tools/objtool/check.c

   1 /*
   2  * Copyright (C) 2015-2017 Josh Poimboeuf <jpoimboe@redhat.com>
   3  *
   4  * This program is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU General Public License
   6  * as published by the Free Software Foundation; either version 2
   7  * of the License, or (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU General Public License
  15  * along with this program; if not, see <http://www.gnu.org/licenses/>.
  16  */
  17
  18 #include <string.h>
  19 #include <stdlib.h>
  20
  21 #include "check.h"
  22 #include "elf.h"
  23 #include "special.h"
  24 #include "arch.h"
  25 #include "warn.h"
  26
  27 #include <linux/hashtable.h>
  28 #include <linux/kernel.h>
  29
  30 #define STATE_FP_SAVED          0x1
  31 #define STATE_FP_SETUP          0x2
  32 #define STATE_FENTRY            0x4
  33
  34 struct alternative {
  35         struct list_head list;
  36         struct instruction *insn;
  37 };
  38
  39 const char *objname;
  40 static bool nofp;
  41
  42 static struct instruction *find_insn(struct objtool_file *file,
  43                                      struct section *sec, unsigned long offset)
  44 {
  45         struct instruction *insn;
  46
  47         hash_for_each_possible(file->insn_hash, insn, hash, offset)
  48                 if (insn->sec == sec && insn->offset == offset)
  49                         return insn;
  50
  51         return NULL;
  52 }
  53
  54 static struct instruction *next_insn_same_sec(struct objtool_file *file,
  55                                               struct instruction *insn)
  56 {
  57         struct instruction *next = list_next_entry(insn, list);
  58
  59         if (&next->list == &file->insn_list || next->sec != insn->sec)
  60                 return NULL;
  61
  62         return next;
  63 }
  64
  65 static bool gcov_enabled(struct objtool_file *file)
  66 {
  67         struct section *sec;
  68         struct symbol *sym;
  69
  70         list_for_each_entry(sec, &file->elf->sections, list)
  71                 list_for_each_entry(sym, &sec->symbol_list, list)
  72                         if (!strncmp(sym->name, "__gcov_.", 8))
  73                                 return true;
  74
  75         return false;
  76 }
  77
  78 #define for_each_insn(file, insn)                                       \
  79         list_for_each_entry(insn, &file->insn_list, list)
  80
  81 #define func_for_each_insn(file, func, insn)                            \
  82         for (insn = find_insn(file, func->sec, func->offset);           \
  83              insn && &insn->list != &file->insn_list &&                 \
  84                 insn->sec == func->sec &&                               \
  85                 insn->offset < func->offset + func->len;                \
  86              insn = list_next_entry(insn, list))
  87
  88 #define func_for_each_insn_continue_reverse(file, func, insn)           \
  89         for (insn = list_prev_entry(insn, list);                        \
  90              &insn->list != &file->insn_list &&                         \
  91                 insn->sec == func->sec && insn->offset >= func->offset; \
  92              insn = list_prev_entry(insn, list))
  93
  94 #define sec_for_each_insn_from(file, insn)                              \
  95         for (; insn; insn = next_insn_same_sec(file, insn))
  96
  97
  98 /*
  99  * Check if the function has been manually whitelisted with the
 100  * STACK_FRAME_NON_STANDARD macro, or if it should be automatically whitelisted
 101  * due to its use of a context switching instruction.
 102  */
 103 static bool ignore_func(struct objtool_file *file, struct symbol *func)
 104 {
 105         struct rela *rela;
 106         struct instruction *insn;
 107
 108         /* check for STACK_FRAME_NON_STANDARD */
 109         if (file->whitelist && file->whitelist->rela)
 110                 list_for_each_entry(rela, &file->whitelist->rela->rela_list, list) {
 111                         if (rela->sym->type == STT_SECTION &&
 112                             rela->sym->sec == func->sec &&
 113                             rela->addend == func->offset)
 114                                 return true;
 115                         if (rela->sym->type == STT_FUNC && rela->sym == func)
 116                                 return true;
 117                 }
 118
 119         /* check if it has a context switching instruction */
 120         func_for_each_insn(file, func, insn)
 121                 if (insn->type == INSN_CONTEXT_SWITCH)
 122                         return true;
 123
 124         return false;
 125 }
 126
 127 /*
 128  * This checks to see if the given function is a "noreturn" function.
 129  *
 130  * For global functions which are outside the scope of this object file, we
 131  * have to keep a manual list of them.
 132  *
 133  * For local functions, we have to detect them manually by simply looking for
 134  * the lack of a return instruction.
 135  *
 136  * Returns:
 137  *  -1: error
 138  *   0: no dead end
 139  *   1: dead end
 140  */
 141 static int __dead_end_function(struct objtool_file *file, struct symbol *func,
 142                                int recursion)
 143 {
 144         int i;
 145         struct instruction *insn;
 146         bool empty = true;
 147
 148         /*
 149          * Unfortunately these have to be hard coded because the noreturn
 150          * attribute isn't provided in ELF data.
 151          */
 152         static const char * const global_noreturns[] = {
 153                 "__stack_chk_fail",
 154                 "panic",
 155                 "do_exit",
 156                 "do_task_dead",
 157                 "__module_put_and_exit",
 158                 "complete_and_exit",
 159                 "kvm_spurious_fault",
 160                 "__reiserfs_panic",
 161                 "lbug_with_loc",
 162                 "fortify_panic",
 163         };
 164
 165         if (func->bind == STB_WEAK)
 166                 return 0;
 167
 168         if (func->bind == STB_GLOBAL)
 169                 for (i = 0; i < ARRAY_SIZE(global_noreturns); i++)
 170                         if (!strcmp(func->name, global_noreturns[i]))
 171                                 return 1;
 172
 173         if (!func->sec)
 174                 return 0;
 175
 176         func_for_each_insn(file, func, insn) {
 177                 empty = false;
 178
 179                 if (insn->type == INSN_RETURN)
 180                         return 0;
 181         }
 182
 183         if (empty)
 184                 return 0;
 185
 186         /*
 187          * A function can have a sibling call instead of a return.  In that
 188          * case, the function's dead-end status depends on whether the target
 189          * of the sibling call returns.
 190          */
 191         func_for_each_insn(file, func, insn) {
 192                 if (insn->sec != func->sec ||
 193                     insn->offset >= func->offset + func->len)
 194                         break;
 195
 196                 if (insn->type == INSN_JUMP_UNCONDITIONAL) {
 197                         struct instruction *dest = insn->jump_dest;
 198                         struct symbol *dest_func;
 199
 200                         if (!dest)
 201                                 /* sibling call to another file */
 202                                 return 0;
 203
 204                         if (dest->sec != func->sec ||
 205                             dest->offset < func->offset ||
 206                             dest->offset >= func->offset + func->len) {
 207                                 /* local sibling call */
 208                                 dest_func = find_symbol_by_offset(dest->sec,
 209                                                                   dest->offset);
 210                                 if (!dest_func)
 211                                         continue;
 212
 213                                 if (recursion == 5) {
 214                                         WARN_FUNC("infinite recursion (objtool bug!)",
 215                                                   dest->sec, dest->offset);
 216                                         return -1;
 217                                 }
 218
 219                                 return __dead_end_function(file, dest_func,
 220                                                            recursion + 1);
 221                         }
 222                 }
 223
 224                 if (insn->type == INSN_JUMP_DYNAMIC && list_empty(&insn->alts))
 225                         /* sibling call */
 226                         return 0;
 227         }
 228
 229         return 1;
 230 }
 231
 232 static int dead_end_function(struct objtool_file *file, struct symbol *func)
 233 {
 234         return __dead_end_function(file, func, 0);
 235 }
 236
 237 /*
 238  * Call the arch-specific instruction decoder for all the instructions and add
 239  * them to the global instruction list.
 240  */
 241 static int decode_instructions(struct objtool_file *file)
 242 {
 243         struct section *sec;
 244         struct symbol *func;
 245         unsigned long offset;
 246         struct instruction *insn;
 247         int ret;
 248
 249         list_for_each_entry(sec, &file->elf->sections, list) {
 250
 251                 if (!(sec->sh.sh_flags & SHF_EXECINSTR))
 252                         continue;
 253
 254                 for (offset = 0; offset < sec->len; offset += insn->len) {
 255                         insn = malloc(sizeof(*insn));
 256                         memset(insn, 0, sizeof(*insn));
 257
 258                         INIT_LIST_HEAD(&insn->alts);
 259                         insn->sec = sec;
 260                         insn->offset = offset;
 261
 262                         ret = arch_decode_instruction(file->elf, sec, offset,
 263                                                       sec->len - offset,
 264                                                       &insn->len, &insn->type,
 265                                                       &insn->immediate);
 266                         if (ret)
 267                                 return ret;
 268
 269                         if (!insn->type || insn->type > INSN_LAST) {
 270                                 WARN_FUNC("invalid instruction type %d",
 271                                           insn->sec, insn->offset, insn->type);
 272                                 return -1;
 273                         }
 274
 275                         hash_add(file->insn_hash, &insn->hash, insn->offset);
 276                         list_add_tail(&insn->list, &file->insn_list);
 277                 }
 278
 279                 list_for_each_entry(func, &sec->symbol_list, list) {
 280                         if (func->type != STT_FUNC)
 281                                 continue;
 282
 283                         if (!find_insn(file, sec, func->offset)) {
 284                                 WARN("%s(): can't find starting instruction",
 285                                      func->name);
 286                                 return -1;
 287                         }
 288
 289                         func_for_each_insn(file, func, insn)
 290                                 if (!insn->func)
 291                                         insn->func = func;
 292                 }
 293         }
 294
 295         return 0;
 296 }
 297
 298 /*
 299  * Find all uses of the unreachable() macro, which are code path dead ends.
 300  */
 301 static int add_dead_ends(struct objtool_file *file)
 302 {
 303         struct section *sec;
 304         struct rela *rela;
 305         struct instruction *insn;
 306         bool found;
 307
 308         sec = find_section_by_name(file->elf, ".rela.discard.unreachable");
 309         if (!sec)
 310                 return 0;
 311
 312         list_for_each_entry(rela, &sec->rela_list, list) {
 313                 if (rela->sym->type != STT_SECTION) {
 314                         WARN("unexpected relocation symbol type in %s", sec->name);
 315                         return -1;
 316                 }
 317                 insn = find_insn(file, rela->sym->sec, rela->addend);
 318                 if (insn)
 319                         insn = list_prev_entry(insn, list);
 320                 else if (rela->addend == rela->sym->sec->len) {
 321                         found = false;
 322                         list_for_each_entry_reverse(insn, &file->insn_list, list) {
 323                                 if (insn->sec == rela->sym->sec) {
 324                                         found = true;
 325                                         break;
 326                                 }
 327                         }
 328
 329                         if (!found) {
 330                                 WARN("can't find unreachable insn at %s+0x%x",
 331                                      rela->sym->sec->name, rela->addend);
 332                                 return -1;
 333                         }
 334                 } else {
 335                         WARN("can't find unreachable insn at %s+0x%x",
 336                              rela->sym->sec->name, rela->addend);
 337                         return -1;
 338                 }
 339
 340                 insn->dead_end = true;
 341         }
 342
 343         return 0;
 344 }
 345
 346 /*
 347  * Warnings shouldn't be reported for ignored functions.
 348  */
 349 static void add_ignores(struct objtool_file *file)
 350 {
 351         struct instruction *insn;
 352         struct section *sec;
 353         struct symbol *func;
 354
 355         list_for_each_entry(sec, &file->elf->sections, list) {
 356                 list_for_each_entry(func, &sec->symbol_list, list) {
 357                         if (func->type != STT_FUNC)
 358                                 continue;
 359
 360                         if (!ignore_func(file, func))
 361                                 continue;
 362
 363                         func_for_each_insn(file, func, insn)
 364                                 insn->visited = true;
 365                 }
 366         }
 367 }
 368
 369 /*
 370  * Find the destination instructions for all jumps.
 371  */
 372 static int add_jump_destinations(struct objtool_file *file)
 373 {
 374         struct instruction *insn;
 375         struct rela *rela;
 376         struct section *dest_sec;
 377         unsigned long dest_off;
 378
 379         for_each_insn(file, insn) {
 380                 if (insn->type != INSN_JUMP_CONDITIONAL &&
 381                     insn->type != INSN_JUMP_UNCONDITIONAL)
 382                         continue;
 383
 384                 /* skip ignores */
 385                 if (insn->visited)
 386                         continue;
 387
 388                 rela = find_rela_by_dest_range(insn->sec, insn->offset,
 389                                                insn->len);
 390                 if (!rela) {
 391                         dest_sec = insn->sec;
 392                         dest_off = insn->offset + insn->len + insn->immediate;
 393                 } else if (rela->sym->type == STT_SECTION) {
 394                         dest_sec = rela->sym->sec;
 395                         dest_off = rela->addend + 4;
 396                 } else if (rela->sym->sec->idx) {
 397                         dest_sec = rela->sym->sec;
 398                         dest_off = rela->sym->sym.st_value + rela->addend + 4;
 399                 } else {
 400                         /* sibling call */
 401                         insn->jump_dest = 0;
 402                         continue;
 403                 }
 404
 405                 insn->jump_dest = find_insn(file, dest_sec, dest_off);
 406                 if (!insn->jump_dest) {
 407
 408                         /*
 409                          * This is a special case where an alt instruction
 410                          * jumps past the end of the section.  These are
 411                          * handled later in handle_group_alt().
 412                          */
 413                         if (!strcmp(insn->sec->name, ".altinstr_replacement"))
 414                                 continue;
 415
 416                         WARN_FUNC("can't find jump dest instruction at %s+0x%lx",
 417                                   insn->sec, insn->offset, dest_sec->name,
 418                                   dest_off);
 419                         return -1;
 420                 }
 421         }
 422
 423         return 0;
 424 }
 425
 426 /*
 427  * Find the destination instructions for all calls.
 428  */
 429 static int add_call_destinations(struct objtool_file *file)
 430 {
 431         struct instruction *insn;
 432         unsigned long dest_off;
 433         struct rela *rela;
 434
 435         for_each_insn(file, insn) {
 436                 if (insn->type != INSN_CALL)
 437                         continue;
 438
 439                 rela = find_rela_by_dest_range(insn->sec, insn->offset,
 440                                                insn->len);
 441                 if (!rela) {
 442                         dest_off = insn->offset + insn->len + insn->immediate;
 443                         insn->call_dest = find_symbol_by_offset(insn->sec,
 444                                                                 dest_off);
 445                         if (!insn->call_dest) {
 446                                 WARN_FUNC("can't find call dest symbol at offset 0x%lx",
 447                                           insn->sec, insn->offset, dest_off);
 448                                 return -1;
 449                         }
 450                 } else if (rela->sym->type == STT_SECTION) {
 451                         insn->call_dest = find_symbol_by_offset(rela->sym->sec,
 452                                                                 rela->addend+4);
 453                         if (!insn->call_dest ||
 454                             insn->call_dest->type != STT_FUNC) {
 455                                 WARN_FUNC("can't find call dest symbol at %s+0x%x",
 456                                           insn->sec, insn->offset,
 457                                           rela->sym->sec->name,
 458                                           rela->addend + 4);
 459                                 return -1;
 460                         }
 461                 } else
 462                         insn->call_dest = rela->sym;
 463         }
 464
 465         return 0;
 466 }
 467
 468 /*
 469  * The .alternatives section requires some extra special care, over and above
 470  * what other special sections require:
 471  *
 472  * 1. Because alternatives are patched in-place, we need to insert a fake jump
 473  *    instruction at the end so that validate_branch() skips all the original
 474  *    replaced instructions when validating the new instruction path.
 475  *
 476  * 2. An added wrinkle is that the new instruction length might be zero.  In
 477  *    that case the old instructions are replaced with noops.  We simulate that
 478  *    by creating a fake jump as the only new instruction.
 479  *
 480  * 3. In some cases, the alternative section includes an instruction which
 481  *    conditionally jumps to the _end_ of the entry.  We have to modify these
 482  *    jumps' destinations to point back to .text rather than the end of the
 483  *    entry in .altinstr_replacement.
 484  *
 485  * 4. It has been requested that we don't validate the !POPCNT feature path
 486  *    which is a "very very small percentage of machines".
 487  */
 488 static int handle_group_alt(struct objtool_file *file,
 489                             struct special_alt *special_alt,
 490                             struct instruction *orig_insn,
 491                             struct instruction **new_insn)
 492 {
 493         struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump;
 494         unsigned long dest_off;
 495
 496         last_orig_insn = NULL;
 497         insn = orig_insn;
 498         sec_for_each_insn_from(file, insn) {
 499                 if (insn->offset >= special_alt->orig_off + special_alt->orig_len)
 500                         break;
 501
 502                 if (special_alt->skip_orig)
 503                         insn->type = INSN_NOP;
 504
 505                 insn->alt_group = true;
 506                 last_orig_insn = insn;
 507         }
 508
 509         if (!next_insn_same_sec(file, last_orig_insn)) {
 510                 WARN("%s: don't know how to handle alternatives at end of section",
 511                      special_alt->orig_sec->name);
 512                 return -1;
 513         }
 514
 515         fake_jump = malloc(sizeof(*fake_jump));
 516         if (!fake_jump) {
 517                 WARN("malloc failed");
 518                 return -1;
 519         }
 520         memset(fake_jump, 0, sizeof(*fake_jump));
 521         INIT_LIST_HEAD(&fake_jump->alts);
 522         fake_jump->sec = special_alt->new_sec;
 523         fake_jump->offset = -1;
 524         fake_jump->type = INSN_JUMP_UNCONDITIONAL;
 525         fake_jump->jump_dest = list_next_entry(last_orig_insn, list);
 526
 527         if (!special_alt->new_len) {
 528                 *new_insn = fake_jump;
 529                 return 0;
 530         }
 531
 532         last_new_insn = NULL;
 533         insn = *new_insn;
 534         sec_for_each_insn_from(file, insn) {
 535                 if (insn->offset >= special_alt->new_off + special_alt->new_len)
 536                         break;
 537
 538                 last_new_insn = insn;
 539
 540                 if (insn->type != INSN_JUMP_CONDITIONAL &&
 541                     insn->type != INSN_JUMP_UNCONDITIONAL)
 542                         continue;
 543
 544                 if (!insn->immediate)
 545                         continue;
 546
 547                 dest_off = insn->offset + insn->len + insn->immediate;
 548                 if (dest_off == special_alt->new_off + special_alt->new_len)
 549                         insn->jump_dest = fake_jump;
 550
 551                 if (!insn->jump_dest) {
 552                         WARN_FUNC("can't find alternative jump destination",
 553                                   insn->sec, insn->offset);
 554                         return -1;
 555                 }
 556         }
 557
 558         if (!last_new_insn) {
 559                 WARN_FUNC("can't find last new alternative instruction",
 560                           special_alt->new_sec, special_alt->new_off);
 561                 return -1;
 562         }
 563
 564         list_add(&fake_jump->list, &last_new_insn->list);
 565
 566         return 0;
 567 }
 568
 569 /*
 570  * A jump table entry can either convert a nop to a jump or a jump to a nop.
 571  * If the original instruction is a jump, make the alt entry an effective nop
 572  * by just skipping the original instruction.
 573  */
 574 static int handle_jump_alt(struct objtool_file *file,
 575                            struct special_alt *special_alt,
 576                            struct instruction *orig_insn,
 577                            struct instruction **new_insn)
 578 {
 579         if (orig_insn->type == INSN_NOP)
 580                 return 0;
 581
 582         if (orig_insn->type != INSN_JUMP_UNCONDITIONAL) {
 583                 WARN_FUNC("unsupported instruction at jump label",
 584                           orig_insn->sec, orig_insn->offset);
 585                 return -1;
 586         }
 587
 588         *new_insn = list_next_entry(orig_insn, list);
 589         return 0;
 590 }
 591
 592 /*
 593  * Read all the special sections which have alternate instructions which can be
 594  * patched in or redirected to at runtime.  Each instruction having alternate
 595  * instruction(s) has them added to its insn->alts list, which will be
 596  * traversed in validate_branch().
 597  */
 598 static int add_special_section_alts(struct objtool_file *file)
 599 {
 600         struct list_head special_alts;
 601         struct instruction *orig_insn, *new_insn;
 602         struct special_alt *special_alt, *tmp;
 603         struct alternative *alt;
 604         int ret;
 605
 606         ret = special_get_alts(file->elf, &special_alts);
 607         if (ret)
 608                 return ret;
 609
 610         list_for_each_entry_safe(special_alt, tmp, &special_alts, list) {
 611                 alt = malloc(sizeof(*alt));
 612                 if (!alt) {
 613                         WARN("malloc failed");
 614                         ret = -1;
 615                         goto out;
 616                 }
 617
 618                 orig_insn = find_insn(file, special_alt->orig_sec,
 619                                       special_alt->orig_off);
 620                 if (!orig_insn) {
 621                         WARN_FUNC("special: can't find orig instruction",
 622                                   special_alt->orig_sec, special_alt->orig_off);
 623                         ret = -1;
 624                         goto out;
 625                 }
 626
 627                 new_insn = NULL;
 628                 if (!special_alt->group || special_alt->new_len) {
 629                         new_insn = find_insn(file, special_alt->new_sec,
 630                                              special_alt->new_off);
 631                         if (!new_insn) {
 632                                 WARN_FUNC("special: can't find new instruction",
 633                                           special_alt->new_sec,
 634                                           special_alt->new_off);
 635                                 ret = -1;
 636                                 goto out;
 637                         }
 638                 }
 639
 640                 if (special_alt->group) {
 641                         ret = handle_group_alt(file, special_alt, orig_insn,
 642                                                &new_insn);
 643                         if (ret)
 644                                 goto out;
 645                 } else if (special_alt->jump_or_nop) {
 646                         ret = handle_jump_alt(file, special_alt, orig_insn,
 647                                               &new_insn);
 648                         if (ret)
 649                                 goto out;
 650                 }
 651
 652                 alt->insn = new_insn;
 653                 list_add_tail(&alt->list, &orig_insn->alts);
 654
 655                 list_del(&special_alt->list);
 656                 free(special_alt);
 657         }
 658
 659 out:
 660         return ret;
 661 }
 662
 663 static int add_switch_table(struct objtool_file *file, struct symbol *func,
 664                             struct instruction *insn, struct rela *table,
 665                             struct rela *next_table)
 666 {
 667         struct rela *rela = table;
 668         struct instruction *alt_insn;
 669         struct alternative *alt;
 670
 671         list_for_each_entry_from(rela, &file->rodata->rela->rela_list, list) {
 672                 if (rela == next_table)
 673                         break;
 674
 675                 if (rela->sym->sec != insn->sec ||
 676                     rela->addend <= func->offset ||
 677                     rela->addend >= func->offset + func->len)
 678                         break;
 679
 680                 alt_insn = find_insn(file, insn->sec, rela->addend);
 681                 if (!alt_insn) {
 682                         WARN("%s: can't find instruction at %s+0x%x",
 683                              file->rodata->rela->name, insn->sec->name,
 684                              rela->addend);
 685                         return -1;
 686                 }
 687
 688                 alt = malloc(sizeof(*alt));
 689                 if (!alt) {
 690                         WARN("malloc failed");
 691                         return -1;
 692                 }
 693
 694                 alt->insn = alt_insn;
 695                 list_add_tail(&alt->list, &insn->alts);
 696         }
 697
 698         return 0;
 699 }
 700
 701 /*
 702  * find_switch_table() - Given a dynamic jump, find the switch jump table in
 703  * .rodata associated with it.
 704  *
 705  * There are 3 basic patterns:
 706  *
 707  * 1. jmpq *[rodata addr](,%reg,8)
 708  *
 709  *    This is the most common case by far.  It jumps to an address in a simple
 710  *    jump table which is stored in .rodata.
 711  *
 712  * 2. jmpq *[rodata addr](%rip)
 713  *
 714  *    This is caused by a rare GCC quirk, currently only seen in three driver
 715  *    functions in the kernel, only with certain obscure non-distro configs.
 716  *
 717  *    As part of an optimization, GCC makes a copy of an existing switch jump
 718  *    table, modifies it, and then hard-codes the jump (albeit with an indirect
 719  *    jump) to use a single entry in the table.  The rest of the jump table and
 720  *    some of its jump targets remain as dead code.
 721  *
 722  *    In such a case we can just crudely ignore all unreachable instruction
 723  *    warnings for the entire object file.  Ideally we would just ignore them
 724  *    for the function, but that would require redesigning the code quite a
 725  *    bit.  And honestly that's just not worth doing: unreachable instruction
 726  *    warnings are of questionable value anyway, and this is such a rare issue.
 727  *
 728  * 3. mov [rodata addr],%reg1
 729  *    ... some instructions ...
 730  *    jmpq *(%reg1,%reg2,8)
 731  *
 732  *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
 733  *    writing, there are 11 occurrences of it in the allmodconfig kernel.
 734  *
 735  *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
 736  *    ensure the same register is used in the mov and jump instructions.
 737  */
 738 static struct rela *find_switch_table(struct objtool_file *file,
 739                                       struct symbol *func,
 740                                       struct instruction *insn)
 741 {
 742         struct rela *text_rela, *rodata_rela;
 743         struct instruction *orig_insn = insn;
 744
 745         text_rela = find_rela_by_dest_range(insn->sec, insn->offset, insn->len);
 746         if (text_rela && text_rela->sym == file->rodata->sym) {
 747                 /* case 1 */
 748                 rodata_rela = find_rela_by_dest(file->rodata,
 749                                                 text_rela->addend);
 750                 if (rodata_rela)
 751                         return rodata_rela;
 752
 753                 /* case 2 */
 754                 rodata_rela = find_rela_by_dest(file->rodata,
 755                                                 text_rela->addend + 4);
 756                 if (!rodata_rela)
 757                         return NULL;
 758                 file->ignore_unreachables = true;
 759                 return rodata_rela;
 760         }
 761
 762         /* case 3 */
 763         func_for_each_insn_continue_reverse(file, func, insn) {
 764                 if (insn->type == INSN_JUMP_DYNAMIC)
 765                         break;
 766
 767                 /* allow small jumps within the range */
 768                 if (insn->type == INSN_JUMP_UNCONDITIONAL &&
 769                     insn->jump_dest &&
 770                     (insn->jump_dest->offset <= insn->offset ||
 771                      insn->jump_dest->offset > orig_insn->offset))
 772                     break;
 773
 774                 /* look for a relocation which references .rodata */
 775                 text_rela = find_rela_by_dest_range(insn->sec, insn->offset,
 776                                                     insn->len);
 777                 if (!text_rela || text_rela->sym != file->rodata->sym)
 778                         continue;
 779
 780                 /*
 781                  * Make sure the .rodata address isn't associated with a
 782                  * symbol.  gcc jump tables are anonymous data.
 783                  */
 784                 if (find_symbol_containing(file->rodata, text_rela->addend))
 785                         continue;
 786
 787                 return find_rela_by_dest(file->rodata, text_rela->addend);
 788         }
 789
 790         return NULL;
 791 }
 792
 793 static int add_func_switch_tables(struct objtool_file *file,
 794                                   struct symbol *func)
 795 {
 796         struct instruction *insn, *prev_jump = NULL;
 797         struct rela *rela, *prev_rela = NULL;
 798         int ret;
 799
 800         func_for_each_insn(file, func, insn) {
 801                 if (insn->type != INSN_JUMP_DYNAMIC)
 802                         continue;
 803
 804                 rela = find_switch_table(file, func, insn);
 805                 if (!rela)
 806                         continue;
 807
 808                 /*
 809                  * We found a switch table, but we don't know yet how big it
 810                  * is.  Don't add it until we reach the end of the function or
 811                  * the beginning of another switch table in the same function.
 812                  */
 813                 if (prev_jump) {
 814                         ret = add_switch_table(file, func, prev_jump, prev_rela,
 815                                                rela);
 816                         if (ret)
 817                                 return ret;
 818                 }
 819
 820                 prev_jump = insn;
 821                 prev_rela = rela;
 822         }
 823
 824         if (prev_jump) {
 825                 ret = add_switch_table(file, func, prev_jump, prev_rela, NULL);
 826                 if (ret)
 827                         return ret;
 828         }
 829
 830         return 0;
 831 }
 832
 833 /*
 834  * For some switch statements, gcc generates a jump table in the .rodata
 835  * section which contains a list of addresses within the function to jump to.
 836  * This finds these jump tables and adds them to the insn->alts lists.
 837  */
 838 static int add_switch_table_alts(struct objtool_file *file)
 839 {
 840         struct section *sec;
 841         struct symbol *func;
 842         int ret;
 843
 844         if (!file->rodata || !file->rodata->rela)
 845                 return 0;
 846
 847         list_for_each_entry(sec, &file->elf->sections, list) {
 848                 list_for_each_entry(func, &sec->symbol_list, list) {
 849                         if (func->type != STT_FUNC)
 850                                 continue;
 851
 852                         ret = add_func_switch_tables(file, func);
 853                         if (ret)
 854                                 return ret;
 855                 }
 856         }
 857
 858         return 0;
 859 }
 860
 861 static int decode_sections(struct objtool_file *file)
 862 {
 863         int ret;
 864
 865         ret = decode_instructions(file);
 866         if (ret)
 867                 return ret;
 868
 869         ret = add_dead_ends(file);
 870         if (ret)
 871                 return ret;
 872
 873         add_ignores(file);
 874
 875         ret = add_jump_destinations(file);
 876         if (ret)
 877                 return ret;
 878
 879         ret = add_call_destinations(file);
 880         if (ret)
 881                 return ret;
 882
 883         ret = add_special_section_alts(file);
 884         if (ret)
 885                 return ret;
 886
 887         ret = add_switch_table_alts(file);
 888         if (ret)
 889                 return ret;
 890
 891         return 0;
 892 }
 893
 894 static bool is_fentry_call(struct instruction *insn)
 895 {
 896         if (insn->type == INSN_CALL &&
 897             insn->call_dest->type == STT_NOTYPE &&
 898             !strcmp(insn->call_dest->name, "__fentry__"))
 899                 return true;
 900
 901         return false;
 902 }
 903
 904 static bool has_modified_stack_frame(struct instruction *insn)
 905 {
 906         return (insn->state & STATE_FP_SAVED) ||
 907                (insn->state & STATE_FP_SETUP);
 908 }
 909
 910 static bool has_valid_stack_frame(struct instruction *insn)
 911 {
 912         return (insn->state & STATE_FP_SAVED) &&
 913                (insn->state & STATE_FP_SETUP);
 914 }
 915
 916 static unsigned int frame_state(unsigned long state)
 917 {
 918         return (state & (STATE_FP_SAVED | STATE_FP_SETUP));
 919 }
 920
 921 /*
 922  * Follow the branch starting at the given instruction, and recursively follow
 923  * any other branches (jumps).  Meanwhile, track the frame pointer state at
 924  * each instruction and validate all the rules described in
 925  * tools/objtool/Documentation/stack-validation.txt.
 926  */
 927 static int validate_branch(struct objtool_file *file,
 928                            struct instruction *first, unsigned char first_state)
 929 {
 930         struct alternative *alt;
 931         struct instruction *insn;
 932         struct section *sec;
 933         struct symbol *func = NULL;
 934         unsigned char state;
 935         int ret;
 936
 937         insn = first;
 938         sec = insn->sec;
 939         state = first_state;
 940
 941         if (insn->alt_group && list_empty(&insn->alts)) {
 942                 WARN_FUNC("don't know how to handle branch to middle of alternative instruction group",
 943                           sec, insn->offset);
 944                 return 1;
 945         }
 946
 947         while (1) {
 948                 if (file->c_file && insn->func) {
 949                         if (func && func != insn->func) {
 950                                 WARN("%s() falls through to next function %s()",
 951                                      func->name, insn->func->name);
 952                                 return 1;
 953                         }
 954
 955                         func = insn->func;
 956                 }
 957
 958                 if (insn->visited) {
 959                         if (frame_state(insn->state) != frame_state(state)) {
 960                                 WARN_FUNC("frame pointer state mismatch",
 961                                           sec, insn->offset);
 962                                 return 1;
 963                         }
 964
 965                         return 0;
 966                 }
 967
 968                 insn->visited = true;
 969                 insn->state = state;
 970
 971                 list_for_each_entry(alt, &insn->alts, list) {
 972                         ret = validate_branch(file, alt->insn, state);
 973                         if (ret)
 974                                 return 1;
 975                 }
 976
 977                 switch (insn->type) {
 978
 979                 case INSN_FP_SAVE:
 980                         if (!nofp) {
 981                                 if (state & STATE_FP_SAVED) {
 982                                         WARN_FUNC("duplicate frame pointer save",
 983                                                   sec, insn->offset);
 984                                         return 1;
 985                                 }
 986                                 state |= STATE_FP_SAVED;
 987                         }
 988                         break;
 989
 990                 case INSN_FP_SETUP:
 991                         if (!nofp) {
 992                                 if (state & STATE_FP_SETUP) {
 993                                         WARN_FUNC("duplicate frame pointer setup",
 994                                                   sec, insn->offset);
 995                                         return 1;
 996                                 }
 997                                 state |= STATE_FP_SETUP;
 998                         }
 999                         break;
1000
1001                 case INSN_FP_RESTORE:
1002                         if (!nofp) {
1003                                 if (has_valid_stack_frame(insn))
1004                                         state &= ~STATE_FP_SETUP;
1005
1006                                 state &= ~STATE_FP_SAVED;
1007                         }
1008                         break;
1009
1010                 case INSN_RETURN:
1011                         if (!nofp && has_modified_stack_frame(insn)) {
1012                                 WARN_FUNC("return without frame pointer restore",
1013                                           sec, insn->offset);
1014                                 return 1;
1015                         }
1016                         return 0;
1017
1018                 case INSN_CALL:
1019                         if (is_fentry_call(insn)) {
1020                                 state |= STATE_FENTRY;
1021                                 break;
1022                         }
1023
1024                         ret = dead_end_function(file, insn->call_dest);
1025                         if (ret == 1)
1026                                 return 0;
1027                         if (ret == -1)
1028                                 return 1;
1029
1030                         /* fallthrough */
1031                 case INSN_CALL_DYNAMIC:
1032                         if (!nofp && !has_valid_stack_frame(insn)) {
1033                                 WARN_FUNC("call without frame pointer save/setup",
1034                                           sec, insn->offset);
1035                                 return 1;
1036                         }
1037                         break;
1038
1039                 case INSN_JUMP_CONDITIONAL:
1040                 case INSN_JUMP_UNCONDITIONAL:
1041                         if (insn->jump_dest) {
1042                                 ret = validate_branch(file, insn->jump_dest,
1043                                                       state);
1044                                 if (ret)
1045                                         return 1;
1046                         } else if (has_modified_stack_frame(insn)) {
1047                                 WARN_FUNC("sibling call from callable instruction with changed frame pointer",
1048                                           sec, insn->offset);
1049                                 return 1;
1050                         } /* else it's a sibling call */
1051
1052                         if (insn->type == INSN_JUMP_UNCONDITIONAL)
1053                                 return 0;
1054
1055                         break;
1056
1057                 case INSN_JUMP_DYNAMIC:
1058                         if (list_empty(&insn->alts) &&
1059                             has_modified_stack_frame(insn)) {
1060                                 WARN_FUNC("sibling call from callable instruction with changed frame pointer",
1061                                           sec, insn->offset);
1062                                 return 1;
1063                         }
1064
1065                         return 0;
1066
1067                 default:
1068                         break;
1069                 }
1070
1071                 if (insn->dead_end)
1072                         return 0;
1073
1074                 insn = next_insn_same_sec(file, insn);
1075                 if (!insn) {
1076                         WARN("%s: unexpected end of section", sec->name);
1077                         return 1;
1078                 }
1079         }
1080
1081         return 0;
1082 }
1083
1084 static bool is_kasan_insn(struct instruction *insn)
1085 {
1086         return (insn->type == INSN_CALL &&
1087                 !strcmp(insn->call_dest->name, "__asan_handle_no_return"));
1088 }
1089
1090 static bool is_ubsan_insn(struct instruction *insn)
1091 {
1092         return (insn->type == INSN_CALL &&
1093                 !strcmp(insn->call_dest->name,
1094                         "__ubsan_handle_builtin_unreachable"));
1095 }
1096
1097 static bool ignore_unreachable_insn(struct symbol *func,
1098                                     struct instruction *insn)
1099 {
1100         int i;
1101
1102         if (insn->type == INSN_NOP)
1103                 return true;
1104
1105         /*
1106          * Check if this (or a subsequent) instruction is related to
1107          * CONFIG_UBSAN or CONFIG_KASAN.
1108          *
1109          * End the search at 5 instructions to avoid going into the weeds.
1110          */
1111         for (i = 0; i < 5; i++) {
1112
1113                 if (is_kasan_insn(insn) || is_ubsan_insn(insn))
1114                         return true;
1115
1116                 if (insn->type == INSN_JUMP_UNCONDITIONAL && insn->jump_dest) {
1117                         insn = insn->jump_dest;
1118                         continue;
1119                 }
1120
1121                 if (insn->offset + insn->len >= func->offset + func->len)
1122                         break;
1123                 insn = list_next_entry(insn, list);
1124         }
1125
1126         return false;
1127 }
1128
1129 static int validate_functions(struct objtool_file *file)
1130 {
1131         struct section *sec;
1132         struct symbol *func;
1133         struct instruction *insn;
1134         int ret, warnings = 0;
1135
1136         list_for_each_entry(sec, &file->elf->sections, list) {
1137                 list_for_each_entry(func, &sec->symbol_list, list) {
1138                         if (func->type != STT_FUNC)
1139                                 continue;
1140
1141                         insn = find_insn(file, sec, func->offset);
1142                         if (!insn)
1143                                 continue;
1144
1145                         ret = validate_branch(file, insn, 0);
1146                         warnings += ret;
1147                 }
1148         }
1149
1150         list_for_each_entry(sec, &file->elf->sections, list) {
1151                 list_for_each_entry(func, &sec->symbol_list, list) {
1152                         if (func->type != STT_FUNC)
1153                                 continue;
1154
1155                         func_for_each_insn(file, func, insn) {
1156                                 if (insn->visited)
1157                                         continue;
1158
1159                                 insn->visited = true;
1160
1161                                 if (file->ignore_unreachables || warnings ||
1162                                     ignore_unreachable_insn(func, insn))
1163                                         continue;
1164
1165                                 /*
1166                                  * gcov produces a lot of unreachable
1167                                  * instructions.  If we get an unreachable
1168                                  * warning and the file has gcov enabled, just
1169                                  * ignore it, and all other such warnings for
1170                                  * the file.
1171                                  */
1172                                 if (!file->ignore_unreachables &&
1173                                     gcov_enabled(file)) {
1174                                         file->ignore_unreachables = true;
1175                                         continue;
1176                                 }
1177
1178                                 WARN_FUNC("function has unreachable instruction", insn->sec, insn->offset);
1179                                 warnings++;
1180                         }
1181                 }
1182         }
1183
1184         return warnings;
1185 }
1186
1187 static int validate_uncallable_instructions(struct objtool_file *file)
1188 {
1189         struct instruction *insn;
1190         int warnings = 0;
1191
1192         for_each_insn(file, insn) {
1193                 if (!insn->visited && insn->type == INSN_RETURN) {
1194                         WARN_FUNC("return instruction outside of a callable function",
1195                                   insn->sec, insn->offset);
1196                         warnings++;
1197                 }
1198         }
1199
1200         return warnings;
1201 }
1202
1203 static void cleanup(struct objtool_file *file)
1204 {
1205         struct instruction *insn, *tmpinsn;
1206         struct alternative *alt, *tmpalt;
1207
1208         list_for_each_entry_safe(insn, tmpinsn, &file->insn_list, list) {
1209                 list_for_each_entry_safe(alt, tmpalt, &insn->alts, list) {
1210                         list_del(&alt->list);
1211                         free(alt);
1212                 }
1213                 list_del(&insn->list);
1214                 hash_del(&insn->hash);
1215                 free(insn);
1216         }
1217         elf_close(file->elf);
1218 }
1219
1220 int check(const char *_objname, bool _nofp)
1221 {
1222         struct objtool_file file;
1223         int ret, warnings = 0;
1224
1225         objname = _objname;
1226         nofp = _nofp;
1227
1228         file.elf = elf_open(objname);
1229         if (!file.elf) {
1230                 fprintf(stderr, "error reading elf file %s\n", objname);
1231                 return 1;
1232         }
1233
1234         INIT_LIST_HEAD(&file.insn_list);
1235         hash_init(file.insn_hash);
1236         file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
1237         file.rodata = find_section_by_name(file.elf, ".rodata");
1238         file.ignore_unreachables = false;
1239         file.c_file = find_section_by_name(file.elf, ".comment");
1240
1241         ret = decode_sections(&file);
1242         if (ret < 0)
1243                 goto out;
1244         warnings += ret;
1245
1246         ret = validate_functions(&file);
1247         if (ret < 0)
1248                 goto out;
1249         warnings += ret;
1250
1251         ret = validate_uncallable_instructions(&file);
1252         if (ret < 0)
1253                 goto out;
1254         warnings += ret;
1255
1256 out:
1257         cleanup(&file);
1258
1259         /* ignore warnings for now until we get all the code cleaned up */
1260         if (ret || warnings)
1261                 return 0;
1262         return 0;
1263 }