arch/x86/kernel/unwind_orc.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 #include <linux/module.h>
   3 #include <linux/sort.h>
   4 #include <asm/ptrace.h>
   5 #include <asm/stacktrace.h>
   6 #include <asm/unwind.h>
   7 #include <asm/orc_types.h>
   8 #include <asm/orc_lookup.h>
   9
  10 #define orc_warn(fmt, ...) \
  11         printk_deferred_once(KERN_WARNING "WARNING: " fmt, ##__VA_ARGS__)
  12
  13 #define orc_warn_current(args...)                                       \
  14 ({                                                                      \
  15         if (state->task == current)                                     \
  16                 orc_warn(args);                                         \
  17 })
  18
  19 extern int __start_orc_unwind_ip[];
  20 extern int __stop_orc_unwind_ip[];
  21 extern struct orc_entry __start_orc_unwind[];
  22 extern struct orc_entry __stop_orc_unwind[];
  23
  24 static bool orc_init __ro_after_init;
  25 static unsigned int lookup_num_blocks __ro_after_init;
  26
  27 static inline unsigned long orc_ip(const int *ip)
  28 {
  29         return (unsigned long)ip + *ip;
  30 }
  31
  32 static struct orc_entry *__orc_find(int *ip_table, struct orc_entry *u_table,
  33                                     unsigned int num_entries, unsigned long ip)
  34 {
  35         int *first = ip_table;
  36         int *last = ip_table + num_entries - 1;
  37         int *mid = first, *found = first;
  38
  39         if (!num_entries)
  40                 return NULL;
  41
  42         /*
  43          * Do a binary range search to find the rightmost duplicate of a given
  44          * starting address.  Some entries are section terminators which are
  45          * "weak" entries for ensuring there are no gaps.  They should be
  46          * ignored when they conflict with a real entry.
  47          */
  48         while (first <= last) {
  49                 mid = first + ((last - first) / 2);
  50
  51                 if (orc_ip(mid) <= ip) {
  52                         found = mid;
  53                         first = mid + 1;
  54                 } else
  55                         last = mid - 1;
  56         }
  57
  58         return u_table + (found - ip_table);
  59 }
  60
  61 #ifdef CONFIG_MODULES
  62 static struct orc_entry *orc_module_find(unsigned long ip)
  63 {
  64         struct module *mod;
  65
  66         mod = __module_address(ip);
  67         if (!mod || !mod->arch.orc_unwind || !mod->arch.orc_unwind_ip)
  68                 return NULL;
  69         return __orc_find(mod->arch.orc_unwind_ip, mod->arch.orc_unwind,
  70                           mod->arch.num_orcs, ip);
  71 }
  72 #else
  73 static struct orc_entry *orc_module_find(unsigned long ip)
  74 {
  75         return NULL;
  76 }
  77 #endif
  78
  79 #ifdef CONFIG_DYNAMIC_FTRACE
  80 static struct orc_entry *orc_find(unsigned long ip);
  81
  82 /*
  83  * Ftrace dynamic trampolines do not have orc entries of their own.
  84  * But they are copies of the ftrace entries that are static and
  85  * defined in ftrace_*.S, which do have orc entries.
  86  *
  87  * If the unwinder comes across a ftrace trampoline, then find the
  88  * ftrace function that was used to create it, and use that ftrace
  89  * function's orc entry, as the placement of the return code in
  90  * the stack will be identical.
  91  */
  92 static struct orc_entry *orc_ftrace_find(unsigned long ip)
  93 {
  94         struct ftrace_ops *ops;
  95         unsigned long caller;
  96
  97         ops = ftrace_ops_trampoline(ip);
  98         if (!ops)
  99                 return NULL;
 100
 101         if (ops->flags & FTRACE_OPS_FL_SAVE_REGS)
 102                 caller = (unsigned long)ftrace_regs_call;
 103         else
 104                 caller = (unsigned long)ftrace_call;
 105
 106         /* Prevent unlikely recursion */
 107         if (ip == caller)
 108                 return NULL;
 109
 110         return orc_find(caller);
 111 }
 112 #else
 113 static struct orc_entry *orc_ftrace_find(unsigned long ip)
 114 {
 115         return NULL;
 116 }
 117 #endif
 118
 119 /*
 120  * If we crash with IP==0, the last successfully executed instruction
 121  * was probably an indirect function call with a NULL function pointer,
 122  * and we don't have unwind information for NULL.
 123  * This hardcoded ORC entry for IP==0 allows us to unwind from a NULL function
 124  * pointer into its parent and then continue normally from there.
 125  */
 126 static struct orc_entry null_orc_entry = {
 127         .sp_offset = sizeof(long),
 128         .sp_reg = ORC_REG_SP,
 129         .bp_reg = ORC_REG_UNDEFINED,
 130         .type = ORC_TYPE_CALL
 131 };
 132
 133 /* Fake frame pointer entry -- used as a fallback for generated code */
 134 static struct orc_entry orc_fp_entry = {
 135         .type           = ORC_TYPE_CALL,
 136         .sp_reg         = ORC_REG_BP,
 137         .sp_offset      = 16,
 138         .bp_reg         = ORC_REG_PREV_SP,
 139         .bp_offset      = -16,
 140         .end            = 0,
 141 };
 142
 143 static struct orc_entry *orc_find(unsigned long ip)
 144 {
 145         static struct orc_entry *orc;
 146
 147         if (ip == 0)
 148                 return &null_orc_entry;
 149
 150         /* For non-init vmlinux addresses, use the fast lookup table: */
 151         if (ip >= LOOKUP_START_IP && ip < LOOKUP_STOP_IP) {
 152                 unsigned int idx, start, stop;
 153
 154                 idx = (ip - LOOKUP_START_IP) / LOOKUP_BLOCK_SIZE;
 155
 156                 if (unlikely((idx >= lookup_num_blocks-1))) {
 157                         orc_warn("WARNING: bad lookup idx: idx=%u num=%u ip=%pB\n",
 158                                  idx, lookup_num_blocks, (void *)ip);
 159                         return NULL;
 160                 }
 161
 162                 start = orc_lookup[idx];
 163                 stop = orc_lookup[idx + 1] + 1;
 164
 165                 if (unlikely((__start_orc_unwind + start >= __stop_orc_unwind) ||
 166                              (__start_orc_unwind + stop > __stop_orc_unwind))) {
 167                         orc_warn("WARNING: bad lookup value: idx=%u num=%u start=%u stop=%u ip=%pB\n",
 168                                  idx, lookup_num_blocks, start, stop, (void *)ip);
 169                         return NULL;
 170                 }
 171
 172                 return __orc_find(__start_orc_unwind_ip + start,
 173                                   __start_orc_unwind + start, stop - start, ip);
 174         }
 175
 176         /* vmlinux .init slow lookup: */
 177         if (init_kernel_text(ip))
 178                 return __orc_find(__start_orc_unwind_ip, __start_orc_unwind,
 179                                   __stop_orc_unwind_ip - __start_orc_unwind_ip, ip);
 180
 181         /* Module lookup: */
 182         orc = orc_module_find(ip);
 183         if (orc)
 184                 return orc;
 185
 186         return orc_ftrace_find(ip);
 187 }
 188
 189 #ifdef CONFIG_MODULES
 190
 191 static DEFINE_MUTEX(sort_mutex);
 192 static int *cur_orc_ip_table = __start_orc_unwind_ip;
 193 static struct orc_entry *cur_orc_table = __start_orc_unwind;
 194
 195 static void orc_sort_swap(void *_a, void *_b, int size)
 196 {
 197         struct orc_entry *orc_a, *orc_b;
 198         struct orc_entry orc_tmp;
 199         int *a = _a, *b = _b, tmp;
 200         int delta = _b - _a;
 201
 202         /* Swap the .orc_unwind_ip entries: */
 203         tmp = *a;
 204         *a = *b + delta;
 205         *b = tmp - delta;
 206
 207         /* Swap the corresponding .orc_unwind entries: */
 208         orc_a = cur_orc_table + (a - cur_orc_ip_table);
 209         orc_b = cur_orc_table + (b - cur_orc_ip_table);
 210         orc_tmp = *orc_a;
 211         *orc_a = *orc_b;
 212         *orc_b = orc_tmp;
 213 }
 214
 215 static int orc_sort_cmp(const void *_a, const void *_b)
 216 {
 217         struct orc_entry *orc_a;
 218         const int *a = _a, *b = _b;
 219         unsigned long a_val = orc_ip(a);
 220         unsigned long b_val = orc_ip(b);
 221
 222         if (a_val > b_val)
 223                 return 1;
 224         if (a_val < b_val)
 225                 return -1;
 226
 227         /*
 228          * The "weak" section terminator entries need to always be on the left
 229          * to ensure the lookup code skips them in favor of real entries.
 230          * These terminator entries exist to handle any gaps created by
 231          * whitelisted .o files which didn't get objtool generation.
 232          */
 233         orc_a = cur_orc_table + (a - cur_orc_ip_table);
 234         return orc_a->sp_reg == ORC_REG_UNDEFINED && !orc_a->end ? -1 : 1;
 235 }
 236
 237 void unwind_module_init(struct module *mod, void *_orc_ip, size_t orc_ip_size,
 238                         void *_orc, size_t orc_size)
 239 {
 240         int *orc_ip = _orc_ip;
 241         struct orc_entry *orc = _orc;
 242         unsigned int num_entries = orc_ip_size / sizeof(int);
 243
 244         WARN_ON_ONCE(orc_ip_size % sizeof(int) != 0 ||
 245                      orc_size % sizeof(*orc) != 0 ||
 246                      num_entries != orc_size / sizeof(*orc));
 247
 248         /*
 249          * The 'cur_orc_*' globals allow the orc_sort_swap() callback to
 250          * associate an .orc_unwind_ip table entry with its corresponding
 251          * .orc_unwind entry so they can both be swapped.
 252          */
 253         mutex_lock(&sort_mutex);
 254         cur_orc_ip_table = orc_ip;
 255         cur_orc_table = orc;
 256         sort(orc_ip, num_entries, sizeof(int), orc_sort_cmp, orc_sort_swap);
 257         mutex_unlock(&sort_mutex);
 258
 259         mod->arch.orc_unwind_ip = orc_ip;
 260         mod->arch.orc_unwind = orc;
 261         mod->arch.num_orcs = num_entries;
 262 }
 263 #endif
 264
 265 void __init unwind_init(void)
 266 {
 267         size_t orc_ip_size = (void *)__stop_orc_unwind_ip - (void *)__start_orc_unwind_ip;
 268         size_t orc_size = (void *)__stop_orc_unwind - (void *)__start_orc_unwind;
 269         size_t num_entries = orc_ip_size / sizeof(int);
 270         struct orc_entry *orc;
 271         int i;
 272
 273         if (!num_entries || orc_ip_size % sizeof(int) != 0 ||
 274             orc_size % sizeof(struct orc_entry) != 0 ||
 275             num_entries != orc_size / sizeof(struct orc_entry)) {
 276                 orc_warn("WARNING: Bad or missing .orc_unwind table.  Disabling unwinder.\n");
 277                 return;
 278         }
 279
 280         /*
 281          * Note, the orc_unwind and orc_unwind_ip tables were already
 282          * sorted at build time via the 'sorttable' tool.
 283          * It's ready for binary search straight away, no need to sort it.
 284          */
 285
 286         /* Initialize the fast lookup table: */
 287         lookup_num_blocks = orc_lookup_end - orc_lookup;
 288         for (i = 0; i < lookup_num_blocks-1; i++) {
 289                 orc = __orc_find(__start_orc_unwind_ip, __start_orc_unwind,
 290                                  num_entries,
 291                                  LOOKUP_START_IP + (LOOKUP_BLOCK_SIZE * i));
 292                 if (!orc) {
 293                         orc_warn("WARNING: Corrupt .orc_unwind table.  Disabling unwinder.\n");
 294                         return;
 295                 }
 296
 297                 orc_lookup[i] = orc - __start_orc_unwind;
 298         }
 299
 300         /* Initialize the ending block: */
 301         orc = __orc_find(__start_orc_unwind_ip, __start_orc_unwind, num_entries,
 302                          LOOKUP_STOP_IP);
 303         if (!orc) {
 304                 orc_warn("WARNING: Corrupt .orc_unwind table.  Disabling unwinder.\n");
 305                 return;
 306         }
 307         orc_lookup[lookup_num_blocks-1] = orc - __start_orc_unwind;
 308
 309         orc_init = true;
 310 }
 311
 312 unsigned long unwind_get_return_address(struct unwind_state *state)
 313 {
 314         if (unwind_done(state))
 315                 return 0;
 316
 317         return __kernel_text_address(state->ip) ? state->ip : 0;
 318 }
 319 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 320
 321 unsigned long *unwind_get_return_address_ptr(struct unwind_state *state)
 322 {
 323         if (unwind_done(state))
 324                 return NULL;
 325
 326         if (state->regs)
 327                 return &state->regs->ip;
 328
 329         if (state->sp)
 330                 return (unsigned long *)state->sp - 1;
 331
 332         return NULL;
 333 }
 334
 335 static bool stack_access_ok(struct unwind_state *state, unsigned long _addr,
 336                             size_t len)
 337 {
 338         struct stack_info *info = &state->stack_info;
 339         void *addr = (void *)_addr;
 340
 341         if (!on_stack(info, addr, len) &&
 342             (get_stack_info(addr, state->task, info, &state->stack_mask)))
 343                 return false;
 344
 345         return true;
 346 }
 347
 348 static bool deref_stack_reg(struct unwind_state *state, unsigned long addr,
 349                             unsigned long *val)
 350 {
 351         if (!stack_access_ok(state, addr, sizeof(long)))
 352                 return false;
 353
 354         *val = READ_ONCE_NOCHECK(*(unsigned long *)addr);
 355         return true;
 356 }
 357
 358 static bool deref_stack_regs(struct unwind_state *state, unsigned long addr,
 359                              unsigned long *ip, unsigned long *sp)
 360 {
 361         struct pt_regs *regs = (struct pt_regs *)addr;
 362
 363         /* x86-32 support will be more complicated due to the &regs->sp hack */
 364         BUILD_BUG_ON(IS_ENABLED(CONFIG_X86_32));
 365
 366         if (!stack_access_ok(state, addr, sizeof(struct pt_regs)))
 367                 return false;
 368
 369         *ip = regs->ip;
 370         *sp = regs->sp;
 371         return true;
 372 }
 373
 374 static bool deref_stack_iret_regs(struct unwind_state *state, unsigned long addr,
 375                                   unsigned long *ip, unsigned long *sp)
 376 {
 377         struct pt_regs *regs = (void *)addr - IRET_FRAME_OFFSET;
 378
 379         if (!stack_access_ok(state, addr, IRET_FRAME_SIZE))
 380                 return false;
 381
 382         *ip = regs->ip;
 383         *sp = regs->sp;
 384         return true;
 385 }
 386
 387 /*
 388  * If state->regs is non-NULL, and points to a full pt_regs, just get the reg
 389  * value from state->regs.
 390  *
 391  * Otherwise, if state->regs just points to IRET regs, and the previous frame
 392  * had full regs, it's safe to get the value from the previous regs.  This can
 393  * happen when early/late IRQ entry code gets interrupted by an NMI.
 394  */
 395 static bool get_reg(struct unwind_state *state, unsigned int reg_off,
 396                     unsigned long *val)
 397 {
 398         unsigned int reg = reg_off/8;
 399
 400         if (!state->regs)
 401                 return false;
 402
 403         if (state->full_regs) {
 404                 *val = ((unsigned long *)state->regs)[reg];
 405                 return true;
 406         }
 407
 408         if (state->prev_regs) {
 409                 *val = ((unsigned long *)state->prev_regs)[reg];
 410                 return true;
 411         }
 412
 413         return false;
 414 }
 415
 416 bool unwind_next_frame(struct unwind_state *state)
 417 {
 418         unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp;
 419         enum stack_type prev_type = state->stack_info.type;
 420         struct orc_entry *orc;
 421         bool indirect = false;
 422
 423         if (unwind_done(state))
 424                 return false;
 425
 426         /* Don't let modules unload while we're reading their ORC data. */
 427         preempt_disable();
 428
 429         /* End-of-stack check for user tasks: */
 430         if (state->regs && user_mode(state->regs))
 431                 goto the_end;
 432
 433         /*
 434          * Find the orc_entry associated with the text address.
 435          *
 436          * Decrement call return addresses by one so they work for sibling
 437          * calls and calls to noreturn functions.
 438          */
 439         orc = orc_find(state->signal ? state->ip : state->ip - 1);
 440         if (!orc) {
 441                 /*
 442                  * As a fallback, try to assume this code uses a frame pointer.
 443                  * This is useful for generated code, like BPF, which ORC
 444                  * doesn't know about.  This is just a guess, so the rest of
 445                  * the unwind is no longer considered reliable.
 446                  */
 447                 orc = &orc_fp_entry;
 448                 state->error = true;
 449         }
 450
 451         /* End-of-stack check for kernel threads: */
 452         if (orc->sp_reg == ORC_REG_UNDEFINED) {
 453                 if (!orc->end)
 454                         goto err;
 455
 456                 goto the_end;
 457         }
 458
 459         /* Find the previous frame's stack: */
 460         switch (orc->sp_reg) {
 461         case ORC_REG_SP:
 462                 sp = state->sp + orc->sp_offset;
 463                 break;
 464
 465         case ORC_REG_BP:
 466                 sp = state->bp + orc->sp_offset;
 467                 break;
 468
 469         case ORC_REG_SP_INDIRECT:
 470                 sp = state->sp + orc->sp_offset;
 471                 indirect = true;
 472                 break;
 473
 474         case ORC_REG_BP_INDIRECT:
 475                 sp = state->bp + orc->sp_offset;
 476                 indirect = true;
 477                 break;
 478
 479         case ORC_REG_R10:
 480                 if (!get_reg(state, offsetof(struct pt_regs, r10), &sp)) {
 481                         orc_warn_current("missing R10 value at %pB\n",
 482                                          (void *)state->ip);
 483                         goto err;
 484                 }
 485                 break;
 486
 487         case ORC_REG_R13:
 488                 if (!get_reg(state, offsetof(struct pt_regs, r13), &sp)) {
 489                         orc_warn_current("missing R13 value at %pB\n",
 490                                          (void *)state->ip);
 491                         goto err;
 492                 }
 493                 break;
 494
 495         case ORC_REG_DI:
 496                 if (!get_reg(state, offsetof(struct pt_regs, di), &sp)) {
 497                         orc_warn_current("missing RDI value at %pB\n",
 498                                          (void *)state->ip);
 499                         goto err;
 500                 }
 501                 break;
 502
 503         case ORC_REG_DX:
 504                 if (!get_reg(state, offsetof(struct pt_regs, dx), &sp)) {
 505                         orc_warn_current("missing DX value at %pB\n",
 506                                          (void *)state->ip);
 507                         goto err;
 508                 }
 509                 break;
 510
 511         default:
 512                 orc_warn("unknown SP base reg %d at %pB\n",
 513                          orc->sp_reg, (void *)state->ip);
 514                 goto err;
 515         }
 516
 517         if (indirect) {
 518                 if (!deref_stack_reg(state, sp, &sp))
 519                         goto err;
 520         }
 521
 522         /* Find IP, SP and possibly regs: */
 523         switch (orc->type) {
 524         case ORC_TYPE_CALL:
 525                 ip_p = sp - sizeof(long);
 526
 527                 if (!deref_stack_reg(state, ip_p, &state->ip))
 528                         goto err;
 529
 530                 state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
 531                                                   state->ip, (void *)ip_p);
 532
 533                 state->sp = sp;
 534                 state->regs = NULL;
 535                 state->prev_regs = NULL;
 536                 state->signal = false;
 537                 break;
 538
 539         case ORC_TYPE_REGS:
 540                 if (!deref_stack_regs(state, sp, &state->ip, &state->sp)) {
 541                         orc_warn_current("can't access registers at %pB\n",
 542                                          (void *)orig_ip);
 543                         goto err;
 544                 }
 545
 546                 state->regs = (struct pt_regs *)sp;
 547                 state->prev_regs = NULL;
 548                 state->full_regs = true;
 549                 state->signal = true;
 550                 break;
 551
 552         case ORC_TYPE_REGS_IRET:
 553                 if (!deref_stack_iret_regs(state, sp, &state->ip, &state->sp)) {
 554                         orc_warn_current("can't access iret registers at %pB\n",
 555                                          (void *)orig_ip);
 556                         goto err;
 557                 }
 558
 559                 if (state->full_regs)
 560                         state->prev_regs = state->regs;
 561                 state->regs = (void *)sp - IRET_FRAME_OFFSET;
 562                 state->full_regs = false;
 563                 state->signal = true;
 564                 break;
 565
 566         default:
 567                 orc_warn("unknown .orc_unwind entry type %d at %pB\n",
 568                          orc->type, (void *)orig_ip);
 569                 goto err;
 570         }
 571
 572         /* Find BP: */
 573         switch (orc->bp_reg) {
 574         case ORC_REG_UNDEFINED:
 575                 if (get_reg(state, offsetof(struct pt_regs, bp), &tmp))
 576                         state->bp = tmp;
 577                 break;
 578
 579         case ORC_REG_PREV_SP:
 580                 if (!deref_stack_reg(state, sp + orc->bp_offset, &state->bp))
 581                         goto err;
 582                 break;
 583
 584         case ORC_REG_BP:
 585                 if (!deref_stack_reg(state, state->bp + orc->bp_offset, &state->bp))
 586                         goto err;
 587                 break;
 588
 589         default:
 590                 orc_warn("unknown BP base reg %d for ip %pB\n",
 591                          orc->bp_reg, (void *)orig_ip);
 592                 goto err;
 593         }
 594
 595         /* Prevent a recursive loop due to bad ORC data: */
 596         if (state->stack_info.type == prev_type &&
 597             on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
 598             state->sp <= prev_sp) {
 599                 orc_warn_current("stack going in the wrong direction? at %pB\n",
 600                                  (void *)orig_ip);
 601                 goto err;
 602         }
 603
 604         preempt_enable();
 605         return true;
 606
 607 err:
 608         state->error = true;
 609
 610 the_end:
 611         preempt_enable();
 612         state->stack_info.type = STACK_TYPE_UNKNOWN;
 613         return false;
 614 }
 615 EXPORT_SYMBOL_GPL(unwind_next_frame);
 616
 617 void __unwind_start(struct unwind_state *state, struct task_struct *task,
 618                     struct pt_regs *regs, unsigned long *first_frame)
 619 {
 620         memset(state, 0, sizeof(*state));
 621         state->task = task;
 622
 623         if (!orc_init)
 624                 goto err;
 625
 626         /*
 627          * Refuse to unwind the stack of a task while it's executing on another
 628          * CPU.  This check is racy, but that's ok: the unwinder has other
 629          * checks to prevent it from going off the rails.
 630          */
 631         if (task_on_another_cpu(task))
 632                 goto err;
 633
 634         if (regs) {
 635                 if (user_mode(regs))
 636                         goto the_end;
 637
 638                 state->ip = regs->ip;
 639                 state->sp = regs->sp;
 640                 state->bp = regs->bp;
 641                 state->regs = regs;
 642                 state->full_regs = true;
 643                 state->signal = true;
 644
 645         } else if (task == current) {
 646                 asm volatile("lea (%%rip), %0\n\t"
 647                              "mov %%rsp, %1\n\t"
 648                              "mov %%rbp, %2\n\t"
 649                              : "=r" (state->ip), "=r" (state->sp),
 650                                "=r" (state->bp));
 651
 652         } else {
 653                 struct inactive_task_frame *frame = (void *)task->thread.sp;
 654
 655                 state->sp = task->thread.sp;
 656                 state->bp = READ_ONCE_NOCHECK(frame->bp);
 657                 state->ip = READ_ONCE_NOCHECK(frame->ret_addr);
 658         }
 659
 660         if (get_stack_info((unsigned long *)state->sp, state->task,
 661                            &state->stack_info, &state->stack_mask)) {
 662                 /*
 663                  * We weren't on a valid stack.  It's possible that
 664                  * we overflowed a valid stack into a guard page.
 665                  * See if the next page up is valid so that we can
 666                  * generate some kind of backtrace if this happens.
 667                  */
 668                 void *next_page = (void *)PAGE_ALIGN((unsigned long)state->sp);
 669                 state->error = true;
 670                 if (get_stack_info(next_page, state->task, &state->stack_info,
 671                                    &state->stack_mask))
 672                         return;
 673         }
 674
 675         /*
 676          * The caller can provide the address of the first frame directly
 677          * (first_frame) or indirectly (regs->sp) to indicate which stack frame
 678          * to start unwinding at.  Skip ahead until we reach it.
 679          */
 680
 681         /* When starting from regs, skip the regs frame: */
 682         if (regs) {
 683                 unwind_next_frame(state);
 684                 return;
 685         }
 686
 687         /* Otherwise, skip ahead to the user-specified starting frame: */
 688         while (!unwind_done(state) &&
 689                (!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
 690                         state->sp < (unsigned long)first_frame))
 691                 unwind_next_frame(state);
 692
 693         return;
 694
 695 err:
 696         state->error = true;
 697 the_end:
 698         state->stack_info.type = STACK_TYPE_UNKNOWN;
 699 }
 700 EXPORT_SYMBOL_GPL(__unwind_start);