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