skip-callchain-idx.c

  1// SPDX-License-Identifier: GPL-2.0-or-later
  2/*
  3 * Use DWARF Debug information to skip unnecessary callchain entries.
  4 *
  5 * Copyright (C) 2014 Sukadev Bhattiprolu, IBM Corporation.
  6 * Copyright (C) 2014 Ulrich Weigand, IBM Corporation.
  7 */
  8#include <inttypes.h>
  9#include <dwarf.h>
 10#include <elfutils/libdwfl.h>
 11
 12#include "util/thread.h"
 13#include "util/callchain.h"
 14#include "util/debug.h"
 15#include "util/dso.h"
 16#include "util/event.h" // struct ip_callchain
 17#include "util/map.h"
 18#include "util/symbol.h"
 19
 20/*
 21 * When saving the callchain on Power, the kernel conservatively saves
 22 * excess entries in the callchain. A few of these entries are needed
 23 * in some cases but not others. If the unnecessary entries are not
 24 * ignored, we end up with duplicate arcs in the call-graphs. Use
 25 * DWARF debug information to skip over any unnecessary callchain
 26 * entries.
 27 *
 28 * See function header for arch_adjust_callchain() below for more details.
 29 *
 30 * The libdwfl code in this file is based on code from elfutils
 31 * (libdwfl/argp-std.c, libdwfl/tests/addrcfi.c, etc).
 32 */
 33static char *debuginfo_path;
 34
 35static const Dwfl_Callbacks offline_callbacks = {
 36	.debuginfo_path = &debuginfo_path,
 37	.find_debuginfo = dwfl_standard_find_debuginfo,
 38	.section_address = dwfl_offline_section_address,
 39};
 40
 41
 42/*
 43 * Use the DWARF expression for the Call-frame-address and determine
 44 * if return address is in LR and if a new frame was allocated.
 45 */
 46static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
 47{
 48	Dwarf_Op ops_mem[3];
 49	Dwarf_Op dummy;
 50	Dwarf_Op *ops = &dummy;
 51	size_t nops;
 52	int result;
 53
 54	result = dwarf_frame_register(frame, ra_regno, ops_mem, &ops, &nops);
 55	if (result < 0) {
 56		pr_debug("dwarf_frame_register() %s\n", dwarf_errmsg(-1));
 57		return -1;
 58	}
 59
 60	/*
 61	 * Check if return address is on the stack. If return address
 62	 * is in a register (typically R0), it is yet to be saved on
 63	 * the stack.
 64	 */
 65	if ((nops != 0 || ops != NULL) &&
 66		!(nops == 1 && ops[0].atom == DW_OP_regx &&
 67			ops[0].number2 == 0 && ops[0].offset == 0))
 68		return 0;
 69
 70	/*
 71	 * Return address is in LR. Check if a frame was allocated
 72	 * but not-yet used.
 73	 */
 74	result = dwarf_frame_cfa(frame, &ops, &nops);
 75	if (result < 0) {
 76		pr_debug("dwarf_frame_cfa() returns %d, %s\n", result,
 77					dwarf_errmsg(-1));
 78		return -1;
 79	}
 80
 81	/*
 82	 * If call frame address is in r1, no new frame was allocated.
 83	 */
 84	if (nops == 1 && ops[0].atom == DW_OP_bregx && ops[0].number == 1 &&
 85				ops[0].number2 == 0)
 86		return 1;
 87
 88	/*
 89	 * A new frame was allocated but has not yet been used.
 90	 */
 91	return 2;
 92}
 93
 94/*
 95 * Get the DWARF frame from the .eh_frame section.
 96 */
 97static Dwarf_Frame *get_eh_frame(Dwfl_Module *mod, Dwarf_Addr pc)
 98{
 99	int		result;
100	Dwarf_Addr	bias;
101	Dwarf_CFI	*cfi;
102	Dwarf_Frame	*frame;
103
104	cfi = dwfl_module_eh_cfi(mod, &bias);
105	if (!cfi) {
106		pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
107		return NULL;
108	}
109
110	result = dwarf_cfi_addrframe(cfi, pc-bias, &frame);
111	if (result) {
112		pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
113		return NULL;
114	}
115
116	return frame;
117}
118
119/*
120 * Get the DWARF frame from the .debug_frame section.
121 */
122static Dwarf_Frame *get_dwarf_frame(Dwfl_Module *mod, Dwarf_Addr pc)
123{
124	Dwarf_CFI       *cfi;
125	Dwarf_Addr      bias;
126	Dwarf_Frame     *frame;
127	int             result;
128
129	cfi = dwfl_module_dwarf_cfi(mod, &bias);
130	if (!cfi) {
131		pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
132		return NULL;
133	}
134
135	result = dwarf_cfi_addrframe(cfi, pc-bias, &frame);
136	if (result) {
137		pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
138		return NULL;
139	}
140
141	return frame;
142}
143
144/*
145 * Return:
146 *	0 if return address for the program counter @pc is on stack
147 *	1 if return address is in LR and no new stack frame was allocated
148 *	2 if return address is in LR and a new frame was allocated (but not
149 *		yet used)
150 *	-1 in case of errors
151 */
152static int check_return_addr(struct dso *dso, u64 map_start, Dwarf_Addr pc)
153{
154	int		rc = -1;
155	Dwfl		*dwfl;
156	Dwfl_Module	*mod;
157	Dwarf_Frame	*frame;
158	int		ra_regno;
159	Dwarf_Addr	start = pc;
160	Dwarf_Addr	end = pc;
161	bool		signalp;
162	const char	*exec_file = dso__long_name(dso);
163
164	dwfl = RC_CHK_ACCESS(dso)->dwfl;
165
166	if (!dwfl) {
167		dwfl = dwfl_begin(&offline_callbacks);
168		if (!dwfl) {
169			pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1));
170			return -1;
171		}
172
173		mod = dwfl_report_elf(dwfl, exec_file, exec_file, -1,
174						map_start, false);
175		if (!mod) {
176			pr_debug("dwfl_report_elf() failed %s\n",
177						dwarf_errmsg(-1));
178			/*
179			 * We normally cache the DWARF debug info and never
180			 * call dwfl_end(). But to prevent fd leak, free in
181			 * case of error.
182			 */
183			dwfl_end(dwfl);
184			goto out;
185		}
186		RC_CHK_ACCESS(dso)->dwfl = dwfl;
187	}
188
189	mod = dwfl_addrmodule(dwfl, pc);
190	if (!mod) {
191		pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1));
192		goto out;
193	}
194
195	/*
196	 * To work with split debug info files (eg: glibc), check both
197	 * .eh_frame and .debug_frame sections of the ELF header.
198	 */
199	frame = get_eh_frame(mod, pc);
200	if (!frame) {
201		frame = get_dwarf_frame(mod, pc);
202		if (!frame)
203			goto out;
204	}
205
206	ra_regno = dwarf_frame_info(frame, &start, &end, &signalp);
207	if (ra_regno < 0) {
208		pr_debug("Return address register unavailable: %s\n",
209				dwarf_errmsg(-1));
210		goto out;
211	}
212
213	rc = check_return_reg(ra_regno, frame);
214
215out:
216	return rc;
217}
218
219/*
220 * The callchain saved by the kernel always includes the link register (LR).
221 *
222 *	0:	PERF_CONTEXT_USER
223 *	1:	Program counter (Next instruction pointer)
224 *	2:	LR value
225 *	3:	Caller's caller
226 *	4:	...
227 *
228 * The value in LR is only needed when it holds a return address. If the
229 * return address is on the stack, we should ignore the LR value.
230 *
231 * Further, when the return address is in the LR, if a new frame was just
232 * allocated but the LR was not saved into it, then the LR contains the
233 * caller, slot 4: contains the caller's caller and the contents of slot 3:
234 * (chain->ips[3]) is undefined and must be ignored.
235 *
236 * Use DWARF debug information to determine if any entries need to be skipped.
237 *
238 * Return:
239 *	index:	of callchain entry that needs to be ignored (if any)
240 *	-1	if no entry needs to be ignored or in case of errors
241 */
242int arch_skip_callchain_idx(struct thread *thread, struct ip_callchain *chain)
243{
244	struct addr_location al;
245	struct dso *dso = NULL;
246	int rc;
247	u64 ip;
248	u64 skip_slot = -1;
249
250	if (!chain || chain->nr < 3)
251		return skip_slot;
252
253	addr_location__init(&al);
254	ip = chain->ips[1];
255
256	thread__find_symbol(thread, PERF_RECORD_MISC_USER, ip, &al);
257
258	if (al.map)
259		dso = map__dso(al.map);
260
261	if (!dso) {
262		pr_debug("%" PRIx64 " dso is NULL\n", ip);
263		addr_location__exit(&al);
264		return skip_slot;
265	}
266
267	rc = check_return_addr(dso, map__start(al.map), ip);
268
269	pr_debug("[DSO %s, sym %s, ip 0x%" PRIx64 "] rc %d\n",
270		dso__long_name(dso), al.sym->name, ip, rc);
271
272	if (rc == 0) {
273		/*
274		 * Return address on stack. Ignore LR value in callchain
275		 */
276		skip_slot = 2;
277	} else if (rc == 2) {
278		/*
279		 * New frame allocated but return address still in LR.
280		 * Ignore the caller's caller entry in callchain.
281		 */
282		skip_slot = 3;
283	}
284
285	addr_location__exit(&al);
286	return skip_slot;
287}