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 19 /* 20 * When saving the callchain on Power, the kernel conservatively saves 21 * excess entries in the callchain. A few of these entries are needed 22 * in some cases but not others. If the unnecessary entries are not 23 * ignored, we end up with duplicate arcs in the call-graphs. Use 24 * DWARF debug information to skip over any unnecessary callchain 25 * entries. 26 * 27 * See function header for arch_adjust_callchain() below for more details. 28 * 29 * The libdwfl code in this file is based on code from elfutils 30 * (libdwfl/argp-std.c, libdwfl/tests/addrcfi.c, etc). 31 */ 32 static char *debuginfo_path; 33 34 static const Dwfl_Callbacks offline_callbacks = { 35 .debuginfo_path = &debuginfo_path, 36 .find_debuginfo = dwfl_standard_find_debuginfo, 37 .section_address = dwfl_offline_section_address, 38 }; 39 40 41 /* 42 * Use the DWARF expression for the Call-frame-address and determine 43 * if return address is in LR and if a new frame was allocated. 44 */ 45 static int check_return_reg(int ra_regno, Dwarf_Frame *frame) 46 { 47 Dwarf_Op ops_mem[2]; 48 Dwarf_Op dummy; 49 Dwarf_Op *ops = &dummy; 50 size_t nops; 51 int result; 52 53 result = dwarf_frame_register(frame, ra_regno, ops_mem, &ops, &nops); 54 if (result < 0) { 55 pr_debug("dwarf_frame_register() %s\n", dwarf_errmsg(-1)); 56 return -1; 57 } 58 59 /* 60 * Check if return address is on the stack. 61 */ 62 if (nops != 0 || ops != NULL) 63 return 0; 64 65 /* 66 * Return address is in LR. Check if a frame was allocated 67 * but not-yet used. 68 */ 69 result = dwarf_frame_cfa(frame, &ops, &nops); 70 if (result < 0) { 71 pr_debug("dwarf_frame_cfa() returns %d, %s\n", result, 72 dwarf_errmsg(-1)); 73 return -1; 74 } 75 76 /* 77 * If call frame address is in r1, no new frame was allocated. 78 */ 79 if (nops == 1 && ops[0].atom == DW_OP_bregx && ops[0].number == 1 && 80 ops[0].number2 == 0) 81 return 1; 82 83 /* 84 * A new frame was allocated but has not yet been used. 85 */ 86 return 2; 87 } 88 89 /* 90 * Get the DWARF frame from the .eh_frame section. 91 */ 92 static Dwarf_Frame *get_eh_frame(Dwfl_Module *mod, Dwarf_Addr pc) 93 { 94 int result; 95 Dwarf_Addr bias; 96 Dwarf_CFI *cfi; 97 Dwarf_Frame *frame; 98 99 cfi = dwfl_module_eh_cfi(mod, &bias); 100 if (!cfi) { 101 pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1)); 102 return NULL; 103 } 104 105 result = dwarf_cfi_addrframe(cfi, pc, &frame); 106 if (result) { 107 pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1)); 108 return NULL; 109 } 110 111 return frame; 112 } 113 114 /* 115 * Get the DWARF frame from the .debug_frame section. 116 */ 117 static Dwarf_Frame *get_dwarf_frame(Dwfl_Module *mod, Dwarf_Addr pc) 118 { 119 Dwarf_CFI *cfi; 120 Dwarf_Addr bias; 121 Dwarf_Frame *frame; 122 int result; 123 124 cfi = dwfl_module_dwarf_cfi(mod, &bias); 125 if (!cfi) { 126 pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1)); 127 return NULL; 128 } 129 130 result = dwarf_cfi_addrframe(cfi, pc, &frame); 131 if (result) { 132 pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1)); 133 return NULL; 134 } 135 136 return frame; 137 } 138 139 /* 140 * Return: 141 * 0 if return address for the program counter @pc is on stack 142 * 1 if return address is in LR and no new stack frame was allocated 143 * 2 if return address is in LR and a new frame was allocated (but not 144 * yet used) 145 * -1 in case of errors 146 */ 147 static int check_return_addr(const char *exec_file, Dwarf_Addr pc) 148 { 149 int rc = -1; 150 Dwfl *dwfl; 151 Dwfl_Module *mod; 152 Dwarf_Frame *frame; 153 int ra_regno; 154 Dwarf_Addr start = pc; 155 Dwarf_Addr end = pc; 156 bool signalp; 157 158 dwfl = dwfl_begin(&offline_callbacks); 159 if (!dwfl) { 160 pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1)); 161 return -1; 162 } 163 164 if (dwfl_report_offline(dwfl, "", exec_file, -1) == NULL) { 165 pr_debug("dwfl_report_offline() failed %s\n", dwarf_errmsg(-1)); 166 goto out; 167 } 168 169 mod = dwfl_addrmodule(dwfl, pc); 170 if (!mod) { 171 pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1)); 172 goto out; 173 } 174 175 /* 176 * To work with split debug info files (eg: glibc), check both 177 * .eh_frame and .debug_frame sections of the ELF header. 178 */ 179 frame = get_eh_frame(mod, pc); 180 if (!frame) { 181 frame = get_dwarf_frame(mod, pc); 182 if (!frame) 183 goto out; 184 } 185 186 ra_regno = dwarf_frame_info(frame, &start, &end, &signalp); 187 if (ra_regno < 0) { 188 pr_debug("Return address register unavailable: %s\n", 189 dwarf_errmsg(-1)); 190 goto out; 191 } 192 193 rc = check_return_reg(ra_regno, frame); 194 195 out: 196 dwfl_end(dwfl); 197 return rc; 198 } 199 200 /* 201 * The callchain saved by the kernel always includes the link register (LR). 202 * 203 * 0: PERF_CONTEXT_USER 204 * 1: Program counter (Next instruction pointer) 205 * 2: LR value 206 * 3: Caller's caller 207 * 4: ... 208 * 209 * The value in LR is only needed when it holds a return address. If the 210 * return address is on the stack, we should ignore the LR value. 211 * 212 * Further, when the return address is in the LR, if a new frame was just 213 * allocated but the LR was not saved into it, then the LR contains the 214 * caller, slot 4: contains the caller's caller and the contents of slot 3: 215 * (chain->ips[3]) is undefined and must be ignored. 216 * 217 * Use DWARF debug information to determine if any entries need to be skipped. 218 * 219 * Return: 220 * index: of callchain entry that needs to be ignored (if any) 221 * -1 if no entry needs to be ignored or in case of errors 222 */ 223 int arch_skip_callchain_idx(struct machine *machine, struct thread *thread, 224 struct ip_callchain *chain) 225 { 226 struct addr_location al; 227 struct dso *dso = NULL; 228 int rc; 229 u64 ip; 230 u64 skip_slot = -1; 231 232 if (chain->nr < 3) 233 return skip_slot; 234 235 ip = chain->ips[2]; 236 237 thread__find_addr_location(thread, machine, PERF_RECORD_MISC_USER, 238 MAP__FUNCTION, ip, &al); 239 240 if (al.map) 241 dso = al.map->dso; 242 243 if (!dso) { 244 pr_debug("%" PRIx64 " dso is NULL\n", ip); 245 return skip_slot; 246 } 247 248 rc = check_return_addr(dso->long_name, ip); 249 250 pr_debug("DSO %s, nr %" PRIx64 ", ip 0x%" PRIx64 "rc %d\n", 251 dso->long_name, chain->nr, ip, rc); 252 253 if (rc == 0) { 254 /* 255 * Return address on stack. Ignore LR value in callchain 256 */ 257 skip_slot = 2; 258 } else if (rc == 2) { 259 /* 260 * New frame allocated but return address still in LR. 261 * Ignore the caller's caller entry in callchain. 262 */ 263 skip_slot = 3; 264 } 265 return skip_slot; 266 } 267