tools/perf/util/unwind-libunwind-local.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Post mortem Dwarf CFI based unwinding on top of regs and stack dumps.
   4  *
   5  * Lots of this code have been borrowed or heavily inspired from parts of
   6  * the libunwind 0.99 code which are (amongst other contributors I may have
   7  * forgotten):
   8  *
   9  * Copyright (C) 2002-2007 Hewlett-Packard Co
  10  *      Contributed by David Mosberger-Tang <davidm@hpl.hp.com>
  11  *
  12  * And the bugs have been added by:
  13  *
  14  * Copyright (C) 2010, Frederic Weisbecker <fweisbec@gmail.com>
  15  * Copyright (C) 2012, Jiri Olsa <jolsa@redhat.com>
  16  *
  17  */
  18
  19 #include <elf.h>
  20 #include <errno.h>
  21 #include <gelf.h>
  22 #include <fcntl.h>
  23 #include <inttypes.h>
  24 #include <string.h>
  25 #include <unistd.h>
  26 #include <sys/mman.h>
  27 #include <linux/list.h>
  28 #ifndef REMOTE_UNWIND_LIBUNWIND
  29 #include <libunwind.h>
  30 #include <libunwind-ptrace.h>
  31 #endif
  32 #include "callchain.h"
  33 #include "thread.h"
  34 #include "session.h"
  35 #include "perf_regs.h"
  36 #include "unwind.h"
  37 #include "symbol.h"
  38 #include "util.h"
  39 #include "debug.h"
  40 #include "asm/bug.h"
  41 #include "dso.h"
  42
  43 extern int
  44 UNW_OBJ(dwarf_search_unwind_table) (unw_addr_space_t as,
  45                                     unw_word_t ip,
  46                                     unw_dyn_info_t *di,
  47                                     unw_proc_info_t *pi,
  48                                     int need_unwind_info, void *arg);
  49
  50 #define dwarf_search_unwind_table UNW_OBJ(dwarf_search_unwind_table)
  51
  52 extern int
  53 UNW_OBJ(dwarf_find_debug_frame) (int found, unw_dyn_info_t *di_debug,
  54                                  unw_word_t ip,
  55                                  unw_word_t segbase,
  56                                  const char *obj_name, unw_word_t start,
  57                                  unw_word_t end);
  58
  59 #define dwarf_find_debug_frame UNW_OBJ(dwarf_find_debug_frame)
  60
  61 #define DW_EH_PE_FORMAT_MASK    0x0f    /* format of the encoded value */
  62 #define DW_EH_PE_APPL_MASK      0x70    /* how the value is to be applied */
  63
  64 /* Pointer-encoding formats: */
  65 #define DW_EH_PE_omit           0xff
  66 #define DW_EH_PE_ptr            0x00    /* pointer-sized unsigned value */
  67 #define DW_EH_PE_udata4         0x03    /* unsigned 32-bit value */
  68 #define DW_EH_PE_udata8         0x04    /* unsigned 64-bit value */
  69 #define DW_EH_PE_sdata4         0x0b    /* signed 32-bit value */
  70 #define DW_EH_PE_sdata8         0x0c    /* signed 64-bit value */
  71
  72 /* Pointer-encoding application: */
  73 #define DW_EH_PE_absptr         0x00    /* absolute value */
  74 #define DW_EH_PE_pcrel          0x10    /* rel. to addr. of encoded value */
  75
  76 /*
  77  * The following are not documented by LSB v1.3, yet they are used by
  78  * GCC, presumably they aren't documented by LSB since they aren't
  79  * used on Linux:
  80  */
  81 #define DW_EH_PE_funcrel        0x40    /* start-of-procedure-relative */
  82 #define DW_EH_PE_aligned        0x50    /* aligned pointer */
  83
  84 /* Flags intentionaly not handled, since they're not needed:
  85  * #define DW_EH_PE_indirect      0x80
  86  * #define DW_EH_PE_uleb128       0x01
  87  * #define DW_EH_PE_udata2        0x02
  88  * #define DW_EH_PE_sleb128       0x09
  89  * #define DW_EH_PE_sdata2        0x0a
  90  * #define DW_EH_PE_textrel       0x20
  91  * #define DW_EH_PE_datarel       0x30
  92  */
  93
  94 struct unwind_info {
  95         struct perf_sample      *sample;
  96         struct machine          *machine;
  97         struct thread           *thread;
  98 };
  99
 100 #define dw_read(ptr, type, end) ({      \
 101         type *__p = (type *) ptr;       \
 102         type  __v;                      \
 103         if ((__p + 1) > (type *) end)   \
 104                 return -EINVAL;         \
 105         __v = *__p++;                   \
 106         ptr = (typeof(ptr)) __p;        \
 107         __v;                            \
 108         })
 109
 110 static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
 111                                    u8 encoding)
 112 {
 113         u8 *cur = *p;
 114         *val = 0;
 115
 116         switch (encoding) {
 117         case DW_EH_PE_omit:
 118                 *val = 0;
 119                 goto out;
 120         case DW_EH_PE_ptr:
 121                 *val = dw_read(cur, unsigned long, end);
 122                 goto out;
 123         default:
 124                 break;
 125         }
 126
 127         switch (encoding & DW_EH_PE_APPL_MASK) {
 128         case DW_EH_PE_absptr:
 129                 break;
 130         case DW_EH_PE_pcrel:
 131                 *val = (unsigned long) cur;
 132                 break;
 133         default:
 134                 return -EINVAL;
 135         }
 136
 137         if ((encoding & 0x07) == 0x00)
 138                 encoding |= DW_EH_PE_udata4;
 139
 140         switch (encoding & DW_EH_PE_FORMAT_MASK) {
 141         case DW_EH_PE_sdata4:
 142                 *val += dw_read(cur, s32, end);
 143                 break;
 144         case DW_EH_PE_udata4:
 145                 *val += dw_read(cur, u32, end);
 146                 break;
 147         case DW_EH_PE_sdata8:
 148                 *val += dw_read(cur, s64, end);
 149                 break;
 150         case DW_EH_PE_udata8:
 151                 *val += dw_read(cur, u64, end);
 152                 break;
 153         default:
 154                 return -EINVAL;
 155         }
 156
 157  out:
 158         *p = cur;
 159         return 0;
 160 }
 161
 162 #define dw_read_encoded_value(ptr, end, enc) ({                 \
 163         u64 __v;                                                \
 164         if (__dw_read_encoded_value(&ptr, end, &__v, enc)) {    \
 165                 return -EINVAL;                                 \
 166         }                                                       \
 167         __v;                                                    \
 168         })
 169
 170 static u64 elf_section_offset(int fd, const char *name)
 171 {
 172         Elf *elf;
 173         GElf_Ehdr ehdr;
 174         GElf_Shdr shdr;
 175         u64 offset = 0;
 176
 177         elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
 178         if (elf == NULL)
 179                 return 0;
 180
 181         do {
 182                 if (gelf_getehdr(elf, &ehdr) == NULL)
 183                         break;
 184
 185                 if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
 186                         break;
 187
 188                 offset = shdr.sh_offset;
 189         } while (0);
 190
 191         elf_end(elf);
 192         return offset;
 193 }
 194
 195 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 196 static int elf_is_exec(int fd, const char *name)
 197 {
 198         Elf *elf;
 199         GElf_Ehdr ehdr;
 200         int retval = 0;
 201
 202         elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
 203         if (elf == NULL)
 204                 return 0;
 205         if (gelf_getehdr(elf, &ehdr) == NULL)
 206                 goto out;
 207
 208         retval = (ehdr.e_type == ET_EXEC);
 209
 210 out:
 211         elf_end(elf);
 212         pr_debug("unwind: elf_is_exec(%s): %d\n", name, retval);
 213         return retval;
 214 }
 215 #endif
 216
 217 struct table_entry {
 218         u32 start_ip_offset;
 219         u32 fde_offset;
 220 };
 221
 222 struct eh_frame_hdr {
 223         unsigned char version;
 224         unsigned char eh_frame_ptr_enc;
 225         unsigned char fde_count_enc;
 226         unsigned char table_enc;
 227
 228         /*
 229          * The rest of the header is variable-length and consists of the
 230          * following members:
 231          *
 232          *      encoded_t eh_frame_ptr;
 233          *      encoded_t fde_count;
 234          */
 235
 236         /* A single encoded pointer should not be more than 8 bytes. */
 237         u64 enc[2];
 238
 239         /*
 240          * struct {
 241          *    encoded_t start_ip;
 242          *    encoded_t fde_addr;
 243          * } binary_search_table[fde_count];
 244          */
 245         char data[0];
 246 } __packed;
 247
 248 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
 249                                u64 offset, u64 *table_data, u64 *segbase,
 250                                u64 *fde_count)
 251 {
 252         struct eh_frame_hdr hdr;
 253         u8 *enc = (u8 *) &hdr.enc;
 254         u8 *end = (u8 *) &hdr.data;
 255         ssize_t r;
 256
 257         r = dso__data_read_offset(dso, machine, offset,
 258                                   (u8 *) &hdr, sizeof(hdr));
 259         if (r != sizeof(hdr))
 260                 return -EINVAL;
 261
 262         /* We dont need eh_frame_ptr, just skip it. */
 263         dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
 264
 265         *fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
 266         *segbase    = offset;
 267         *table_data = (enc - (u8 *) &hdr) + offset;
 268         return 0;
 269 }
 270
 271 static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
 272                                      u64 *table_data, u64 *segbase,
 273                                      u64 *fde_count)
 274 {
 275         int ret = -EINVAL, fd;
 276         u64 offset = dso->data.eh_frame_hdr_offset;
 277
 278         if (offset == 0) {
 279                 fd = dso__data_get_fd(dso, machine);
 280                 if (fd < 0)
 281                         return -EINVAL;
 282
 283                 /* Check the .eh_frame section for unwinding info */
 284                 offset = elf_section_offset(fd, ".eh_frame_hdr");
 285                 dso->data.eh_frame_hdr_offset = offset;
 286                 dso__data_put_fd(dso);
 287         }
 288
 289         if (offset)
 290                 ret = unwind_spec_ehframe(dso, machine, offset,
 291                                           table_data, segbase,
 292                                           fde_count);
 293
 294         return ret;
 295 }
 296
 297 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 298 static int read_unwind_spec_debug_frame(struct dso *dso,
 299                                         struct machine *machine, u64 *offset)
 300 {
 301         int fd;
 302         u64 ofs = dso->data.debug_frame_offset;
 303
 304         /* debug_frame can reside in:
 305          *  - dso
 306          *  - debug pointed by symsrc_filename
 307          *  - gnu_debuglink, which doesn't necessary
 308          *    has to be pointed by symsrc_filename
 309          */
 310         if (ofs == 0) {
 311                 fd = dso__data_get_fd(dso, machine);
 312                 if (fd >= 0) {
 313                         ofs = elf_section_offset(fd, ".debug_frame");
 314                         dso__data_put_fd(dso);
 315                 }
 316
 317                 if (ofs <= 0) {
 318                         fd = open(dso->symsrc_filename, O_RDONLY);
 319                         if (fd >= 0) {
 320                                 ofs = elf_section_offset(fd, ".debug_frame");
 321                                 close(fd);
 322                         }
 323                 }
 324
 325                 if (ofs <= 0) {
 326                         char *debuglink = malloc(PATH_MAX);
 327                         int ret = 0;
 328
 329                         ret = dso__read_binary_type_filename(
 330                                 dso, DSO_BINARY_TYPE__DEBUGLINK,
 331                                 machine->root_dir, debuglink, PATH_MAX);
 332                         if (!ret) {
 333                                 fd = open(debuglink, O_RDONLY);
 334                                 if (fd >= 0) {
 335                                         ofs = elf_section_offset(fd,
 336                                                         ".debug_frame");
 337                                         close(fd);
 338                                 }
 339                         }
 340                         if (ofs > 0) {
 341                                 if (dso->symsrc_filename != NULL) {
 342                                         pr_warning(
 343                                                 "%s: overwrite symsrc(%s,%s)\n",
 344                                                         __func__,
 345                                                         dso->symsrc_filename,
 346                                                         debuglink);
 347                                         free(dso->symsrc_filename);
 348                                 }
 349                                 dso->symsrc_filename = debuglink;
 350                         } else {
 351                                 free(debuglink);
 352                         }
 353                 }
 354
 355                 dso->data.debug_frame_offset = ofs;
 356         }
 357
 358         *offset = ofs;
 359         if (*offset)
 360                 return 0;
 361
 362         return -EINVAL;
 363 }
 364 #endif
 365
 366 static struct map *find_map(unw_word_t ip, struct unwind_info *ui)
 367 {
 368         struct addr_location al;
 369
 370         thread__find_addr_map(ui->thread, PERF_RECORD_MISC_USER,
 371                               MAP__FUNCTION, ip, &al);
 372         if (!al.map) {
 373                 /*
 374                  * We've seen cases (softice) where DWARF unwinder went
 375                  * through non executable mmaps, which we need to lookup
 376                  * in MAP__VARIABLE tree.
 377                  */
 378                 thread__find_addr_map(ui->thread, PERF_RECORD_MISC_USER,
 379                                       MAP__VARIABLE, ip, &al);
 380         }
 381         return al.map;
 382 }
 383
 384 static int
 385 find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
 386                int need_unwind_info, void *arg)
 387 {
 388         struct unwind_info *ui = arg;
 389         struct map *map;
 390         unw_dyn_info_t di;
 391         u64 table_data, segbase, fde_count;
 392         int ret = -EINVAL;
 393
 394         map = find_map(ip, ui);
 395         if (!map || !map->dso)
 396                 return -EINVAL;
 397
 398         pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
 399
 400         /* Check the .eh_frame section for unwinding info */
 401         if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
 402                                        &table_data, &segbase, &fde_count)) {
 403                 memset(&di, 0, sizeof(di));
 404                 di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
 405                 di.start_ip = map->start;
 406                 di.end_ip   = map->end;
 407                 di.u.rti.segbase    = map->start + segbase - map->pgoff;
 408                 di.u.rti.table_data = map->start + table_data - map->pgoff;
 409                 di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
 410                                       / sizeof(unw_word_t);
 411                 ret = dwarf_search_unwind_table(as, ip, &di, pi,
 412                                                 need_unwind_info, arg);
 413         }
 414
 415 #ifndef NO_LIBUNWIND_DEBUG_FRAME
 416         /* Check the .debug_frame section for unwinding info */
 417         if (ret < 0 &&
 418             !read_unwind_spec_debug_frame(map->dso, ui->machine, &segbase)) {
 419                 int fd = dso__data_get_fd(map->dso, ui->machine);
 420                 int is_exec = elf_is_exec(fd, map->dso->name);
 421                 unw_word_t base = is_exec ? 0 : map->start;
 422                 const char *symfile;
 423
 424                 if (fd >= 0)
 425                         dso__data_put_fd(map->dso);
 426
 427                 symfile = map->dso->symsrc_filename ?: map->dso->name;
 428
 429                 memset(&di, 0, sizeof(di));
 430                 if (dwarf_find_debug_frame(0, &di, ip, base, symfile,
 431                                            map->start, map->end))
 432                         return dwarf_search_unwind_table(as, ip, &di, pi,
 433                                                          need_unwind_info, arg);
 434         }
 435 #endif
 436
 437         return ret;
 438 }
 439
 440 static int access_fpreg(unw_addr_space_t __maybe_unused as,
 441                         unw_regnum_t __maybe_unused num,
 442                         unw_fpreg_t __maybe_unused *val,
 443                         int __maybe_unused __write,
 444                         void __maybe_unused *arg)
 445 {
 446         pr_err("unwind: access_fpreg unsupported\n");
 447         return -UNW_EINVAL;
 448 }
 449
 450 static int get_dyn_info_list_addr(unw_addr_space_t __maybe_unused as,
 451                                   unw_word_t __maybe_unused *dil_addr,
 452                                   void __maybe_unused *arg)
 453 {
 454         return -UNW_ENOINFO;
 455 }
 456
 457 static int resume(unw_addr_space_t __maybe_unused as,
 458                   unw_cursor_t __maybe_unused *cu,
 459                   void __maybe_unused *arg)
 460 {
 461         pr_err("unwind: resume unsupported\n");
 462         return -UNW_EINVAL;
 463 }
 464
 465 static int
 466 get_proc_name(unw_addr_space_t __maybe_unused as,
 467               unw_word_t __maybe_unused addr,
 468                 char __maybe_unused *bufp, size_t __maybe_unused buf_len,
 469                 unw_word_t __maybe_unused *offp, void __maybe_unused *arg)
 470 {
 471         pr_err("unwind: get_proc_name unsupported\n");
 472         return -UNW_EINVAL;
 473 }
 474
 475 static int access_dso_mem(struct unwind_info *ui, unw_word_t addr,
 476                           unw_word_t *data)
 477 {
 478         struct map *map;
 479         ssize_t size;
 480
 481         map = find_map(addr, ui);
 482         if (!map) {
 483                 pr_debug("unwind: no map for %lx\n", (unsigned long)addr);
 484                 return -1;
 485         }
 486
 487         if (!map->dso)
 488                 return -1;
 489
 490         size = dso__data_read_addr(map->dso, map, ui->machine,
 491                                    addr, (u8 *) data, sizeof(*data));
 492
 493         return !(size == sizeof(*data));
 494 }
 495
 496 static int access_mem(unw_addr_space_t __maybe_unused as,
 497                       unw_word_t addr, unw_word_t *valp,
 498                       int __write, void *arg)
 499 {
 500         struct unwind_info *ui = arg;
 501         struct stack_dump *stack = &ui->sample->user_stack;
 502         u64 start, end;
 503         int offset;
 504         int ret;
 505
 506         /* Don't support write, probably not needed. */
 507         if (__write || !stack || !ui->sample->user_regs.regs) {
 508                 *valp = 0;
 509                 return 0;
 510         }
 511
 512         ret = perf_reg_value(&start, &ui->sample->user_regs,
 513                              LIBUNWIND__ARCH_REG_SP);
 514         if (ret)
 515                 return ret;
 516
 517         end = start + stack->size;
 518
 519         /* Check overflow. */
 520         if (addr + sizeof(unw_word_t) < addr)
 521                 return -EINVAL;
 522
 523         if (addr < start || addr + sizeof(unw_word_t) >= end) {
 524                 ret = access_dso_mem(ui, addr, valp);
 525                 if (ret) {
 526                         pr_debug("unwind: access_mem %p not inside range"
 527                                  " 0x%" PRIx64 "-0x%" PRIx64 "\n",
 528                                  (void *) (uintptr_t) addr, start, end);
 529                         *valp = 0;
 530                         return ret;
 531                 }
 532                 return 0;
 533         }
 534
 535         offset = addr - start;
 536         *valp  = *(unw_word_t *)&stack->data[offset];
 537         pr_debug("unwind: access_mem addr %p val %lx, offset %d\n",
 538                  (void *) (uintptr_t) addr, (unsigned long)*valp, offset);
 539         return 0;
 540 }
 541
 542 static int access_reg(unw_addr_space_t __maybe_unused as,
 543                       unw_regnum_t regnum, unw_word_t *valp,
 544                       int __write, void *arg)
 545 {
 546         struct unwind_info *ui = arg;
 547         int id, ret;
 548         u64 val;
 549
 550         /* Don't support write, I suspect we don't need it. */
 551         if (__write) {
 552                 pr_err("unwind: access_reg w %d\n", regnum);
 553                 return 0;
 554         }
 555
 556         if (!ui->sample->user_regs.regs) {
 557                 *valp = 0;
 558                 return 0;
 559         }
 560
 561         id = LIBUNWIND__ARCH_REG_ID(regnum);
 562         if (id < 0)
 563                 return -EINVAL;
 564
 565         ret = perf_reg_value(&val, &ui->sample->user_regs, id);
 566         if (ret) {
 567                 pr_err("unwind: can't read reg %d\n", regnum);
 568                 return ret;
 569         }
 570
 571         *valp = (unw_word_t) val;
 572         pr_debug("unwind: reg %d, val %lx\n", regnum, (unsigned long)*valp);
 573         return 0;
 574 }
 575
 576 static void put_unwind_info(unw_addr_space_t __maybe_unused as,
 577                             unw_proc_info_t *pi __maybe_unused,
 578                             void *arg __maybe_unused)
 579 {
 580         pr_debug("unwind: put_unwind_info called\n");
 581 }
 582
 583 static int entry(u64 ip, struct thread *thread,
 584                  unwind_entry_cb_t cb, void *arg)
 585 {
 586         struct unwind_entry e;
 587         struct addr_location al;
 588
 589         thread__find_addr_location(thread, PERF_RECORD_MISC_USER,
 590                                    MAP__FUNCTION, ip, &al);
 591
 592         e.ip = al.addr;
 593         e.map = al.map;
 594         e.sym = al.sym;
 595
 596         pr_debug("unwind: %s:ip = 0x%" PRIx64 " (0x%" PRIx64 ")\n",
 597                  al.sym ? al.sym->name : "''",
 598                  ip,
 599                  al.map ? al.map->map_ip(al.map, ip) : (u64) 0);
 600
 601         return cb(&e, arg);
 602 }
 603
 604 static void display_error(int err)
 605 {
 606         switch (err) {
 607         case UNW_EINVAL:
 608                 pr_err("unwind: Only supports local.\n");
 609                 break;
 610         case UNW_EUNSPEC:
 611                 pr_err("unwind: Unspecified error.\n");
 612                 break;
 613         case UNW_EBADREG:
 614                 pr_err("unwind: Register unavailable.\n");
 615                 break;
 616         default:
 617                 break;
 618         }
 619 }
 620
 621 static unw_accessors_t accessors = {
 622         .find_proc_info         = find_proc_info,
 623         .put_unwind_info        = put_unwind_info,
 624         .get_dyn_info_list_addr = get_dyn_info_list_addr,
 625         .access_mem             = access_mem,
 626         .access_reg             = access_reg,
 627         .access_fpreg           = access_fpreg,
 628         .resume                 = resume,
 629         .get_proc_name          = get_proc_name,
 630 };
 631
 632 static int _unwind__prepare_access(struct thread *thread)
 633 {
 634         if (callchain_param.record_mode != CALLCHAIN_DWARF)
 635                 return 0;
 636
 637         thread->addr_space = unw_create_addr_space(&accessors, 0);
 638         if (!thread->addr_space) {
 639                 pr_err("unwind: Can't create unwind address space.\n");
 640                 return -ENOMEM;
 641         }
 642
 643         unw_set_caching_policy(thread->addr_space, UNW_CACHE_GLOBAL);
 644         return 0;
 645 }
 646
 647 static void _unwind__flush_access(struct thread *thread)
 648 {
 649         if (callchain_param.record_mode != CALLCHAIN_DWARF)
 650                 return;
 651
 652         unw_flush_cache(thread->addr_space, 0, 0);
 653 }
 654
 655 static void _unwind__finish_access(struct thread *thread)
 656 {
 657         if (callchain_param.record_mode != CALLCHAIN_DWARF)
 658                 return;
 659
 660         unw_destroy_addr_space(thread->addr_space);
 661 }
 662
 663 static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
 664                        void *arg, int max_stack)
 665 {
 666         u64 val;
 667         unw_word_t ips[max_stack];
 668         unw_addr_space_t addr_space;
 669         unw_cursor_t c;
 670         int ret, i = 0;
 671
 672         ret = perf_reg_value(&val, &ui->sample->user_regs,
 673                              LIBUNWIND__ARCH_REG_IP);
 674         if (ret)
 675                 return ret;
 676
 677         ips[i++] = (unw_word_t) val;
 678
 679         /*
 680          * If we need more than one entry, do the DWARF
 681          * unwind itself.
 682          */
 683         if (max_stack - 1 > 0) {
 684                 WARN_ONCE(!ui->thread, "WARNING: ui->thread is NULL");
 685                 addr_space = ui->thread->addr_space;
 686
 687                 if (addr_space == NULL)
 688                         return -1;
 689
 690                 ret = unw_init_remote(&c, addr_space, ui);
 691                 if (ret)
 692                         display_error(ret);
 693
 694                 while (!ret && (unw_step(&c) > 0) && i < max_stack) {
 695                         unw_get_reg(&c, UNW_REG_IP, &ips[i]);
 696
 697                         /*
 698                          * Decrement the IP for any non-activation frames.
 699                          * this is required to properly find the srcline
 700                          * for caller frames.
 701                          * See also the documentation for dwfl_frame_pc(),
 702                          * which this code tries to replicate.
 703                          */
 704                         if (unw_is_signal_frame(&c) <= 0)
 705                                 --ips[i];
 706
 707                         ++i;
 708                 }
 709
 710                 max_stack = i;
 711         }
 712
 713         /*
 714          * Display what we got based on the order setup.
 715          */
 716         for (i = 0; i < max_stack && !ret; i++) {
 717                 int j = i;
 718
 719                 if (callchain_param.order == ORDER_CALLER)
 720                         j = max_stack - i - 1;
 721                 ret = ips[j] ? entry(ips[j], ui->thread, cb, arg) : 0;
 722         }
 723
 724         return ret;
 725 }
 726
 727 static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 728                         struct thread *thread,
 729                         struct perf_sample *data, int max_stack)
 730 {
 731         struct unwind_info ui = {
 732                 .sample       = data,
 733                 .thread       = thread,
 734                 .machine      = thread->mg->machine,
 735         };
 736
 737         if (!data->user_regs.regs)
 738                 return -EINVAL;
 739
 740         if (max_stack <= 0)
 741                 return -EINVAL;
 742
 743         return get_entries(&ui, cb, arg, max_stack);
 744 }
 745
 746 static struct unwind_libunwind_ops
 747 _unwind_libunwind_ops = {
 748         .prepare_access = _unwind__prepare_access,
 749         .flush_access   = _unwind__flush_access,
 750         .finish_access  = _unwind__finish_access,
 751         .get_entries    = _unwind__get_entries,
 752 };
 753
 754 #ifndef REMOTE_UNWIND_LIBUNWIND
 755 struct unwind_libunwind_ops *
 756 local_unwind_libunwind_ops = &_unwind_libunwind_ops;
 757 #endif