arch/x86/kernel/unwind_orc.c

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 #include <linux/objtool.h>
   3 #include <linux/module.h>
   4 #include <linux/sort.h>
   5 #include <asm/ptrace.h>
   6 #include <asm/stacktrace.h>
   7 #include <asm/unwind.h>
   8 #include <asm/orc_types.h>
   9 #include <asm/orc_lookup.h>
  10
  11 #define orc_warn(fmt, ...) \
  12         printk_deferred_once(KERN_WARNING "WARNING: " fmt, ##__VA_ARGS__)
  13
  14 #define orc_warn_current(args...)                                       \
  15 ({                                                                      \
  16         if (state->task == current)                                     \
  17                 orc_warn(args);                                         \
  18 })
  19
  20 extern int __start_orc_unwind_ip[];
  21 extern int __stop_orc_unwind_ip[];
  22 extern struct orc_entry __start_orc_unwind[];
  23 extern struct orc_entry __stop_orc_unwind[];
  24
  25 static bool orc_init __ro_after_init;
  26 static unsigned int lookup_num_blocks __ro_after_init;
  27
  28 static inline unsigned long orc_ip(const int *ip)
  29 {
  30         return (unsigned long)ip + *ip;
  31 }
  32
  33 static struct orc_entry *__orc_find(int *ip_table, struct orc_entry *u_table,
  34                                     unsigned int num_entries, unsigned long ip)
  35 {
  36         int *first = ip_table;
  37         int *last = ip_table + num_entries - 1;
  38         int *mid = first, *found = first;
  39
  40         if (!num_entries)
  41                 return NULL;
  42
  43         /*
  44          * Do a binary range search to find the rightmost duplicate of a given
  45          * starting address.  Some entries are section terminators which are
  46          * "weak" entries for ensuring there are no gaps.  They should be
  47          * ignored when they conflict with a real entry.
  48          */
  49         while (first <= last) {
  50                 mid = first + ((last - first) / 2);
  51
  52                 if (orc_ip(mid) <= ip) {
  53                         found = mid;
  54                         first = mid + 1;
  55                 } else
  56                         last = mid - 1;
  57         }
  58
  59         return u_table + (found - ip_table);
  60 }
  61
  62 #ifdef CONFIG_MODULES
  63 static struct orc_entry *orc_module_find(unsigned long ip)
  64 {
  65         struct module *mod;
  66
  67         mod = __module_address(ip);
  68         if (!mod || !mod->arch.orc_unwind || !mod->arch.orc_unwind_ip)
  69                 return NULL;
  70         return __orc_find(mod->arch.orc_unwind_ip, mod->arch.orc_unwind,
  71                           mod->arch.num_orcs, ip);
  72 }
  73 #else
  74 static struct orc_entry *orc_module_find(unsigned long ip)
  75 {
  76         return NULL;
  77 }
  78 #endif
  79
  80 #ifdef CONFIG_DYNAMIC_FTRACE
  81 static struct orc_entry *orc_find(unsigned long ip);
  82
  83 /*
  84  * Ftrace dynamic trampolines do not have orc entries of their own.
  85  * But they are copies of the ftrace entries that are static and
  86  * defined in ftrace_*.S, which do have orc entries.
  87  *
  88  * If the unwinder comes across a ftrace trampoline, then find the
  89  * ftrace function that was used to create it, and use that ftrace
  90  * function's orc entry, as the placement of the return code in
  91  * the stack will be identical.
  92  */
  93 static struct orc_entry *orc_ftrace_find(unsigned long ip)
  94 {
  95         struct ftrace_ops *ops;
  96         unsigned long caller;
  97
  98         ops = ftrace_ops_trampoline(ip);
  99         if (!ops)
 100                 return NULL;
 101
 102         if (ops->flags & FTRACE_OPS_FL_SAVE_REGS)
 103                 caller = (unsigned long)ftrace_regs_call;
 104         else
 105                 caller = (unsigned long)ftrace_call;
 106
 107         /* Prevent unlikely recursion */
 108         if (ip == caller)
 109                 return NULL;
 110
 111         return orc_find(caller);
 112 }
 113 #else
 114 static struct orc_entry *orc_ftrace_find(unsigned long ip)
 115 {
 116         return NULL;
 117 }
 118 #endif
 119
 120 /*
 121  * If we crash with IP==0, the last successfully executed instruction
 122  * was probably an indirect function call with a NULL function pointer,
 123  * and we don't have unwind information for NULL.
 124  * This hardcoded ORC entry for IP==0 allows us to unwind from a NULL function
 125  * pointer into its parent and then continue normally from there.
 126  */
 127 static struct orc_entry null_orc_entry = {
 128         .sp_offset = sizeof(long),
 129         .sp_reg = ORC_REG_SP,
 130         .bp_reg = ORC_REG_UNDEFINED,
 131         .type = UNWIND_HINT_TYPE_CALL
 132 };
 133
 134 /* Fake frame pointer entry -- used as a fallback for generated code */
 135 static struct orc_entry orc_fp_entry = {
 136         .type           = UNWIND_HINT_TYPE_CALL,
 137         .sp_reg         = ORC_REG_BP,
 138         .sp_offset      = 16,
 139         .bp_reg         = ORC_REG_PREV_SP,
 140         .bp_offset      = -16,
 141         .end            = 0,
 142 };
 143
 144 static struct orc_entry *orc_find(unsigned long ip)
 145 {
 146         static struct orc_entry *orc;
 147
 148         if (ip == 0)
 149                 return &null_orc_entry;
 150
 151         /* For non-init vmlinux addresses, use the fast lookup table: */
 152         if (ip >= LOOKUP_START_IP && ip < LOOKUP_STOP_IP) {
 153                 unsigned int idx, start, stop;
 154
 155                 idx = (ip - LOOKUP_START_IP) / LOOKUP_BLOCK_SIZE;
 156
 157                 if (unlikely((idx >= lookup_num_blocks-1))) {
 158                         orc_warn("WARNING: bad lookup idx: idx=%u num=%u ip=%pB\n",
 159                                  idx, lookup_num_blocks, (void *)ip);
 160                         return NULL;
 161                 }
 162
 163                 start = orc_lookup[idx];
 164                 stop = orc_lookup[idx + 1] + 1;
 165
 166                 if (unlikely((__start_orc_unwind + start >= __stop_orc_unwind) ||
 167                              (__start_orc_unwind + stop > __stop_orc_unwind))) {
 168                         orc_warn("WARNING: bad lookup value: idx=%u num=%u start=%u stop=%u ip=%pB\n",
 169                                  idx, lookup_num_blocks, start, stop, (void *)ip);
 170                         return NULL;
 171                 }
 172
 173                 return __orc_find(__start_orc_unwind_ip + start,
 174                                   __start_orc_unwind + start, stop - start, ip);
 175         }
 176
 177         /* vmlinux .init slow lookup: */
 178         if (init_kernel_text(ip))
 179                 return __orc_find(__start_orc_unwind_ip, __start_orc_unwind,
 180                                   __stop_orc_unwind_ip - __start_orc_unwind_ip, ip);
 181
 182         /* Module lookup: */
 183         orc = orc_module_find(ip);
 184         if (orc)
 185                 return orc;
 186
 187         return orc_ftrace_find(ip);
 188 }
 189
 190 #ifdef CONFIG_MODULES
 191
 192 static DEFINE_MUTEX(sort_mutex);
 193 static int *cur_orc_ip_table = __start_orc_unwind_ip;
 194 static struct orc_entry *cur_orc_table = __start_orc_unwind;
 195
 196 static void orc_sort_swap(void *_a, void *_b, int size)
 197 {
 198         struct orc_entry *orc_a, *orc_b;
 199         struct orc_entry orc_tmp;
 200         int *a = _a, *b = _b, tmp;
 201         int delta = _b - _a;
 202
 203         /* Swap the .orc_unwind_ip entries: */
 204         tmp = *a;
 205         *a = *b + delta;
 206         *b = tmp - delta;
 207
 208         /* Swap the corresponding .orc_unwind entries: */
 209         orc_a = cur_orc_table + (a - cur_orc_ip_table);
 210         orc_b = cur_orc_table + (b - cur_orc_ip_table);
 211         orc_tmp = *orc_a;
 212         *orc_a = *orc_b;
 213         *orc_b = orc_tmp;
 214 }
 215
 216 static int orc_sort_cmp(const void *_a, const void *_b)
 217 {
 218         struct orc_entry *orc_a;
 219         const int *a = _a, *b = _b;
 220         unsigned long a_val = orc_ip(a);
 221         unsigned long b_val = orc_ip(b);
 222
 223         if (a_val > b_val)
 224                 return 1;
 225         if (a_val < b_val)
 226                 return -1;
 227
 228         /*
 229          * The "weak" section terminator entries need to always be on the left
 230          * to ensure the lookup code skips them in favor of real entries.
 231          * These terminator entries exist to handle any gaps created by
 232          * whitelisted .o files which didn't get objtool generation.
 233          */
 234         orc_a = cur_orc_table + (a - cur_orc_ip_table);
 235         return orc_a->sp_reg == ORC_REG_UNDEFINED && !orc_a->end ? -1 : 1;
 236 }
 237
 238 void unwind_module_init(struct module *mod, void *_orc_ip, size_t orc_ip_size,
 239                         void *_orc, size_t orc_size)
 240 {
 241         int *orc_ip = _orc_ip;
 242         struct orc_entry *orc = _orc;
 243         unsigned int num_entries = orc_ip_size / sizeof(int);
 244
 245         WARN_ON_ONCE(orc_ip_size % sizeof(int) != 0 ||
 246                      orc_size % sizeof(*orc) != 0 ||
 247                      num_entries != orc_size / sizeof(*orc));
 248
 249         /*
 250          * The 'cur_orc_*' globals allow the orc_sort_swap() callback to
 251          * associate an .orc_unwind_ip table entry with its corresponding
 252          * .orc_unwind entry so they can both be swapped.
 253          */
 254         mutex_lock(&sort_mutex);
 255         cur_orc_ip_table = orc_ip;
 256         cur_orc_table = orc;
 257         sort(orc_ip, num_entries, sizeof(int), orc_sort_cmp, orc_sort_swap);
 258         mutex_unlock(&sort_mutex);
 259
 260         mod->arch.orc_unwind_ip = orc_ip;
 261         mod->arch.orc_unwind = orc;
 262         mod->arch.num_orcs = num_entries;
 263 }
 264 #endif
 265
 266 void __init unwind_init(void)
 267 {
 268         size_t orc_ip_size = (void *)__stop_orc_unwind_ip - (void *)__start_orc_unwind_ip;
 269         size_t orc_size = (void *)__stop_orc_unwind - (void *)__start_orc_unwind;
 270         size_t num_entries = orc_ip_size / sizeof(int);
 271         struct orc_entry *orc;
 272         int i;
 273
 274         if (!num_entries || orc_ip_size % sizeof(int) != 0 ||
 275             orc_size % sizeof(struct orc_entry) != 0 ||
 276             num_entries != orc_size / sizeof(struct orc_entry)) {
 277                 orc_warn("WARNING: Bad or missing .orc_unwind table.  Disabling unwinder.\n");
 278                 return;
 279         }
 280
 281         /*
 282          * Note, the orc_unwind and orc_unwind_ip tables were already
 283          * sorted at build time via the 'sorttable' tool.
 284          * It's ready for binary search straight away, no need to sort it.
 285          */
 286
 287         /* Initialize the fast lookup table: */
 288         lookup_num_blocks = orc_lookup_end - orc_lookup;
 289         for (i = 0; i < lookup_num_blocks-1; i++) {
 290                 orc = __orc_find(__start_orc_unwind_ip, __start_orc_unwind,
 291                                  num_entries,
 292                                  LOOKUP_START_IP + (LOOKUP_BLOCK_SIZE * i));
 293                 if (!orc) {
 294                         orc_warn("WARNING: Corrupt .orc_unwind table.  Disabling unwinder.\n");
 295                         return;
 296                 }
 297
 298                 orc_lookup[i] = orc - __start_orc_unwind;
 299         }
 300
 301         /* Initialize the ending block: */
 302         orc = __orc_find(__start_orc_unwind_ip, __start_orc_unwind, num_entries,
 303                          LOOKUP_STOP_IP);
 304         if (!orc) {
 305                 orc_warn("WARNING: Corrupt .orc_unwind table.  Disabling unwinder.\n");
 306                 return;
 307         }
 308         orc_lookup[lookup_num_blocks-1] = orc - __start_orc_unwind;
 309
 310         orc_init = true;
 311 }
 312
 313 unsigned long unwind_get_return_address(struct unwind_state *state)
 314 {
 315         if (unwind_done(state))
 316                 return 0;
 317
 318         return __kernel_text_address(state->ip) ? state->ip : 0;
 319 }
 320 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 321
 322 unsigned long *unwind_get_return_address_ptr(struct unwind_state *state)
 323 {
 324         struct task_struct *task = state->task;
 325
 326         if (unwind_done(state))
 327                 return NULL;
 328
 329         if (state->regs)
 330                 return &state->regs->ip;
 331
 332         if (task != current && state->sp == task->thread.sp) {
 333                 struct inactive_task_frame *frame = (void *)task->thread.sp;
 334                 return &frame->ret_addr;
 335         }
 336
 337         if (state->sp)
 338                 return (unsigned long *)state->sp - 1;
 339
 340         return NULL;
 341 }
 342
 343 static bool stack_access_ok(struct unwind_state *state, unsigned long _addr,
 344                             size_t len)
 345 {
 346         struct stack_info *info = &state->stack_info;
 347         void *addr = (void *)_addr;
 348
 349         if (!on_stack(info, addr, len) &&
 350             (get_stack_info(addr, state->task, info, &state->stack_mask)))
 351                 return false;
 352
 353         return true;
 354 }
 355
 356 static bool deref_stack_reg(struct unwind_state *state, unsigned long addr,
 357                             unsigned long *val)
 358 {
 359         if (!stack_access_ok(state, addr, sizeof(long)))
 360                 return false;
 361
 362         *val = READ_ONCE_NOCHECK(*(unsigned long *)addr);
 363         return true;
 364 }
 365
 366 static bool deref_stack_regs(struct unwind_state *state, unsigned long addr,
 367                              unsigned long *ip, unsigned long *sp)
 368 {
 369         struct pt_regs *regs = (struct pt_regs *)addr;
 370
 371         /* x86-32 support will be more complicated due to the &regs->sp hack */
 372         BUILD_BUG_ON(IS_ENABLED(CONFIG_X86_32));
 373
 374         if (!stack_access_ok(state, addr, sizeof(struct pt_regs)))
 375                 return false;
 376
 377         *ip = regs->ip;
 378         *sp = regs->sp;
 379         return true;
 380 }
 381
 382 static bool deref_stack_iret_regs(struct unwind_state *state, unsigned long addr,
 383                                   unsigned long *ip, unsigned long *sp)
 384 {
 385         struct pt_regs *regs = (void *)addr - IRET_FRAME_OFFSET;
 386
 387         if (!stack_access_ok(state, addr, IRET_FRAME_SIZE))
 388                 return false;
 389
 390         *ip = regs->ip;
 391         *sp = regs->sp;
 392         return true;
 393 }
 394
 395 /*
 396  * If state->regs is non-NULL, and points to a full pt_regs, just get the reg
 397  * value from state->regs.
 398  *
 399  * Otherwise, if state->regs just points to IRET regs, and the previous frame
 400  * had full regs, it's safe to get the value from the previous regs.  This can
 401  * happen when early/late IRQ entry code gets interrupted by an NMI.
 402  */
 403 static bool get_reg(struct unwind_state *state, unsigned int reg_off,
 404                     unsigned long *val)
 405 {
 406         unsigned int reg = reg_off/8;
 407
 408         if (!state->regs)
 409                 return false;
 410
 411         if (state->full_regs) {
 412                 *val = ((unsigned long *)state->regs)[reg];
 413                 return true;
 414         }
 415
 416         if (state->prev_regs) {
 417                 *val = ((unsigned long *)state->prev_regs)[reg];
 418                 return true;
 419         }
 420
 421         return false;
 422 }
 423
 424 bool unwind_next_frame(struct unwind_state *state)
 425 {
 426         unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp;
 427         enum stack_type prev_type = state->stack_info.type;
 428         struct orc_entry *orc;
 429         bool indirect = false;
 430
 431         if (unwind_done(state))
 432                 return false;
 433
 434         /* Don't let modules unload while we're reading their ORC data. */
 435         preempt_disable();
 436
 437         /* End-of-stack check for user tasks: */
 438         if (state->regs && user_mode(state->regs))
 439                 goto the_end;
 440
 441         /*
 442          * Find the orc_entry associated with the text address.
 443          *
 444          * For a call frame (as opposed to a signal frame), state->ip points to
 445          * the instruction after the call.  That instruction's stack layout
 446          * could be different from the call instruction's layout, for example
 447          * if the call was to a noreturn function.  So get the ORC data for the
 448          * call instruction itself.
 449          */
 450         orc = orc_find(state->signal ? state->ip : state->ip - 1);
 451         if (!orc) {
 452                 /*
 453                  * As a fallback, try to assume this code uses a frame pointer.
 454                  * This is useful for generated code, like BPF, which ORC
 455                  * doesn't know about.  This is just a guess, so the rest of
 456                  * the unwind is no longer considered reliable.
 457                  */
 458                 orc = &orc_fp_entry;
 459                 state->error = true;
 460         }
 461
 462         /* End-of-stack check for kernel threads: */
 463         if (orc->sp_reg == ORC_REG_UNDEFINED) {
 464                 if (!orc->end)
 465                         goto err;
 466
 467                 goto the_end;
 468         }
 469
 470         /* Find the previous frame's stack: */
 471         switch (orc->sp_reg) {
 472         case ORC_REG_SP:
 473                 sp = state->sp + orc->sp_offset;
 474                 break;
 475
 476         case ORC_REG_BP:
 477                 sp = state->bp + orc->sp_offset;
 478                 break;
 479
 480         case ORC_REG_SP_INDIRECT:
 481                 sp = state->sp + orc->sp_offset;
 482                 indirect = true;
 483                 break;
 484
 485         case ORC_REG_BP_INDIRECT:
 486                 sp = state->bp + orc->sp_offset;
 487                 indirect = true;
 488                 break;
 489
 490         case ORC_REG_R10:
 491                 if (!get_reg(state, offsetof(struct pt_regs, r10), &sp)) {
 492                         orc_warn_current("missing R10 value at %pB\n",
 493                                          (void *)state->ip);
 494                         goto err;
 495                 }
 496                 break;
 497
 498         case ORC_REG_R13:
 499                 if (!get_reg(state, offsetof(struct pt_regs, r13), &sp)) {
 500                         orc_warn_current("missing R13 value at %pB\n",
 501                                          (void *)state->ip);
 502                         goto err;
 503                 }
 504                 break;
 505
 506         case ORC_REG_DI:
 507                 if (!get_reg(state, offsetof(struct pt_regs, di), &sp)) {
 508                         orc_warn_current("missing RDI value at %pB\n",
 509                                          (void *)state->ip);
 510                         goto err;
 511                 }
 512                 break;
 513
 514         case ORC_REG_DX:
 515                 if (!get_reg(state, offsetof(struct pt_regs, dx), &sp)) {
 516                         orc_warn_current("missing DX value at %pB\n",
 517                                          (void *)state->ip);
 518                         goto err;
 519                 }
 520                 break;
 521
 522         default:
 523                 orc_warn("unknown SP base reg %d at %pB\n",
 524                          orc->sp_reg, (void *)state->ip);
 525                 goto err;
 526         }
 527
 528         if (indirect) {
 529                 if (!deref_stack_reg(state, sp, &sp))
 530                         goto err;
 531         }
 532
 533         /* Find IP, SP and possibly regs: */
 534         switch (orc->type) {
 535         case UNWIND_HINT_TYPE_CALL:
 536                 ip_p = sp - sizeof(long);
 537
 538                 if (!deref_stack_reg(state, ip_p, &state->ip))
 539                         goto err;
 540
 541                 state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx,
 542                                                   state->ip, (void *)ip_p);
 543
 544                 state->sp = sp;
 545                 state->regs = NULL;
 546                 state->prev_regs = NULL;
 547                 state->signal = false;
 548                 break;
 549
 550         case UNWIND_HINT_TYPE_REGS:
 551                 if (!deref_stack_regs(state, sp, &state->ip, &state->sp)) {
 552                         orc_warn_current("can't access registers at %pB\n",
 553                                          (void *)orig_ip);
 554                         goto err;
 555                 }
 556
 557                 state->regs = (struct pt_regs *)sp;
 558                 state->prev_regs = NULL;
 559                 state->full_regs = true;
 560                 state->signal = true;
 561                 break;
 562
 563         case UNWIND_HINT_TYPE_REGS_PARTIAL:
 564                 if (!deref_stack_iret_regs(state, sp, &state->ip, &state->sp)) {
 565                         orc_warn_current("can't access iret registers at %pB\n",
 566                                          (void *)orig_ip);
 567                         goto err;
 568                 }
 569
 570                 if (state->full_regs)
 571                         state->prev_regs = state->regs;
 572                 state->regs = (void *)sp - IRET_FRAME_OFFSET;
 573                 state->full_regs = false;
 574                 state->signal = true;
 575                 break;
 576
 577         default:
 578                 orc_warn("unknown .orc_unwind entry type %d at %pB\n",
 579                          orc->type, (void *)orig_ip);
 580                 goto err;
 581         }
 582
 583         /* Find BP: */
 584         switch (orc->bp_reg) {
 585         case ORC_REG_UNDEFINED:
 586                 if (get_reg(state, offsetof(struct pt_regs, bp), &tmp))
 587                         state->bp = tmp;
 588                 break;
 589
 590         case ORC_REG_PREV_SP:
 591                 if (!deref_stack_reg(state, sp + orc->bp_offset, &state->bp))
 592                         goto err;
 593                 break;
 594
 595         case ORC_REG_BP:
 596                 if (!deref_stack_reg(state, state->bp + orc->bp_offset, &state->bp))
 597                         goto err;
 598                 break;
 599
 600         default:
 601                 orc_warn("unknown BP base reg %d for ip %pB\n",
 602                          orc->bp_reg, (void *)orig_ip);
 603                 goto err;
 604         }
 605
 606         /* Prevent a recursive loop due to bad ORC data: */
 607         if (state->stack_info.type == prev_type &&
 608             on_stack(&state->stack_info, (void *)state->sp, sizeof(long)) &&
 609             state->sp <= prev_sp) {
 610                 orc_warn_current("stack going in the wrong direction? at %pB\n",
 611                                  (void *)orig_ip);
 612                 goto err;
 613         }
 614
 615         preempt_enable();
 616         return true;
 617
 618 err:
 619         state->error = true;
 620
 621 the_end:
 622         preempt_enable();
 623         state->stack_info.type = STACK_TYPE_UNKNOWN;
 624         return false;
 625 }
 626 EXPORT_SYMBOL_GPL(unwind_next_frame);
 627
 628 void __unwind_start(struct unwind_state *state, struct task_struct *task,
 629                     struct pt_regs *regs, unsigned long *first_frame)
 630 {
 631         memset(state, 0, sizeof(*state));
 632         state->task = task;
 633
 634         if (!orc_init)
 635                 goto err;
 636
 637         /*
 638          * Refuse to unwind the stack of a task while it's executing on another
 639          * CPU.  This check is racy, but that's ok: the unwinder has other
 640          * checks to prevent it from going off the rails.
 641          */
 642         if (task_on_another_cpu(task))
 643                 goto err;
 644
 645         if (regs) {
 646                 if (user_mode(regs))
 647                         goto the_end;
 648
 649                 state->ip = regs->ip;
 650                 state->sp = regs->sp;
 651                 state->bp = regs->bp;
 652                 state->regs = regs;
 653                 state->full_regs = true;
 654                 state->signal = true;
 655
 656         } else if (task == current) {
 657                 asm volatile("lea (%%rip), %0\n\t"
 658                              "mov %%rsp, %1\n\t"
 659                              "mov %%rbp, %2\n\t"
 660                              : "=r" (state->ip), "=r" (state->sp),
 661                                "=r" (state->bp));
 662
 663         } else {
 664                 struct inactive_task_frame *frame = (void *)task->thread.sp;
 665
 666                 state->sp = task->thread.sp;
 667                 state->bp = READ_ONCE_NOCHECK(frame->bp);
 668                 state->ip = READ_ONCE_NOCHECK(frame->ret_addr);
 669                 state->signal = (void *)state->ip == ret_from_fork;
 670         }
 671
 672         if (get_stack_info((unsigned long *)state->sp, state->task,
 673                            &state->stack_info, &state->stack_mask)) {
 674                 /*
 675                  * We weren't on a valid stack.  It's possible that
 676                  * we overflowed a valid stack into a guard page.
 677                  * See if the next page up is valid so that we can
 678                  * generate some kind of backtrace if this happens.
 679                  */
 680                 void *next_page = (void *)PAGE_ALIGN((unsigned long)state->sp);
 681                 state->error = true;
 682                 if (get_stack_info(next_page, state->task, &state->stack_info,
 683                                    &state->stack_mask))
 684                         return;
 685         }
 686
 687         /*
 688          * The caller can provide the address of the first frame directly
 689          * (first_frame) or indirectly (regs->sp) to indicate which stack frame
 690          * to start unwinding at.  Skip ahead until we reach it.
 691          */
 692
 693         /* When starting from regs, skip the regs frame: */
 694         if (regs) {
 695                 unwind_next_frame(state);
 696                 return;
 697         }
 698
 699         /* Otherwise, skip ahead to the user-specified starting frame: */
 700         while (!unwind_done(state) &&
 701                (!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
 702                         state->sp < (unsigned long)first_frame))
 703                 unwind_next_frame(state);
 704
 705         return;
 706
 707 err:
 708         state->error = true;
 709 the_end:
 710         state->stack_info.type = STACK_TYPE_UNKNOWN;
 711 }
 712 EXPORT_SYMBOL_GPL(__unwind_start);