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