1 /*
2 * Use DWARF Debug information to skip unnecessary callchain entries.
3 *
4 * Copyright (C) 2014 Sukadev Bhattiprolu, IBM Corporation.
5 * Copyright (C) 2014 Ulrich Weigand, IBM Corporation.
6 *
7 * This program is free software; you can redistribute it and/or
8 * modify it under the terms of the GNU General Public License
9 * as published by the Free Software Foundation; either version
10 * 2 of the License, or (at your option) any later version.
11 */
12 #include <inttypes.h>
13 #include <dwarf.h>
14 #include <elfutils/libdwfl.h>
15
16 #include "util/thread.h"
17 #include "util/callchain.h"
18 #include "util/debug.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 */
33 static char *debuginfo_path;
34
35 static 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 */
check_return_reg(int ra_regno,Dwarf_Frame * frame)46 static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
47 {
48 Dwarf_Op ops_mem[2];
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.
62 */
63 if (nops != 0 || ops != NULL)
64 return 0;
65
66 /*
67 * Return address is in LR. Check if a frame was allocated
68 * but not-yet used.
69 */
70 result = dwarf_frame_cfa(frame, &ops, &nops);
71 if (result < 0) {
72 pr_debug("dwarf_frame_cfa() returns %d, %s\n", result,
73 dwarf_errmsg(-1));
74 return -1;
75 }
76
77 /*
78 * If call frame address is in r1, no new frame was allocated.
79 */
80 if (nops == 1 && ops[0].atom == DW_OP_bregx && ops[0].number == 1 &&
81 ops[0].number2 == 0)
82 return 1;
83
84 /*
85 * A new frame was allocated but has not yet been used.
86 */
87 return 2;
88 }
89
90 /*
91 * Get the DWARF frame from the .eh_frame section.
92 */
get_eh_frame(Dwfl_Module * mod,Dwarf_Addr pc)93 static Dwarf_Frame *get_eh_frame(Dwfl_Module *mod, Dwarf_Addr pc)
94 {
95 int result;
96 Dwarf_Addr bias;
97 Dwarf_CFI *cfi;
98 Dwarf_Frame *frame;
99
100 cfi = dwfl_module_eh_cfi(mod, &bias);
101 if (!cfi) {
102 pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
103 return NULL;
104 }
105
106 result = dwarf_cfi_addrframe(cfi, pc, &frame);
107 if (result) {
108 pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
109 return NULL;
110 }
111
112 return frame;
113 }
114
115 /*
116 * Get the DWARF frame from the .debug_frame section.
117 */
get_dwarf_frame(Dwfl_Module * mod,Dwarf_Addr pc)118 static Dwarf_Frame *get_dwarf_frame(Dwfl_Module *mod, Dwarf_Addr pc)
119 {
120 Dwarf_CFI *cfi;
121 Dwarf_Addr bias;
122 Dwarf_Frame *frame;
123 int result;
124
125 cfi = dwfl_module_dwarf_cfi(mod, &bias);
126 if (!cfi) {
127 pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
128 return NULL;
129 }
130
131 result = dwarf_cfi_addrframe(cfi, pc, &frame);
132 if (result) {
133 pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
134 return NULL;
135 }
136
137 return frame;
138 }
139
140 /*
141 * Return:
142 * 0 if return address for the program counter @pc is on stack
143 * 1 if return address is in LR and no new stack frame was allocated
144 * 2 if return address is in LR and a new frame was allocated (but not
145 * yet used)
146 * -1 in case of errors
147 */
check_return_addr(const char * exec_file,Dwarf_Addr pc)148 static int check_return_addr(const char *exec_file, Dwarf_Addr pc)
149 {
150 int rc = -1;
151 Dwfl *dwfl;
152 Dwfl_Module *mod;
153 Dwarf_Frame *frame;
154 int ra_regno;
155 Dwarf_Addr start = pc;
156 Dwarf_Addr end = pc;
157 bool signalp;
158
159 dwfl = dwfl_begin(&offline_callbacks);
160 if (!dwfl) {
161 pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1));
162 return -1;
163 }
164
165 if (dwfl_report_offline(dwfl, "", exec_file, -1) == NULL) {
166 pr_debug("dwfl_report_offline() failed %s\n", dwarf_errmsg(-1));
167 goto out;
168 }
169
170 mod = dwfl_addrmodule(dwfl, pc);
171 if (!mod) {
172 pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1));
173 goto out;
174 }
175
176 /*
177 * To work with split debug info files (eg: glibc), check both
178 * .eh_frame and .debug_frame sections of the ELF header.
179 */
180 frame = get_eh_frame(mod, pc);
181 if (!frame) {
182 frame = get_dwarf_frame(mod, pc);
183 if (!frame)
184 goto out;
185 }
186
187 ra_regno = dwarf_frame_info(frame, &start, &end, &signalp);
188 if (ra_regno < 0) {
189 pr_debug("Return address register unavailable: %s\n",
190 dwarf_errmsg(-1));
191 goto out;
192 }
193
194 rc = check_return_reg(ra_regno, frame);
195
196 out:
197 dwfl_end(dwfl);
198 return rc;
199 }
200
201 /*
202 * The callchain saved by the kernel always includes the link register (LR).
203 *
204 * 0: PERF_CONTEXT_USER
205 * 1: Program counter (Next instruction pointer)
206 * 2: LR value
207 * 3: Caller's caller
208 * 4: ...
209 *
210 * The value in LR is only needed when it holds a return address. If the
211 * return address is on the stack, we should ignore the LR value.
212 *
213 * Further, when the return address is in the LR, if a new frame was just
214 * allocated but the LR was not saved into it, then the LR contains the
215 * caller, slot 4: contains the caller's caller and the contents of slot 3:
216 * (chain->ips[3]) is undefined and must be ignored.
217 *
218 * Use DWARF debug information to determine if any entries need to be skipped.
219 *
220 * Return:
221 * index: of callchain entry that needs to be ignored (if any)
222 * -1 if no entry needs to be ignored or in case of errors
223 */
arch_skip_callchain_idx(struct machine * machine,struct thread * thread,struct ip_callchain * chain)224 int arch_skip_callchain_idx(struct machine *machine, struct thread *thread,
225 struct ip_callchain *chain)
226 {
227 struct addr_location al;
228 struct dso *dso = NULL;
229 int rc;
230 u64 ip;
231 u64 skip_slot = -1;
232
233 if (chain->nr < 3)
234 return skip_slot;
235
236 ip = chain->ips[2];
237
238 thread__find_addr_location(thread, machine, PERF_RECORD_MISC_USER,
239 MAP__FUNCTION, ip, &al);
240
241 if (al.map)
242 dso = al.map->dso;
243
244 if (!dso) {
245 pr_debug("%" PRIx64 " dso is NULL\n", ip);
246 return skip_slot;
247 }
248
249 rc = check_return_addr(dso->long_name, ip);
250
251 pr_debug("DSO %s, nr %" PRIx64 ", ip 0x%" PRIx64 "rc %d\n",
252 dso->long_name, chain->nr, ip, rc);
253
254 if (rc == 0) {
255 /*
256 * Return address on stack. Ignore LR value in callchain
257 */
258 skip_slot = 2;
259 } else if (rc == 2) {
260 /*
261 * New frame allocated but return address still in LR.
262 * Ignore the caller's caller entry in callchain.
263 */
264 skip_slot = 3;
265 }
266 return skip_slot;
267 }
268