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