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