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 */ 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 */ 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 */ 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 */ 148 static int check_return_addr(struct dso *dso, 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 = dso->dwfl; 160 161 if (!dwfl) { 162 dwfl = dwfl_begin(&offline_callbacks); 163 if (!dwfl) { 164 pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1)); 165 return -1; 166 } 167 168 if (dwfl_report_offline(dwfl, "", dso->long_name, -1) == NULL) { 169 pr_debug("dwfl_report_offline() failed %s\n", 170 dwarf_errmsg(-1)); 171 /* 172 * We normally cache the DWARF debug info and never 173 * call dwfl_end(). But to prevent fd leak, free in 174 * case of error. 175 */ 176 dwfl_end(dwfl); 177 goto out; 178 } 179 dso->dwfl = dwfl; 180 } 181 182 mod = dwfl_addrmodule(dwfl, pc); 183 if (!mod) { 184 pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1)); 185 goto out; 186 } 187 188 /* 189 * To work with split debug info files (eg: glibc), check both 190 * .eh_frame and .debug_frame sections of the ELF header. 191 */ 192 frame = get_eh_frame(mod, pc); 193 if (!frame) { 194 frame = get_dwarf_frame(mod, pc); 195 if (!frame) 196 goto out; 197 } 198 199 ra_regno = dwarf_frame_info(frame, &start, &end, &signalp); 200 if (ra_regno < 0) { 201 pr_debug("Return address register unavailable: %s\n", 202 dwarf_errmsg(-1)); 203 goto out; 204 } 205 206 rc = check_return_reg(ra_regno, frame); 207 208 out: 209 return rc; 210 } 211 212 /* 213 * The callchain saved by the kernel always includes the link register (LR). 214 * 215 * 0: PERF_CONTEXT_USER 216 * 1: Program counter (Next instruction pointer) 217 * 2: LR value 218 * 3: Caller's caller 219 * 4: ... 220 * 221 * The value in LR is only needed when it holds a return address. If the 222 * return address is on the stack, we should ignore the LR value. 223 * 224 * Further, when the return address is in the LR, if a new frame was just 225 * allocated but the LR was not saved into it, then the LR contains the 226 * caller, slot 4: contains the caller's caller and the contents of slot 3: 227 * (chain->ips[3]) is undefined and must be ignored. 228 * 229 * Use DWARF debug information to determine if any entries need to be skipped. 230 * 231 * Return: 232 * index: of callchain entry that needs to be ignored (if any) 233 * -1 if no entry needs to be ignored or in case of errors 234 */ 235 int arch_skip_callchain_idx(struct thread *thread, struct ip_callchain *chain) 236 { 237 struct addr_location al; 238 struct dso *dso = NULL; 239 int rc; 240 u64 ip; 241 u64 skip_slot = -1; 242 243 if (chain->nr < 3) 244 return skip_slot; 245 246 ip = chain->ips[2]; 247 248 thread__find_addr_location(thread, PERF_RECORD_MISC_USER, 249 MAP__FUNCTION, ip, &al); 250 251 if (al.map) 252 dso = al.map->dso; 253 254 if (!dso) { 255 pr_debug("%" PRIx64 " dso is NULL\n", ip); 256 return skip_slot; 257 } 258 259 rc = check_return_addr(dso, ip); 260 261 pr_debug("DSO %s, nr %" PRIx64 ", ip 0x%" PRIx64 "rc %d\n", 262 dso->long_name, chain->nr, ip, rc); 263 264 if (rc == 0) { 265 /* 266 * Return address on stack. Ignore LR value in callchain 267 */ 268 skip_slot = 2; 269 } else if (rc == 2) { 270 /* 271 * New frame allocated but return address still in LR. 272 * Ignore the caller's caller entry in callchain. 273 */ 274 skip_slot = 3; 275 } 276 return skip_slot; 277 } 278