linux/tools/perf/arch/powerpc/util/skip-callchain-idx.c
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   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 * Use DWARF Debug information to skip unnecessary callchain entries.
   4 *
   5 * Copyright (C) 2014 Sukadev Bhattiprolu, IBM Corporation.
   6 * Copyright (C) 2014 Ulrich Weigand, IBM Corporation.
   7 */
   8#include <inttypes.h>
   9#include <dwarf.h>
  10#include <elfutils/libdwfl.h>
  11
  12#include "util/thread.h"
  13#include "util/callchain.h"
  14#include "util/debug.h"
  15#include "util/dso.h"
  16#include "util/event.h" // struct ip_callchain
  17#include "util/map.h"
  18#include "util/symbol.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 */
  33static char *debuginfo_path;
  34
  35static 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 */
  46static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
  47{
  48        Dwarf_Op ops_mem[3];
  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 */
  97static 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 */
 122static 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 */
 152static 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
 215out:
 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 */
 242int 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