// SPDX-License-Identifier: GPL-2.0-or-later
/*
 * Use DWARF Debug information to skip unnecessary callchain entries.
 *
 * Copyright (C) 2014 Sukadev Bhattiprolu, IBM Corporation.
 * Copyright (C) 2014 Ulrich Weigand, IBM Corporation.
 */
#include <inttypes.h>
#include <dwarf.h>
#include <elfutils/libdwfl.h>

#include "util/thread.h"
#include "util/callchain.h"
#include "util/debug.h"
#include "util/dso.h"
#include "util/event.h" // struct ip_callchain
#include "util/map.h"
#include "util/symbol.h"

/*
 * When saving the callchain on Power, the kernel conservatively saves
 * excess entries in the callchain. A few of these entries are needed
 * in some cases but not others. If the unnecessary entries are not
 * ignored, we end up with duplicate arcs in the call-graphs. Use
 * DWARF debug information to skip over any unnecessary callchain
 * entries.
 *
 * See function header for arch_adjust_callchain() below for more details.
 *
 * The libdwfl code in this file is based on code from elfutils
 * (libdwfl/argp-std.c, libdwfl/tests/addrcfi.c, etc).
 */
static char *debuginfo_path;

static const Dwfl_Callbacks offline_callbacks = {
        .debuginfo_path = &debuginfo_path,
        .find_debuginfo = dwfl_standard_find_debuginfo,
        .section_address = dwfl_offline_section_address,
};


/*
 * Use the DWARF expression for the Call-frame-address and determine
 * if return address is in LR and if a new frame was allocated.
 */
static int check_return_reg(int ra_regno, Dwarf_Frame *frame)
{
        Dwarf_Op ops_mem[2];
        Dwarf_Op dummy;
        Dwarf_Op *ops = &dummy;
        size_t nops;
        int result;

        result = dwarf_frame_register(frame, ra_regno, ops_mem, &ops, &nops);
        if (result < 0) {
                pr_debug("dwarf_frame_register() %s\n", dwarf_errmsg(-1));
                return -1;
        }

        /*
         * Check if return address is on the stack. If return address
         * is in a register (typically R0), it is yet to be saved on
         * the stack.
         */
        if ((nops != 0 || ops != NULL) &&
                !(nops == 1 && ops[0].atom == DW_OP_regx &&
                        ops[0].number2 == 0 && ops[0].offset == 0))
                return 0;

        /*
         * Return address is in LR. Check if a frame was allocated
         * but not-yet used.
         */
        result = dwarf_frame_cfa(frame, &ops, &nops);
        if (result < 0) {
                pr_debug("dwarf_frame_cfa() returns %d, %s\n", result,
                                        dwarf_errmsg(-1));
                return -1;
        }

        /*
         * If call frame address is in r1, no new frame was allocated.
         */
        if (nops == 1 && ops[0].atom == DW_OP_bregx && ops[0].number == 1 &&
                                ops[0].number2 == 0)
                return 1;

        /*
         * A new frame was allocated but has not yet been used.
         */
        return 2;
}

/*
 * Get the DWARF frame from the .eh_frame section.
 */
static Dwarf_Frame *get_eh_frame(Dwfl_Module *mod, Dwarf_Addr pc)
{
        int             result;
        Dwarf_Addr      bias;
        Dwarf_CFI       *cfi;
        Dwarf_Frame     *frame;

        cfi = dwfl_module_eh_cfi(mod, &bias);
        if (!cfi) {
                pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
                return NULL;
        }

        result = dwarf_cfi_addrframe(cfi, pc-bias, &frame);
        if (result) {
                pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
                return NULL;
        }

        return frame;
}

/*
 * Get the DWARF frame from the .debug_frame section.
 */
static Dwarf_Frame *get_dwarf_frame(Dwfl_Module *mod, Dwarf_Addr pc)
{
        Dwarf_CFI       *cfi;
        Dwarf_Addr      bias;
        Dwarf_Frame     *frame;
        int             result;

        cfi = dwfl_module_dwarf_cfi(mod, &bias);
        if (!cfi) {
                pr_debug("%s(): no CFI - %s\n", __func__, dwfl_errmsg(-1));
                return NULL;
        }

        result = dwarf_cfi_addrframe(cfi, pc-bias, &frame);
        if (result) {
                pr_debug("%s(): %s\n", __func__, dwfl_errmsg(-1));
                return NULL;
        }

        return frame;
}

/*
 * Return:
 *      0 if return address for the program counter @pc is on stack
 *      1 if return address is in LR and no new stack frame was allocated
 *      2 if return address is in LR and a new frame was allocated (but not
 *              yet used)
 *      -1 in case of errors
 */
static int check_return_addr(struct dso *dso, u64 map_start, Dwarf_Addr pc)
{
        int             rc = -1;
        Dwfl            *dwfl;
        Dwfl_Module     *mod;
        Dwarf_Frame     *frame;
        int             ra_regno;
        Dwarf_Addr      start = pc;
        Dwarf_Addr      end = pc;
        bool            signalp;
        const char      *exec_file = dso->long_name;

        dwfl = dso->dwfl;

        if (!dwfl) {
                dwfl = dwfl_begin(&offline_callbacks);
                if (!dwfl) {
                        pr_debug("dwfl_begin() failed: %s\n", dwarf_errmsg(-1));
                        return -1;
                }

                mod = dwfl_report_elf(dwfl, exec_file, exec_file, -1,
                                                map_start, false);
                if (!mod) {
                        pr_debug("dwfl_report_elf() failed %s\n",
                                                dwarf_errmsg(-1));
                        /*
                         * We normally cache the DWARF debug info and never
                         * call dwfl_end(). But to prevent fd leak, free in
                         * case of error.
                         */
                        dwfl_end(dwfl);
                        goto out;
                }
                dso->dwfl = dwfl;
        }

        mod = dwfl_addrmodule(dwfl, pc);
        if (!mod) {
                pr_debug("dwfl_addrmodule() failed, %s\n", dwarf_errmsg(-1));
                goto out;
        }

        /*
         * To work with split debug info files (eg: glibc), check both
         * .eh_frame and .debug_frame sections of the ELF header.
         */
        frame = get_eh_frame(mod, pc);
        if (!frame) {
                frame = get_dwarf_frame(mod, pc);
                if (!frame)
                        goto out;
        }

        ra_regno = dwarf_frame_info(frame, &start, &end, &signalp);
        if (ra_regno < 0) {
                pr_debug("Return address register unavailable: %s\n",
                                dwarf_errmsg(-1));
                goto out;
        }

        rc = check_return_reg(ra_regno, frame);

out:
        return rc;
}

/*
 * The callchain saved by the kernel always includes the link register (LR).
 *
 *      0:      PERF_CONTEXT_USER
 *      1:      Program counter (Next instruction pointer)
 *      2:      LR value
 *      3:      Caller's caller
 *      4:      ...
 *
 * The value in LR is only needed when it holds a return address. If the
 * return address is on the stack, we should ignore the LR value.
 *
 * Further, when the return address is in the LR, if a new frame was just
 * allocated but the LR was not saved into it, then the LR contains the
 * caller, slot 4: contains the caller's caller and the contents of slot 3:
 * (chain->ips[3]) is undefined and must be ignored.
 *
 * Use DWARF debug information to determine if any entries need to be skipped.
 *
 * Return:
 *      index:  of callchain entry that needs to be ignored (if any)
 *      -1      if no entry needs to be ignored or in case of errors
 */
int arch_skip_callchain_idx(struct thread *thread, struct ip_callchain *chain)
{
        struct addr_location al;
        struct dso *dso = NULL;
        int rc;
        u64 ip;
        u64 skip_slot = -1;

        if (!chain || chain->nr < 3)
                return skip_slot;

        ip = chain->ips[1];

        thread__find_symbol(thread, PERF_RECORD_MISC_USER, ip, &al);

        if (al.map)
                dso = al.map->dso;

        if (!dso) {
                pr_debug("%" PRIx64 " dso is NULL\n", ip);
                return skip_slot;
        }

        rc = check_return_addr(dso, al.map->start, ip);

        pr_debug("[DSO %s, sym %s, ip 0x%" PRIx64 "] rc %d\n",
                                dso->long_name, al.sym->name, ip, rc);

        if (rc == 0) {
                /*
                 * Return address on stack. Ignore LR value in callchain
                 */
                skip_slot = 2;
        } else if (rc == 2) {
                /*
                 * New frame allocated but return address still in LR.
                 * Ignore the caller's caller entry in callchain.
                 */
                skip_slot = 3;
        }
        return skip_slot;
}