tools/objtool/builtin-check.c

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