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 	return thread__find_map(ui->thread, PERF_RECORD_MISC_USER, ip, &al);
370 }
371 
372 static int
373 find_proc_info(unw_addr_space_t as, unw_word_t ip, unw_proc_info_t *pi,
374 	       int need_unwind_info, void *arg)
375 {
376 	struct unwind_info *ui = arg;
377 	struct map *map;
378 	unw_dyn_info_t di;
379 	u64 table_data, segbase, fde_count;
380 	int ret = -EINVAL;
381 
382 	map = find_map(ip, ui);
383 	if (!map || !map->dso)
384 		return -EINVAL;
385 
386 	pr_debug("unwind: find_proc_info dso %s\n", map->dso->name);
387 
388 	/* Check the .eh_frame section for unwinding info */
389 	if (!read_unwind_spec_eh_frame(map->dso, ui->machine,
390 				       &table_data, &segbase, &fde_count)) {
391 		memset(&di, 0, sizeof(di));
392 		di.format   = UNW_INFO_FORMAT_REMOTE_TABLE;
393 		di.start_ip = map->start;
394 		di.end_ip   = map->end;
395 		di.u.rti.segbase    = map->start + segbase - map->pgoff;
396 		di.u.rti.table_data = map->start + table_data - map->pgoff;
397 		di.u.rti.table_len  = fde_count * sizeof(struct table_entry)
398 				      / sizeof(unw_word_t);
399 		ret = dwarf_search_unwind_table(as, ip, &di, pi,
400 						need_unwind_info, arg);
401 	}
402 
403 #ifndef NO_LIBUNWIND_DEBUG_FRAME
404 	/* Check the .debug_frame section for unwinding info */
405 	if (ret < 0 &&
406 	    !read_unwind_spec_debug_frame(map->dso, ui->machine, &segbase)) {
407 		int fd = dso__data_get_fd(map->dso, ui->machine);
408 		int is_exec = elf_is_exec(fd, map->dso->name);
409 		unw_word_t base = is_exec ? 0 : map->start;
410 		const char *symfile;
411 
412 		if (fd >= 0)
413 			dso__data_put_fd(map->dso);
414 
415 		symfile = map->dso->symsrc_filename ?: map->dso->name;
416 
417 		memset(&di, 0, sizeof(di));
418 		if (dwarf_find_debug_frame(0, &di, ip, base, symfile,
419 					   map->start, map->end))
420 			return dwarf_search_unwind_table(as, ip, &di, pi,
421 							 need_unwind_info, arg);
422 	}
423 #endif
424 
425 	return ret;
426 }
427 
428 static int access_fpreg(unw_addr_space_t __maybe_unused as,
429 			unw_regnum_t __maybe_unused num,
430 			unw_fpreg_t __maybe_unused *val,
431 			int __maybe_unused __write,
432 			void __maybe_unused *arg)
433 {
434 	pr_err("unwind: access_fpreg unsupported\n");
435 	return -UNW_EINVAL;
436 }
437 
438 static int get_dyn_info_list_addr(unw_addr_space_t __maybe_unused as,
439 				  unw_word_t __maybe_unused *dil_addr,
440 				  void __maybe_unused *arg)
441 {
442 	return -UNW_ENOINFO;
443 }
444 
445 static int resume(unw_addr_space_t __maybe_unused as,
446 		  unw_cursor_t __maybe_unused *cu,
447 		  void __maybe_unused *arg)
448 {
449 	pr_err("unwind: resume unsupported\n");
450 	return -UNW_EINVAL;
451 }
452 
453 static int
454 get_proc_name(unw_addr_space_t __maybe_unused as,
455 	      unw_word_t __maybe_unused addr,
456 		char __maybe_unused *bufp, size_t __maybe_unused buf_len,
457 		unw_word_t __maybe_unused *offp, void __maybe_unused *arg)
458 {
459 	pr_err("unwind: get_proc_name unsupported\n");
460 	return -UNW_EINVAL;
461 }
462 
463 static int access_dso_mem(struct unwind_info *ui, unw_word_t addr,
464 			  unw_word_t *data)
465 {
466 	struct map *map;
467 	ssize_t size;
468 
469 	map = find_map(addr, ui);
470 	if (!map) {
471 		pr_debug("unwind: no map for %lx\n", (unsigned long)addr);
472 		return -1;
473 	}
474 
475 	if (!map->dso)
476 		return -1;
477 
478 	size = dso__data_read_addr(map->dso, map, ui->machine,
479 				   addr, (u8 *) data, sizeof(*data));
480 
481 	return !(size == sizeof(*data));
482 }
483 
484 static int access_mem(unw_addr_space_t __maybe_unused as,
485 		      unw_word_t addr, unw_word_t *valp,
486 		      int __write, void *arg)
487 {
488 	struct unwind_info *ui = arg;
489 	struct stack_dump *stack = &ui->sample->user_stack;
490 	u64 start, end;
491 	int offset;
492 	int ret;
493 
494 	/* Don't support write, probably not needed. */
495 	if (__write || !stack || !ui->sample->user_regs.regs) {
496 		*valp = 0;
497 		return 0;
498 	}
499 
500 	ret = perf_reg_value(&start, &ui->sample->user_regs,
501 			     LIBUNWIND__ARCH_REG_SP);
502 	if (ret)
503 		return ret;
504 
505 	end = start + stack->size;
506 
507 	/* Check overflow. */
508 	if (addr + sizeof(unw_word_t) < addr)
509 		return -EINVAL;
510 
511 	if (addr < start || addr + sizeof(unw_word_t) >= end) {
512 		ret = access_dso_mem(ui, addr, valp);
513 		if (ret) {
514 			pr_debug("unwind: access_mem %p not inside range"
515 				 " 0x%" PRIx64 "-0x%" PRIx64 "\n",
516 				 (void *) (uintptr_t) addr, start, end);
517 			*valp = 0;
518 			return ret;
519 		}
520 		return 0;
521 	}
522 
523 	offset = addr - start;
524 	*valp  = *(unw_word_t *)&stack->data[offset];
525 	pr_debug("unwind: access_mem addr %p val %lx, offset %d\n",
526 		 (void *) (uintptr_t) addr, (unsigned long)*valp, offset);
527 	return 0;
528 }
529 
530 static int access_reg(unw_addr_space_t __maybe_unused as,
531 		      unw_regnum_t regnum, unw_word_t *valp,
532 		      int __write, void *arg)
533 {
534 	struct unwind_info *ui = arg;
535 	int id, ret;
536 	u64 val;
537 
538 	/* Don't support write, I suspect we don't need it. */
539 	if (__write) {
540 		pr_err("unwind: access_reg w %d\n", regnum);
541 		return 0;
542 	}
543 
544 	if (!ui->sample->user_regs.regs) {
545 		*valp = 0;
546 		return 0;
547 	}
548 
549 	id = LIBUNWIND__ARCH_REG_ID(regnum);
550 	if (id < 0)
551 		return -EINVAL;
552 
553 	ret = perf_reg_value(&val, &ui->sample->user_regs, id);
554 	if (ret) {
555 		pr_err("unwind: can't read reg %d\n", regnum);
556 		return ret;
557 	}
558 
559 	*valp = (unw_word_t) val;
560 	pr_debug("unwind: reg %d, val %lx\n", regnum, (unsigned long)*valp);
561 	return 0;
562 }
563 
564 static void put_unwind_info(unw_addr_space_t __maybe_unused as,
565 			    unw_proc_info_t *pi __maybe_unused,
566 			    void *arg __maybe_unused)
567 {
568 	pr_debug("unwind: put_unwind_info called\n");
569 }
570 
571 static int entry(u64 ip, struct thread *thread,
572 		 unwind_entry_cb_t cb, void *arg)
573 {
574 	struct unwind_entry e;
575 	struct addr_location al;
576 
577 	e.sym = thread__find_symbol(thread, PERF_RECORD_MISC_USER, ip, &al);
578 	e.ip = al.addr;
579 	e.map = al.map;
580 
581 	pr_debug("unwind: %s:ip = 0x%" PRIx64 " (0x%" PRIx64 ")\n",
582 		 al.sym ? al.sym->name : "''",
583 		 ip,
584 		 al.map ? al.map->map_ip(al.map, ip) : (u64) 0);
585 
586 	return cb(&e, arg);
587 }
588 
589 static void display_error(int err)
590 {
591 	switch (err) {
592 	case UNW_EINVAL:
593 		pr_err("unwind: Only supports local.\n");
594 		break;
595 	case UNW_EUNSPEC:
596 		pr_err("unwind: Unspecified error.\n");
597 		break;
598 	case UNW_EBADREG:
599 		pr_err("unwind: Register unavailable.\n");
600 		break;
601 	default:
602 		break;
603 	}
604 }
605 
606 static unw_accessors_t accessors = {
607 	.find_proc_info		= find_proc_info,
608 	.put_unwind_info	= put_unwind_info,
609 	.get_dyn_info_list_addr	= get_dyn_info_list_addr,
610 	.access_mem		= access_mem,
611 	.access_reg		= access_reg,
612 	.access_fpreg		= access_fpreg,
613 	.resume			= resume,
614 	.get_proc_name		= get_proc_name,
615 };
616 
617 static int _unwind__prepare_access(struct thread *thread)
618 {
619 	if (!dwarf_callchain_users)
620 		return 0;
621 	thread->addr_space = unw_create_addr_space(&accessors, 0);
622 	if (!thread->addr_space) {
623 		pr_err("unwind: Can't create unwind address space.\n");
624 		return -ENOMEM;
625 	}
626 
627 	unw_set_caching_policy(thread->addr_space, UNW_CACHE_GLOBAL);
628 	return 0;
629 }
630 
631 static void _unwind__flush_access(struct thread *thread)
632 {
633 	if (!dwarf_callchain_users)
634 		return;
635 	unw_flush_cache(thread->addr_space, 0, 0);
636 }
637 
638 static void _unwind__finish_access(struct thread *thread)
639 {
640 	if (!dwarf_callchain_users)
641 		return;
642 	unw_destroy_addr_space(thread->addr_space);
643 }
644 
645 static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
646 		       void *arg, int max_stack)
647 {
648 	u64 val;
649 	unw_word_t ips[max_stack];
650 	unw_addr_space_t addr_space;
651 	unw_cursor_t c;
652 	int ret, i = 0;
653 
654 	ret = perf_reg_value(&val, &ui->sample->user_regs,
655 			     LIBUNWIND__ARCH_REG_IP);
656 	if (ret)
657 		return ret;
658 
659 	ips[i++] = (unw_word_t) val;
660 
661 	/*
662 	 * If we need more than one entry, do the DWARF
663 	 * unwind itself.
664 	 */
665 	if (max_stack - 1 > 0) {
666 		WARN_ONCE(!ui->thread, "WARNING: ui->thread is NULL");
667 		addr_space = ui->thread->addr_space;
668 
669 		if (addr_space == NULL)
670 			return -1;
671 
672 		ret = unw_init_remote(&c, addr_space, ui);
673 		if (ret)
674 			display_error(ret);
675 
676 		while (!ret && (unw_step(&c) > 0) && i < max_stack) {
677 			unw_get_reg(&c, UNW_REG_IP, &ips[i]);
678 
679 			/*
680 			 * Decrement the IP for any non-activation frames.
681 			 * this is required to properly find the srcline
682 			 * for caller frames.
683 			 * See also the documentation for dwfl_frame_pc(),
684 			 * which this code tries to replicate.
685 			 */
686 			if (unw_is_signal_frame(&c) <= 0)
687 				--ips[i];
688 
689 			++i;
690 		}
691 
692 		max_stack = i;
693 	}
694 
695 	/*
696 	 * Display what we got based on the order setup.
697 	 */
698 	for (i = 0; i < max_stack && !ret; i++) {
699 		int j = i;
700 
701 		if (callchain_param.order == ORDER_CALLER)
702 			j = max_stack - i - 1;
703 		ret = ips[j] ? entry(ips[j], ui->thread, cb, arg) : 0;
704 	}
705 
706 	return ret;
707 }
708 
709 static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
710 			struct thread *thread,
711 			struct perf_sample *data, int max_stack)
712 {
713 	struct unwind_info ui = {
714 		.sample       = data,
715 		.thread       = thread,
716 		.machine      = thread->mg->machine,
717 	};
718 
719 	if (!data->user_regs.regs)
720 		return -EINVAL;
721 
722 	if (max_stack <= 0)
723 		return -EINVAL;
724 
725 	return get_entries(&ui, cb, arg, max_stack);
726 }
727 
728 static struct unwind_libunwind_ops
729 _unwind_libunwind_ops = {
730 	.prepare_access = _unwind__prepare_access,
731 	.flush_access   = _unwind__flush_access,
732 	.finish_access  = _unwind__finish_access,
733 	.get_entries    = _unwind__get_entries,
734 };
735 
736 #ifndef REMOTE_UNWIND_LIBUNWIND
737 struct unwind_libunwind_ops *
738 local_unwind_libunwind_ops = &_unwind_libunwind_ops;
739 #endif
740