "executable file is unrecognized ELF version", 0);
 579       goto fail;
 580     }
 581
 582 #if BACKTRACE_ELF_SIZE == 32
 583 #define BACKTRACE_ELFCLASS ELFCLASS32
 584 #else
 585 #define BACKTRACE_ELFCLASS ELFCLASS64
 586 #endif
 587
 588   if (ehdr.e_ident[EI_CLASS] != BACKTRACE_ELFCLASS)
 589     {
 590       error_callback (data, "executable file is unexpected ELF class", 0);
 591       goto fail;
 592     }
 593
 594   if (ehdr.e_ident[EI_DATA] != ELFDATA2LSB
 595       && ehdr.e_ident[EI_DATA] != ELFDATA2MSB)
 596     {
 597       error_callback (data, "executable file has unknown endianness", 0);
 598       goto fail;
 599     }
 600
 601   /* If the executable is ET_DYN, it is either a PIE, or we are running
 602      directly a shared library with .interp.  We need to wait for
 603      dl_iterate_phdr in that case to determine the actual base_address.  */
 604   if (exe && ehdr.e_type == ET_DYN)
 605     return -1;
 606
 607   shoff = ehdr.e_shoff;
 608   shnum = ehdr.e_shnum;
 609   shstrndx = ehdr.e_shstrndx;
 610
 611   if ((shnum == 0 || shstrndx == SHN_XINDEX)
 612       && shoff != 0)
 613     {
 614       struct backtrace_view shdr_view;
 615       const b_elf_shdr *shdr;
 616
 617       if (!backtrace_get_view (state, descriptor, shoff, sizeof shdr,
 618                                error_callback, data, &shdr_view))
 619         goto fail;
 620
 621       shdr = (const b_elf_shdr *) shdr_view.data;
 622
 623       if (shnum == 0)
 624         shnum = shdr->sh_size;
 625
 626       if (shstrndx == SHN_XINDEX)
 627         {
 628           shstrndx = shdr->sh_link;
 629
 630           /* Versions of the GNU binutils between 2.12 and 2.18 did
 631              not handle objects with more than SHN_LORESERVE sections
 632              correctly.  All large section indexes were offset by
 633              0x100.  There is more information at
 634              http://sourceware.org/bugzilla/show_bug.cgi?id-5900 .
 635              Fortunately these object files are easy to detect, as the
 636              GNU binutils always put the section header string table
 637              near the end of the list of sections.  Thus if the
 638              section header string table index is larger than the
 639              number of sections, then we know we have to subtract
 640              0x100 to get the real section index.  */
 641           if (shstrndx >= shnum && shstrndx >= SHN_LORESERVE + 0x100)
 642             shstrndx -= 0x100;
 643         }
 644
 645       backtrace_release_view (state, &shdr_view, error_callback, data);
 646     }
 647
 648   /* To translate PC to file/line when using DWARF, we need to find
 649      the .debug_info and .debug_line sections.  */
 650
 651   /* Read the section headers, skipping the first one.  */
 652
 653   if (!backtrace_get_view (state, descriptor, shoff + sizeof (b_elf_shdr),
 654                            (shnum - 1) * sizeof (b_elf_shdr),
 655                            error_callback, data, &shdrs_view))
 656     goto fail;
 657   shdrs_view_valid = 1;
 658   shdrs = (const b_elf_shdr *) shdrs_view.data;
 659
 660   /* Read the section names.  */
 661
 662   shstrhdr = &shdrs[shstrndx - 1];
 663   shstr_size = shstrhdr->sh_size;
 664   shstr_off = shstrhdr->sh_offset;
 665
 666   if (!backtrace_get_view (state, descriptor, shstr_off, shstr_size,
 667                            error_callback, data, &names_view))
 668     goto fail;
 669   names_view_valid = 1;
 670   names = (const char *) names_view.data;
 671
 672   symtab_shndx = 0;
 673   dynsym_shndx = 0;
 674
 675   memset (sections, 0, sizeof sections);
 676
 677   /* Look for the symbol table.  */
 678   for (i = 1; i < shnum; ++i)
 679     {
 680       const b_elf_shdr *shdr;
 681       unsigned int sh_name;
 682       const char *name;
 683       int j;
 684
 685       shdr = &shdrs[i - 1];
 686
 687       if (shdr->sh_type == SHT_SYMTAB)
 688         symtab_shndx = i;
 689       else if (shdr->sh_type == SHT_DYNSYM)
 690         dynsym_shndx = i;
 691
 692       sh_name = shdr->sh_name;
 693       if (sh_name >= shstr_size)
 694         {
 695           error_callback (data, "ELF section name out of range", 0);
 696           goto fail;
 697         }
 698
 699       name = names + sh_name;
 700
 701       for (j = 0; j < (int) DEBUG_MAX; ++j)
 702         {
 703           if (strcmp (name, debug_section_names[j]) == 0)
 704             {
 705               sections[j].offset = shdr->sh_offset;
 706               sections[j].size = shdr->sh_size;
 707               break;
 708             }
 709         }
 710     }
 711
 712   if (symtab_shndx == 0)
 713     symtab_shndx = dynsym_shndx;
 714   if (symtab_shndx != 0)
 715     {
 716       const b_elf_shdr *symtab_shdr;
 717       unsigned int strtab_shndx;
 718       const b_elf_shdr *strtab_shdr;
 719       struct elf_syminfo_data *sdata;
 720
 721       symtab_shdr = &shdrs[symtab_shndx - 1];
 722       strtab_shndx = symtab_shdr->sh_link;
 723       if (strtab_shndx >= shnum)
 724         {
 725           error_callback (data,
 726                           "ELF symbol table strtab link out of range", 0);
 727           goto fail;
 728         }
 729       strtab_shdr = &shdrs[strtab_shndx - 1];
 730
 731       if (!backtrace_get_view (state, descriptor, symtab_shdr->sh_offset,
 732                                symtab_shdr->sh_size, error_callback, data,
 733                                &symtab_view))
 734         goto fail;
 735       symtab_view_valid = 1;
 736
 737       if (!backtrace_get_view (state, descriptor, strtab_shdr->sh_offset,
 738                                strtab_shdr->sh_size, error_callback, data,
 739                                &strtab_view))
 740         goto fail;
 741       strtab_view_valid = 1;
 742
 743       sdata = ((struct elf_syminfo_data *)
 744                backtrace_alloc (state, sizeof *sdata, error_callback, data));
 745       if (sdata == NULL)
 746         goto fail;
 747
 748       if (!elf_initialize_syminfo (state, base_address,
 749                                    symtab_view.data, symtab_shdr->sh_size,
 750                                    strtab_view.data, strtab_shdr->sh_size,
 751                                    error_callback, data, sdata))
 752         {
 753           backtrace_free (state, sdata, sizeof *sdata, error_callback, data);
 754           goto fail;
 755         }
 756
 757       /* We no longer need the symbol table, but we hold on to the
 758          string table permanently.  */
 759       backtrace_release_view (state, &symtab_view, error_callback, data);
 760
 761       *found_sym = 1;
 762
 763       elf_add_syminfo_data (state, sdata);
 764     }
 765
 766   /* FIXME: Need to handle compressed debug sections.  */
 767
 768   backtrace_release_view (state, &shdrs_view, error_callback, data);
 769   shdrs_view_valid = 0;
 770   backtrace_release_view (state, &names_view, error_callback, data);
 771   names_view_valid = 0;
 772
 773   /* Read all the debug sections in a single view, since they are
 774      probably adjacent in the file.  We never release this view.  */
 775
 776   min_offset = 0;
 777   max_offset = 0;
 778   for (i = 0; i < (int) DEBUG_MAX; ++i)
 779     {
 780       off_t end;
 781
 782       if (sections[i].size == 0)
 783         continue;
 784       if (min_offset == 0 || sections[i].offset < min_offset)
 785         min_offset = sections[i].offset;
 786       end = sections[i].offset + sections[i].size;
 787       if (end > max_offset)
 788         max_offset = end;
 789     }
 790   if (min_offset == 0 || max_offset == 0)
 791     {
 792       if (!backtrace_close (descriptor, error_callback, data))
 793         goto fail;
 794       *fileline_fn = elf_nodebug;
 795       return 1;
 796     }
 797
 798   if (!backtrace_get_view (state, descriptor, min_offset,
 799                            max_offset - min_offset,
 800                            error_callback, data, &debug_view))
 801     goto fail;
 802   debug_view_valid = 1;
 803
 804   /* We've read all we need from the executable.  */
 805   if (!backtrace_close (descriptor, error_callback, data))
 806     goto fail;
 807   descriptor = -1;
 808
 809   for (i = 0; i < (int) DEBUG_MAX; ++i)
 810     {
 811       if (sections[i].size == 0)
 812         sections[i].data = NULL;
 813       else
 814         sections[i].data = ((const unsigned char *) debug_view.data
 815                             + (sections[i].offset - min_offset));
 816     }
 817
 818   if (!backtrace_dwarf_add (state, base_address,
 819                             sections[DEBUG_INFO].data,
 820                             sections[DEBUG_INFO].size,
 821                             sections[DEBUG_LINE].data,
 822                             sections[DEBUG_LINE].size,
 823                             sections[DEBUG_ABBREV].data,
 824                             sections[DEBUG_ABBREV].size,
 825                             sections[DEBUG_RANGES].data,
 826                             sections[DEBUG_RANGES].size,
 827                             sections[DEBUG_STR].data,
 828                             sections[DEBUG_STR].size,
 829                             ehdr.e_ident[EI_DATA] == ELFDATA2MSB,
 830                             error_callback, data, fileline_fn))
 831     goto fail;
 832
 833   *found_dwarf = 1;
 834
 835   return 1;
 836
 837  fail:
 838   if (shdrs_view_valid)
 839     backtrace_release_view (state, &shdrs_view, error_callback, data);
 840   if (names_view_valid)
 841     backtrace_release_view (state, &names_view, error_callback, data);
 842   if (symtab_view_valid)
 843     backtrace_release_view (state, &symtab_view, error_callback, data);
 844   if (strtab_view_valid)
 845     backtrace_release_view (state, &strtab_view, error_callback, data);
 846   if (debug_view_valid)
 847     backtrace_release_view (state, &debug_view, error_callback, data);
 848   if (descriptor != -1)
 849     backtrace_close (descriptor, error_callback, data);
 850   return 0;
 851 }
 852
 853 /* Data passed to phdr_callback.  */
 854
 855 struct phdr_data
 856 {
 857   struct backtrace_state *state;
 858   backtrace_error_callback error_callback;
 859   void *data;
 860   fileline *fileline_fn;
 861   int *found_sym;
 862   int *found_dwarf;
 863   int exe_descriptor;
 864 };
 865
 866 /* Callback passed to dl_iterate_phdr.  Load debug info from shared
 867    libraries.  */
 868
 869 static int
 870 phdr_callback (struct dl_phdr_info *info, size_t size ATTRIBUTE_UNUSED,
 871                void *pdata)
 872 {
 873   struct phdr_data *pd = (struct phdr_data *) pdata;
 874   int descriptor;
 875   int does_not_exist;
 876   fileline elf_fileline_fn;
 877   int found_dwarf;
 878
 879   /* There is not much we can do if we don't have the module name,
 880      unless executable is ET_DYN, where we expect the very first
 881      phdr_callback to be for the PIE.  */
 882   if (info->dlpi_name == NULL || info->dlpi_name[0] == '\0')
 883     {
 884       if (pd->exe_descriptor == -1)
 885         return 0;
 886       descriptor = pd->exe_descriptor;
 887       pd->exe_descriptor = -1;
 888     }
 889   else
 890     {
 891       if (pd->exe_descriptor != -1)
 892         {
 893           backtrace_close (pd->exe_descriptor, pd->error_callback, pd->data);
 894           pd->exe_descriptor = -1;
 895         }
 896
 897       descriptor = backtrace_open (info->dlpi_name, pd->error_callback,
 898                                    pd->data, &does_not_exist);
 899       if (descriptor < 0)
 900         return 0;
 901     }
 902
 903   if (elf_add (pd->state, descriptor, info->dlpi_addr, pd->error_callback,
 904                pd->data, &elf_fileline_fn, pd->found_sym, &found_dwarf, 0))
 905     {
 906       if (found_dwarf)
 907         {
 908           *pd->found_dwarf = 1;
 909           *pd->fileline_fn = elf_fileline_fn;
 910         }
 911     }
 912
 913   return 0;
 914 }
 915
 916 /* Initialize the backtrace data we need from an ELF executable.  At
 917    the ELF level, all we need to do is find the debug info
 918    sections.  */
 919
 920 int
 921 backtrace_initialize (struct backtrace_state *state, int descriptor,
 922                       backtrace_error_callback error_callback,
 923                       void *data, fileline *fileline_fn)
 924 {
 925   int ret;
 926   int found_sym;
 927   int found_dwarf;
 928   fileline elf_fileline_fn;
 929   struct phdr_data pd;
 930
 931   ret = elf_add (state, descriptor, 0, error_callback, data, &elf_fileline_fn,
 932                  &found_sym, &found_dwarf, 1);
 933   if (!ret)
 934     return 0;
 935
 936   pd.state = state;
 937   pd.error_callback = error_callback;
 938   pd.data = data;
 939   pd.fileline_fn = &elf_fileline_fn;
 940   pd.found_sym = &found_sym;
 941   pd.found_dwarf = &found_dwarf;
 942   pd.exe_descriptor = ret < 0 ? descriptor : -1;
 943
 944   dl_iterate_phdr (phdr_callback, (void *) &pd);
 945
 946   if (!state->threaded)
 947     {
 948       if (found_sym)
 949         state->syminfo_fn = elf_syminfo;
 950       else if (state->syminfo_fn == NULL)
 951         state->syminfo_fn = elf_nosyms;
 952     }
 953   else
 954     {
 955       if (found_sym)
 956         backtrace_atomic_store_pointer (&state->syminfo_fn, elf_syminfo);
 957       else
 958         __sync_bool_compare_and_swap (&state->syminfo_fn, NULL, elf_nosyms);
 959     }
 960
 961   if (!state->threaded)
 962     {
 963       if (state->fileline_fn == NULL || state->fileline_fn == elf_nodebug)
 964         *fileline_fn = elf_fileline_fn;
 965     }
 966   else
 967     {
 968       fileline current_fn;
 969
 970       current_fn = backtrace_atomic_load_pointer (&state->fileline_fn);
 971       if (current_fn == NULL || current_fn == elf_nodebug)
 972         *fileline_fn = elf_fileline_fn;
 973     }
 974
 975   return 1;
 976 }