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 #include <linux/zalloc.h>
29 #ifndef REMOTE_UNWIND_LIBUNWIND
30 #include <libunwind.h>
31 #include <libunwind-ptrace.h>
32 #endif
33 #include "callchain.h"
34 #include "thread.h"
35 #include "session.h"
36 #include "perf_regs.h"
37 #include "unwind.h"
38 #include "map.h"
39 #include "symbol.h"
40 #include "debug.h"
41 #include "asm/bug.h"
42 #include "dso.h"
43 
44 extern int
45 UNW_OBJ(dwarf_search_unwind_table) (unw_addr_space_t as,
46 				    unw_word_t ip,
47 				    unw_dyn_info_t *di,
48 				    unw_proc_info_t *pi,
49 				    int need_unwind_info, void *arg);
50 
51 #define dwarf_search_unwind_table UNW_OBJ(dwarf_search_unwind_table)
52 
53 extern int
54 UNW_OBJ(dwarf_find_debug_frame) (int found, unw_dyn_info_t *di_debug,
55 				 unw_word_t ip,
56 				 unw_word_t segbase,
57 				 const char *obj_name, unw_word_t start,
58 				 unw_word_t end);
59 
60 #define dwarf_find_debug_frame UNW_OBJ(dwarf_find_debug_frame)
61 
62 #define DW_EH_PE_FORMAT_MASK	0x0f	/* format of the encoded value */
63 #define DW_EH_PE_APPL_MASK	0x70	/* how the value is to be applied */
64 
65 /* Pointer-encoding formats: */
66 #define DW_EH_PE_omit		0xff
67 #define DW_EH_PE_ptr		0x00	/* pointer-sized unsigned value */
68 #define DW_EH_PE_udata4		0x03	/* unsigned 32-bit value */
69 #define DW_EH_PE_udata8		0x04	/* unsigned 64-bit value */
70 #define DW_EH_PE_sdata4		0x0b	/* signed 32-bit value */
71 #define DW_EH_PE_sdata8		0x0c	/* signed 64-bit value */
72 
73 /* Pointer-encoding application: */
74 #define DW_EH_PE_absptr		0x00	/* absolute value */
75 #define DW_EH_PE_pcrel		0x10	/* rel. to addr. of encoded value */
76 
77 /*
78  * The following are not documented by LSB v1.3, yet they are used by
79  * GCC, presumably they aren't documented by LSB since they aren't
80  * used on Linux:
81  */
82 #define DW_EH_PE_funcrel	0x40	/* start-of-procedure-relative */
83 #define DW_EH_PE_aligned	0x50	/* aligned pointer */
84 
85 /* Flags intentionally not handled, since they're not needed:
86  * #define DW_EH_PE_indirect      0x80
87  * #define DW_EH_PE_uleb128       0x01
88  * #define DW_EH_PE_udata2        0x02
89  * #define DW_EH_PE_sleb128       0x09
90  * #define DW_EH_PE_sdata2        0x0a
91  * #define DW_EH_PE_textrel       0x20
92  * #define DW_EH_PE_datarel       0x30
93  */
94 
95 struct unwind_info {
96 	struct perf_sample	*sample;
97 	struct machine		*machine;
98 	struct thread		*thread;
99 	bool			 best_effort;
100 };
101 
102 #define dw_read(ptr, type, end) ({	\
103 	type *__p = (type *) ptr;	\
104 	type  __v;			\
105 	if ((__p + 1) > (type *) end)	\
106 		return -EINVAL;		\
107 	__v = *__p++;			\
108 	ptr = (typeof(ptr)) __p;	\
109 	__v;				\
110 	})
111 
112 static int __dw_read_encoded_value(u8 **p, u8 *end, u64 *val,
113 				   u8 encoding)
114 {
115 	u8 *cur = *p;
116 	*val = 0;
117 
118 	switch (encoding) {
119 	case DW_EH_PE_omit:
120 		*val = 0;
121 		goto out;
122 	case DW_EH_PE_ptr:
123 		*val = dw_read(cur, unsigned long, end);
124 		goto out;
125 	default:
126 		break;
127 	}
128 
129 	switch (encoding & DW_EH_PE_APPL_MASK) {
130 	case DW_EH_PE_absptr:
131 		break;
132 	case DW_EH_PE_pcrel:
133 		*val = (unsigned long) cur;
134 		break;
135 	default:
136 		return -EINVAL;
137 	}
138 
139 	if ((encoding & 0x07) == 0x00)
140 		encoding |= DW_EH_PE_udata4;
141 
142 	switch (encoding & DW_EH_PE_FORMAT_MASK) {
143 	case DW_EH_PE_sdata4:
144 		*val += dw_read(cur, s32, end);
145 		break;
146 	case DW_EH_PE_udata4:
147 		*val += dw_read(cur, u32, end);
148 		break;
149 	case DW_EH_PE_sdata8:
150 		*val += dw_read(cur, s64, end);
151 		break;
152 	case DW_EH_PE_udata8:
153 		*val += dw_read(cur, u64, end);
154 		break;
155 	default:
156 		return -EINVAL;
157 	}
158 
159  out:
160 	*p = cur;
161 	return 0;
162 }
163 
164 #define dw_read_encoded_value(ptr, end, enc) ({			\
165 	u64 __v;						\
166 	if (__dw_read_encoded_value(&ptr, end, &__v, enc)) {	\
167 		return -EINVAL;                                 \
168 	}                                                       \
169 	__v;                                                    \
170 	})
171 
172 static int elf_section_address_and_offset(int fd, const char *name, u64 *address, u64 *offset)
173 {
174 	Elf *elf;
175 	GElf_Ehdr ehdr;
176 	GElf_Shdr shdr;
177 	int ret;
178 
179 	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
180 	if (elf == NULL)
181 		return -1;
182 
183 	if (gelf_getehdr(elf, &ehdr) == NULL)
184 		goto out_err;
185 
186 	if (!elf_section_by_name(elf, &ehdr, &shdr, name, NULL))
187 		goto out_err;
188 
189 	*address = shdr.sh_addr;
190 	*offset = shdr.sh_offset;
191 	ret = 0;
192 out_err:
193 	elf_end(elf);
194 	return ret;
195 }
196 
197 #ifndef NO_LIBUNWIND_DEBUG_FRAME
198 static u64 elf_section_offset(int fd, const char *name)
199 {
200 	u64 address, offset;
201 
202 	if (elf_section_address_and_offset(fd, name, &address, &offset))
203 		return 0;
204 
205 	return offset;
206 }
207 #endif
208 
209 static u64 elf_base_address(int fd)
210 {
211 	Elf *elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
212 	GElf_Phdr phdr;
213 	u64 retval = 0;
214 	size_t i, phdrnum = 0;
215 
216 	if (elf == NULL)
217 		return 0;
218 	(void)elf_getphdrnum(elf, &phdrnum);
219 	/* PT_LOAD segments are sorted by p_vaddr, so the first has the minimum p_vaddr. */
220 	for (i = 0; i < phdrnum; i++) {
221 		if (gelf_getphdr(elf, i, &phdr) && phdr.p_type == PT_LOAD) {
222 			retval = phdr.p_vaddr & -getpagesize();
223 			break;
224 		}
225 	}
226 
227 	elf_end(elf);
228 	return retval;
229 }
230 
231 #ifndef NO_LIBUNWIND_DEBUG_FRAME
232 static int elf_is_exec(int fd, const char *name)
233 {
234 	Elf *elf;
235 	GElf_Ehdr ehdr;
236 	int retval = 0;
237 
238 	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
239 	if (elf == NULL)
240 		return 0;
241 	if (gelf_getehdr(elf, &ehdr) == NULL)
242 		goto out;
243 
244 	retval = (ehdr.e_type == ET_EXEC);
245 
246 out:
247 	elf_end(elf);
248 	pr_debug("unwind: elf_is_exec(%s): %d\n", name, retval);
249 	return retval;
250 }
251 #endif
252 
253 struct table_entry {
254 	u32 start_ip_offset;
255 	u32 fde_offset;
256 };
257 
258 struct eh_frame_hdr {
259 	unsigned char version;
260 	unsigned char eh_frame_ptr_enc;
261 	unsigned char fde_count_enc;
262 	unsigned char table_enc;
263 
264 	/*
265 	 * The rest of the header is variable-length and consists of the
266 	 * following members:
267 	 *
268 	 *	encoded_t eh_frame_ptr;
269 	 *	encoded_t fde_count;
270 	 */
271 
272 	/* A single encoded pointer should not be more than 8 bytes. */
273 	u64 enc[2];
274 
275 	/*
276 	 * struct {
277 	 *    encoded_t start_ip;
278 	 *    encoded_t fde_addr;
279 	 * } binary_search_table[fde_count];
280 	 */
281 	char data[];
282 } __packed;
283 
284 static int unwind_spec_ehframe(struct dso *dso, struct machine *machine,
285 			       u64 offset, u64 *table_data_offset, u64 *fde_count)
286 {
287 	struct eh_frame_hdr hdr;
288 	u8 *enc = (u8 *) &hdr.enc;
289 	u8 *end = (u8 *) &hdr.data;
290 	ssize_t r;
291 
292 	r = dso__data_read_offset(dso, machine, offset,
293 				  (u8 *) &hdr, sizeof(hdr));
294 	if (r != sizeof(hdr))
295 		return -EINVAL;
296 
297 	/* We dont need eh_frame_ptr, just skip it. */
298 	dw_read_encoded_value(enc, end, hdr.eh_frame_ptr_enc);
299 
300 	*fde_count  = dw_read_encoded_value(enc, end, hdr.fde_count_enc);
301 	*table_data_offset = enc - (u8 *) &hdr;
302 	return 0;
303 }
304 
305 static int read_unwind_spec_eh_frame(struct dso *dso, struct unwind_info *ui,
306 				     u64 *table_data, u64 *segbase,
307 				     u64 *fde_count)
308 {
309 	struct map *map;
310 	u64 base_addr = UINT64_MAX;
311 	int ret, fd;
312 
313 	if (dso->data.eh_frame_hdr_offset == 0) {
314 		fd = dso__data_get_fd(dso, ui->machine);
315 		if (fd < 0)
316 			return -EINVAL;
317 
318 		/* Check the .eh_frame section for unwinding info */
319 		ret = elf_section_address_and_offset(fd, ".eh_frame_hdr",
320 						     &dso->data.eh_frame_hdr_addr,
321 						     &dso->data.eh_frame_hdr_offset);
322 		dso->data.elf_base_addr = elf_base_address(fd);
323 		dso__data_put_fd(dso);
324 		if (ret || dso->data.eh_frame_hdr_offset == 0)
325 			return -EINVAL;
326 	}
327 
328 	maps__for_each_entry(ui->thread->maps, map) {
329 		if (map->dso == dso && map->start < base_addr)
330 			base_addr = map->start;
331 	}
332 	base_addr -= dso->data.elf_base_addr;
333 	/* Address of .eh_frame_hdr */
334 	*segbase = base_addr + dso->data.eh_frame_hdr_addr;
335 	ret = unwind_spec_ehframe(dso, ui->machine, dso->data.eh_frame_hdr_offset,
336 				   table_data, fde_count);
337 	if (ret)
338 		return ret;
339 	/* binary_search_table offset plus .eh_frame_hdr address */
340 	*table_data += *segbase;
341 	return 0;
342 }
343 
344 #ifndef NO_LIBUNWIND_DEBUG_FRAME
345 static int read_unwind_spec_debug_frame(struct dso *dso,
346 					struct machine *machine, u64 *offset)
347 {
348 	int fd;
349 	u64 ofs = dso->data.debug_frame_offset;
350 
351 	/* debug_frame can reside in:
352 	 *  - dso
353 	 *  - debug pointed by symsrc_filename
354 	 *  - gnu_debuglink, which doesn't necessary
355 	 *    has to be pointed by symsrc_filename
356 	 */
357 	if (ofs == 0) {
358 		fd = dso__data_get_fd(dso, machine);
359 		if (fd >= 0) {
360 			ofs = elf_section_offset(fd, ".debug_frame");
361 			dso__data_put_fd(dso);
362 		}
363 
364 		if (ofs <= 0) {
365 			fd = open(dso->symsrc_filename, O_RDONLY);
366 			if (fd >= 0) {
367 				ofs = elf_section_offset(fd, ".debug_frame");
368 				close(fd);
369 			}
370 		}
371 
372 		if (ofs <= 0) {
373 			char *debuglink = malloc(PATH_MAX);
374 			int ret = 0;
375 
376 			ret = dso__read_binary_type_filename(
377 				dso, DSO_BINARY_TYPE__DEBUGLINK,
378 				machine->root_dir, debuglink, PATH_MAX);
379 			if (!ret) {
380 				fd = open(debuglink, O_RDONLY);
381 				if (fd >= 0) {
382 					ofs = elf_section_offset(fd,
383 							".debug_frame");
384 					close(fd);
385 				}
386 			}
387 			if (ofs > 0) {
388 				if (dso->symsrc_filename != NULL) {
389 					pr_warning(
390 						"%s: overwrite symsrc(%s,%s)\n",
391 							__func__,
392 							dso->symsrc_filename,
393 							debuglink);
394 					zfree(&dso->symsrc_filename);
395 				}
396 				dso->symsrc_filename = debuglink;
397 			} else {
398 				free(debuglink);
399 			}
400 		}
401 
402 		dso->data.debug_frame_offset = ofs;
403 	}
404 
405 	*offset = ofs;
406 	if (*offset)
407 		return 0;
408 
409 	return -EINVAL;
410 }
411 #endif
412 
413 static struct map *find_map(unw_word_t ip, struct unwind_info *ui)
414 {
415 	struct addr_location al;
416 	return thread__find_map(ui->thread, PERF_RECORD_MISC_USER, ip, &al);
417 }
418 
419 static int
420 find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
421 	       int need_unwind_info, void *arg)
422 {
423 	struct unwind_info *ui = arg;
424 	struct map *map;
425 	unw_dyn_info_t di;
426 	u64 table_data, segbase, fde_count;
427 	int ret = -EINVAL;
428 
429 	map = find_map(ip, ui);
430 	if (!map || !map->dso)
431 		return -EINVAL;
432 
433 	pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
434 
435 	/* Check the .eh_frame section for unwinding info */
436 	if (!read_unwind_spec_eh_frame(map->dso, ui,
437 				       &table_data, &segbase, &fde_count)) {
438 		memset(&di, 0, sizeof(di));
439 		di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
440 		di.start_ip = map->start;
441 		di.end_ip   = map->end;
442 		di.u.rti.segbase    = segbase;
443 		di.u.rti.table_data = table_data;
444 		di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
445 				      / sizeof(unw_word_t);
446 		ret = dwarf_search_unwind_table(as, ip, &di, pi,
447 						need_unwind_info, arg);
448 	}
449 
450 #ifndef NO_LIBUNWIND_DEBUG_FRAME
451 	/* Check the .debug_frame section for unwinding info */
452 	if (ret < 0 &&
453 	    !read_unwind_spec_debug_frame(map->dso, ui->machine, &segbase)) {
454 		int fd = dso__data_get_fd(map->dso, ui->machine);
455 		int is_exec = elf_is_exec(fd, map->dso->name);
456 		unw_word_t base = is_exec ? 0 : map->start;
457 		const char *symfile;
458 
459 		if (fd >= 0)
460 			dso__data_put_fd(map->dso);
461 
462 		symfile = map->dso->symsrc_filename ?: map->dso->name;
463 
464 		memset(&di, 0, sizeof(di));
465 		if (dwarf_find_debug_frame(0, &di, ip, base, symfile,
466 					   map->start, map->end))
467 			return dwarf_search_unwind_table(as, ip, &di, pi,
468 							 need_unwind_info, arg);
469 	}
470 #endif
471 
472 	return ret;
473 }
474 
475 static int access_fpreg(unw_addr_space_t __maybe_unused as,
476 			unw_regnum_t __maybe_unused num,
477 			unw_fpreg_t __maybe_unused *val,
478 			int __maybe_unused __write,
479 			void __maybe_unused *arg)
480 {
481 	pr_err("unwind: access_fpreg unsupported\n");
482 	return -UNW_EINVAL;
483 }
484 
485 static int get_dyn_info_list_addr(unw_addr_space_t __maybe_unused as,
486 				  unw_word_t __maybe_unused *dil_addr,
487 				  void __maybe_unused *arg)
488 {
489 	return -UNW_ENOINFO;
490 }
491 
492 static int resume(unw_addr_space_t __maybe_unused as,
493 		  unw_cursor_t __maybe_unused *cu,
494 		  void __maybe_unused *arg)
495 {
496 	pr_err("unwind: resume unsupported\n");
497 	return -UNW_EINVAL;
498 }
499 
500 static int
501 get_proc_name(unw_addr_space_t __maybe_unused as,
502 	      unw_word_t __maybe_unused addr,
503 		char __maybe_unused *bufp, size_t __maybe_unused buf_len,
504 		unw_word_t __maybe_unused *offp, void __maybe_unused *arg)
505 {
506 	pr_err("unwind: get_proc_name unsupported\n");
507 	return -UNW_EINVAL;
508 }
509 
510 static int access_dso_mem(struct unwind_info *ui, unw_word_t addr,
511 			  unw_word_t *data)
512 {
513 	struct map *map;
514 	ssize_t size;
515 
516 	map = find_map(addr, ui);
517 	if (!map) {
518 		pr_debug("unwind: no map for %lx\n", (unsigned long)addr);
519 		return -1;
520 	}
521 
522 	if (!map->dso)
523 		return -1;
524 
525 	size = dso__data_read_addr(map->dso, map, ui->machine,
526 				   addr, (u8 *) data, sizeof(*data));
527 
528 	return !(size == sizeof(*data));
529 }
530 
531 static int access_mem(unw_addr_space_t __maybe_unused as,
532 		      unw_word_t addr, unw_word_t *valp,
533 		      int __write, void *arg)
534 {
535 	struct unwind_info *ui = arg;
536 	struct stack_dump *stack = &ui->sample->user_stack;
537 	u64 start, end;
538 	int offset;
539 	int ret;
540 
541 	/* Don't support write, probably not needed. */
542 	if (__write || !stack || !ui->sample->user_regs.regs) {
543 		*valp = 0;
544 		return 0;
545 	}
546 
547 	ret = perf_reg_value(&start, &ui->sample->user_regs,
548 			     LIBUNWIND__ARCH_REG_SP);
549 	if (ret)
550 		return ret;
551 
552 	end = start + stack->size;
553 
554 	/* Check overflow. */
555 	if (addr + sizeof(unw_word_t) < addr)
556 		return -EINVAL;
557 
558 	if (addr < start || addr + sizeof(unw_word_t) >= end) {
559 		ret = access_dso_mem(ui, addr, valp);
560 		if (ret) {
561 			pr_debug("unwind: access_mem %p not inside range"
562 				 " 0x%" PRIx64 "-0x%" PRIx64 "\n",
563 				 (void *) (uintptr_t) addr, start, end);
564 			*valp = 0;
565 			return ret;
566 		}
567 		return 0;
568 	}
569 
570 	offset = addr - start;
571 	*valp  = *(unw_word_t *)&stack->data[offset];
572 	pr_debug("unwind: access_mem addr %p val %lx, offset %d\n",
573 		 (void *) (uintptr_t) addr, (unsigned long)*valp, offset);
574 	return 0;
575 }
576 
577 static int access_reg(unw_addr_space_t __maybe_unused as,
578 		      unw_regnum_t regnum, unw_word_t *valp,
579 		      int __write, void *arg)
580 {
581 	struct unwind_info *ui = arg;
582 	int id, ret;
583 	u64 val;
584 
585 	/* Don't support write, I suspect we don't need it. */
586 	if (__write) {
587 		pr_err("unwind: access_reg w %d\n", regnum);
588 		return 0;
589 	}
590 
591 	if (!ui->sample->user_regs.regs) {
592 		*valp = 0;
593 		return 0;
594 	}
595 
596 	id = LIBUNWIND__ARCH_REG_ID(regnum);
597 	if (id < 0)
598 		return -EINVAL;
599 
600 	ret = perf_reg_value(&val, &ui->sample->user_regs, id);
601 	if (ret) {
602 		if (!ui->best_effort)
603 			pr_err("unwind: can't read reg %d\n", regnum);
604 		return ret;
605 	}
606 
607 	*valp = (unw_word_t) val;
608 	pr_debug("unwind: reg %d, val %lx\n", regnum, (unsigned long)*valp);
609 	return 0;
610 }
611 
612 static void put_unwind_info(unw_addr_space_t __maybe_unused as,
613 			    unw_proc_info_t *pi __maybe_unused,
614 			    void *arg __maybe_unused)
615 {
616 	pr_debug("unwind: put_unwind_info called\n");
617 }
618 
619 static int entry(u64 ip, struct thread *thread,
620 		 unwind_entry_cb_t cb, void *arg)
621 {
622 	struct unwind_entry e;
623 	struct addr_location al;
624 
625 	e.ms.sym = thread__find_symbol(thread, PERF_RECORD_MISC_USER, ip, &al);
626 	e.ip     = ip;
627 	e.ms.map = al.map;
628 	e.ms.maps = al.maps;
629 
630 	pr_debug("unwind: %s:ip = 0x%" PRIx64 " (0x%" PRIx64 ")\n",
631 		 al.sym ? al.sym->name : "''",
632 		 ip,
633 		 al.map ? al.map->map_ip(al.map, ip) : (u64) 0);
634 
635 	return cb(&e, arg);
636 }
637 
638 static void display_error(int err)
639 {
640 	switch (err) {
641 	case UNW_EINVAL:
642 		pr_err("unwind: Only supports local.\n");
643 		break;
644 	case UNW_EUNSPEC:
645 		pr_err("unwind: Unspecified error.\n");
646 		break;
647 	case UNW_EBADREG:
648 		pr_err("unwind: Register unavailable.\n");
649 		break;
650 	default:
651 		break;
652 	}
653 }
654 
655 static unw_accessors_t accessors = {
656 	.find_proc_info		= find_proc_info,
657 	.put_unwind_info	= put_unwind_info,
658 	.get_dyn_info_list_addr	= get_dyn_info_list_addr,
659 	.access_mem		= access_mem,
660 	.access_reg		= access_reg,
661 	.access_fpreg		= access_fpreg,
662 	.resume			= resume,
663 	.get_proc_name		= get_proc_name,
664 };
665 
666 static int _unwind__prepare_access(struct maps *maps)
667 {
668 	maps->addr_space = unw_create_addr_space(&accessors, 0);
669 	if (!maps->addr_space) {
670 		pr_err("unwind: Can't create unwind address space.\n");
671 		return -ENOMEM;
672 	}
673 
674 	unw_set_caching_policy(maps->addr_space, UNW_CACHE_GLOBAL);
675 	return 0;
676 }
677 
678 static void _unwind__flush_access(struct maps *maps)
679 {
680 	unw_flush_cache(maps->addr_space, 0, 0);
681 }
682 
683 static void _unwind__finish_access(struct maps *maps)
684 {
685 	unw_destroy_addr_space(maps->addr_space);
686 }
687 
688 static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
689 		       void *arg, int max_stack)
690 {
691 	u64 val;
692 	unw_word_t ips[max_stack];
693 	unw_addr_space_t addr_space;
694 	unw_cursor_t c;
695 	int ret, i = 0;
696 
697 	ret = perf_reg_value(&val, &ui->sample->user_regs,
698 			     LIBUNWIND__ARCH_REG_IP);
699 	if (ret)
700 		return ret;
701 
702 	ips[i++] = (unw_word_t) val;
703 
704 	/*
705 	 * If we need more than one entry, do the DWARF
706 	 * unwind itself.
707 	 */
708 	if (max_stack - 1 > 0) {
709 		WARN_ONCE(!ui->thread, "WARNING: ui->thread is NULL");
710 		addr_space = ui->thread->maps->addr_space;
711 
712 		if (addr_space == NULL)
713 			return -1;
714 
715 		ret = unw_init_remote(&c, addr_space, ui);
716 		if (ret && !ui->best_effort)
717 			display_error(ret);
718 
719 		while (!ret && (unw_step(&c) > 0) && i < max_stack) {
720 			unw_get_reg(&c, UNW_REG_IP, &ips[i]);
721 
722 			/*
723 			 * Decrement the IP for any non-activation frames.
724 			 * this is required to properly find the srcline
725 			 * for caller frames.
726 			 * See also the documentation for dwfl_frame_pc(),
727 			 * which this code tries to replicate.
728 			 */
729 			if (unw_is_signal_frame(&c) <= 0)
730 				--ips[i];
731 
732 			++i;
733 		}
734 
735 		max_stack = i;
736 	}
737 
738 	/*
739 	 * Display what we got based on the order setup.
740 	 */
741 	for (i = 0; i < max_stack && !ret; i++) {
742 		int j = i;
743 
744 		if (callchain_param.order == ORDER_CALLER)
745 			j = max_stack - i - 1;
746 		ret = ips[j] ? entry(ips[j], ui->thread, cb, arg) : 0;
747 	}
748 
749 	return ret;
750 }
751 
752 static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
753 			struct thread *thread,
754 			struct perf_sample *data, int max_stack,
755 			bool best_effort)
756 {
757 	struct unwind_info ui = {
758 		.sample       = data,
759 		.thread       = thread,
760 		.machine      = thread->maps->machine,
761 		.best_effort  = best_effort
762 	};
763 
764 	if (!data->user_regs.regs)
765 		return -EINVAL;
766 
767 	if (max_stack <= 0)
768 		return -EINVAL;
769 
770 	return get_entries(&ui, cb, arg, max_stack);
771 }
772 
773 static struct unwind_libunwind_ops
774 _unwind_libunwind_ops = {
775 	.prepare_access = _unwind__prepare_access,
776 	.flush_access   = _unwind__flush_access,
777 	.finish_access  = _unwind__finish_access,
778 	.get_entries    = _unwind__get_entries,
779 };
780 
781 #ifndef REMOTE_UNWIND_LIBUNWIND
782 struct unwind_libunwind_ops *
783 local_unwind_libunwind_ops = &_unwind_libunwind_ops;
784 #endif
785