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(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 */ 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