backtrace.c

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/*
 * Copyright 2011 Tilera Corporation. All Rights Reserved.
 *
 *   This program is free software; you can redistribute it and/or
 *   modify it under the terms of the GNU General Public License
 *   as published by the Free Software Foundation, version 2.
 *
 *   This program is distributed in the hope that it will be useful, but
 *   WITHOUT ANY WARRANTY; without even the implied warranty of
 *   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, GOOD TITLE or
 *   NON INFRINGEMENT.  See the GNU General Public License for
 *   more details.
 */

#include <linux/kernel.h>
#include <linux/string.h>
#include <asm/byteorder.h>
#include <asm/backtrace.h>
#include <asm/tile-desc.h>
#include <arch/abi.h>

#ifdef __tilegx__
#define TILE_MAX_INSTRUCTIONS_PER_BUNDLE TILEGX_MAX_INSTRUCTIONS_PER_BUNDLE
#define tile_decoded_instruction tilegx_decoded_instruction
#define tile_mnemonic tilegx_mnemonic
#define parse_insn_tile parse_insn_tilegx
#define TILE_OPC_IRET TILEGX_OPC_IRET
#define TILE_OPC_ADDI TILEGX_OPC_ADDI
#define TILE_OPC_ADDLI TILEGX_OPC_ADDLI
#define TILE_OPC_INFO TILEGX_OPC_INFO
#define TILE_OPC_INFOL TILEGX_OPC_INFOL
#define TILE_OPC_JRP TILEGX_OPC_JRP
#define TILE_OPC_MOVE TILEGX_OPC_MOVE
#define OPCODE_STORE TILEGX_OPC_ST
typedef long long bt_int_reg_t;
#else
#define TILE_MAX_INSTRUCTIONS_PER_BUNDLE TILEPRO_MAX_INSTRUCTIONS_PER_BUNDLE
#define tile_decoded_instruction tilepro_decoded_instruction
#define tile_mnemonic tilepro_mnemonic
#define parse_insn_tile parse_insn_tilepro
#define TILE_OPC_IRET TILEPRO_OPC_IRET
#define TILE_OPC_ADDI TILEPRO_OPC_ADDI
#define TILE_OPC_ADDLI TILEPRO_OPC_ADDLI
#define TILE_OPC_INFO TILEPRO_OPC_INFO
#define TILE_OPC_INFOL TILEPRO_OPC_INFOL
#define TILE_OPC_JRP TILEPRO_OPC_JRP
#define TILE_OPC_MOVE TILEPRO_OPC_MOVE
#define OPCODE_STORE TILEPRO_OPC_SW
typedef int bt_int_reg_t;
#endif

/* A decoded bundle used for backtracer analysis. */
struct BacktraceBundle {
    tile_bundle_bits bits;
    int num_insns;
    struct tile_decoded_instruction
    insns[TILE_MAX_INSTRUCTIONS_PER_BUNDLE];
};


/* Locates an instruction inside the given bundle that
 * has the specified mnemonic, and whose first 'num_operands_to_match'
 * operands exactly match those in 'operand_values'.
 */
static const struct tile_decoded_instruction *find_matching_insn(
    const struct BacktraceBundle *bundle,
    tile_mnemonic mnemonic,
    const int *operand_values,
    int num_operands_to_match)
{
    int i, j;
    bool match;

    for (i = 0; i < bundle->num_insns; i++) {
        const struct tile_decoded_instruction *insn =
            &bundle->insns[i];

        if (insn->opcode->mnemonic != mnemonic)
            continue;

        match = true;
        for (j = 0; j < num_operands_to_match; j++) {
            if (operand_values[j] != insn->operand_values[j]) {
                match = false;
                break;
            }
        }

        if (match)
            return insn;
    }

    return NULL;
}

/* Does this bundle contain an 'iret' instruction? */
static inline bool bt_has_iret(const struct BacktraceBundle *bundle)
{
    return find_matching_insn(bundle, TILE_OPC_IRET, NULL, 0) != NULL;
}

/* Does this bundle contain an 'addi sp, sp, OFFSET' or
 * 'addli sp, sp, OFFSET' instruction, and if so, what is OFFSET?
 */
static bool bt_has_addi_sp(const struct BacktraceBundle *bundle, int *adjust)
{
    static const int vals[2] = { TREG_SP, TREG_SP };

    const struct tile_decoded_instruction *insn =
        find_matching_insn(bundle, TILE_OPC_ADDI, vals, 2);
    if (insn == NULL)
        insn = find_matching_insn(bundle, TILE_OPC_ADDLI, vals, 2);
#ifdef __tilegx__
    if (insn == NULL)
        insn = find_matching_insn(bundle, TILEGX_OPC_ADDXLI, vals, 2);
    if (insn == NULL)
        insn = find_matching_insn(bundle, TILEGX_OPC_ADDXI, vals, 2);
#endif
    if (insn == NULL)
        return false;

    *adjust = insn->operand_values[2];
    return true;
}

/* Does this bundle contain any 'info OP' or 'infol OP'
 * instruction, and if so, what are their OP?  Note that OP is interpreted
 * as an unsigned value by this code since that's what the caller wants.
 * Returns the number of info ops found.
 */
static int bt_get_info_ops(const struct BacktraceBundle *bundle,
        int operands[MAX_INFO_OPS_PER_BUNDLE])
{
    int num_ops = 0;
    int i;

    for (i = 0; i < bundle->num_insns; i++) {
        const struct tile_decoded_instruction *insn =
            &bundle->insns[i];

        if (insn->opcode->mnemonic == TILE_OPC_INFO ||
            insn->opcode->mnemonic == TILE_OPC_INFOL) {
            operands[num_ops++] = insn->operand_values[0];
        }
    }

    return num_ops;
}

/* Does this bundle contain a jrp instruction, and if so, to which
 * register is it jumping?
 */
static bool bt_has_jrp(const struct BacktraceBundle *bundle, int *target_reg)
{
    const struct tile_decoded_instruction *insn =
        find_matching_insn(bundle, TILE_OPC_JRP, NULL, 0);
    if (insn == NULL)
        return false;

    *target_reg = insn->operand_values[0];
    return true;
}

/* Does this bundle modify the specified register in any way? */
static bool bt_modifies_reg(const struct BacktraceBundle *bundle, int reg)
{
    int i, j;
    for (i = 0; i < bundle->num_insns; i++) {
        const struct tile_decoded_instruction *insn =
            &bundle->insns[i];

        if (insn->opcode->implicitly_written_register == reg)
            return true;

        for (j = 0; j < insn->opcode->num_operands; j++)
            if (insn->operands[j]->is_dest_reg &&
                insn->operand_values[j] == reg)
                return true;
    }

    return false;
}

/* Does this bundle modify sp? */
static inline bool bt_modifies_sp(const struct BacktraceBundle *bundle)
{
    return bt_modifies_reg(bundle, TREG_SP);
}

/* Does this bundle modify lr? */
static inline bool bt_modifies_lr(const struct BacktraceBundle *bundle)
{
    return bt_modifies_reg(bundle, TREG_LR);
}

/* Does this bundle contain the instruction 'move fp, sp'? */
static inline bool bt_has_move_r52_sp(const struct BacktraceBundle *bundle)
{
    static const int vals[2] = { 52, TREG_SP };
    return find_matching_insn(bundle, TILE_OPC_MOVE, vals, 2) != NULL;
}

/* Does this bundle contain a store of lr to sp? */
static inline bool bt_has_sw_sp_lr(const struct BacktraceBundle *bundle)
{
    static const int vals[2] = { TREG_SP, TREG_LR };
    return find_matching_insn(bundle, OPCODE_STORE, vals, 2) != NULL;
}

#ifdef __tilegx__
/* Track moveli values placed into registers. */
static inline void bt_update_moveli(const struct BacktraceBundle *bundle,
                    int moveli_args[])
{
    int i;
    for (i = 0; i < bundle->num_insns; i++) {
        const struct tile_decoded_instruction *insn =
            &bundle->insns[i];

        if (insn->opcode->mnemonic == TILEGX_OPC_MOVELI) {
            int reg = insn->operand_values[0];
            moveli_args[reg] = insn->operand_values[1];
        }
    }
}

/* Does this bundle contain an 'add sp, sp, reg' instruction
 * from a register that we saw a moveli into, and if so, what
 * is the value in the register?
 */
static bool bt_has_add_sp(const struct BacktraceBundle *bundle, int *adjust,
              int moveli_args[])
{
    static const int vals[2] = { TREG_SP, TREG_SP };

    const struct tile_decoded_instruction *insn =
        find_matching_insn(bundle, TILEGX_OPC_ADDX, vals, 2);
    if (insn) {
        int reg = insn->operand_values[2];
        if (moveli_args[reg]) {
            *adjust = moveli_args[reg];
            return true;
        }
    }
    return false;
}
#endif

/* Locates the caller's PC and SP for a program starting at the
 * given address.
 */
static void find_caller_pc_and_caller_sp(CallerLocation *location,
                     const unsigned long start_pc,
                     BacktraceMemoryReader read_memory_func,
                     void *read_memory_func_extra)
{
    /* Have we explicitly decided what the sp is,
     * rather than just the default?
     */
    bool sp_determined = false;

    /* Has any bundle seen so far modified lr? */
    bool lr_modified = false;

    /* Have we seen a move from sp to fp? */
    bool sp_moved_to_r52 = false;

    /* Have we seen a terminating bundle? */
    bool seen_terminating_bundle = false;

    /* Cut down on round-trip reading overhead by reading several
     * bundles at a time.
     */
    tile_bundle_bits prefetched_bundles[32];
    int num_bundles_prefetched = 0;
    int next_bundle = 0;
    unsigned long pc;

#ifdef __tilegx__
    /* Naively try to track moveli values to support addx for -m32. */
    int moveli_args[TILEGX_NUM_REGISTERS] = { 0 };
#endif

    /* Default to assuming that the caller's sp is the current sp.
     * This is necessary to handle the case where we start backtracing
     * right at the end of the epilog.
     */
    location->sp_location = SP_LOC_OFFSET;
    location->sp_offset = 0;

    /* Default to having no idea where the caller PC is. */
    location->pc_location = PC_LOC_UNKNOWN;

    /* Don't even try if the PC is not aligned. */
    if (start_pc % TILE_BUNDLE_ALIGNMENT_IN_BYTES != 0)
        return;

    for (pc = start_pc;; pc += sizeof(tile_bundle_bits)) {

        struct BacktraceBundle bundle;
        int num_info_ops, info_operands[MAX_INFO_OPS_PER_BUNDLE];
        int one_ago, jrp_reg;
        bool has_jrp;

        if (next_bundle >= num_bundles_prefetched) {
            /* Prefetch some bytes, but don't cross a page
             * boundary since that might cause a read failure we
             * don't care about if we only need the first few
             * bytes. Note: we don't care what the actual page
             * size is; using the minimum possible page size will
             * prevent any problems.
             */
            unsigned int bytes_to_prefetch = 4096 - (pc & 4095);
            if (bytes_to_prefetch > sizeof prefetched_bundles)
                bytes_to_prefetch = sizeof prefetched_bundles;

            if (!read_memory_func(prefetched_bundles, pc,
                          bytes_to_prefetch,
                          read_memory_func_extra)) {
                if (pc == start_pc) {
                    /* The program probably called a bad
                     * address, such as a NULL pointer.
                     * So treat this as if we are at the
                     * start of the function prolog so the
                     * backtrace will show how we got here.
                     */
                    location->pc_location = PC_LOC_IN_LR;
                    return;
                }

                /* Unreadable address. Give up. */
                break;
            }

            next_bundle = 0;
            num_bundles_prefetched =
                bytes_to_prefetch / sizeof(tile_bundle_bits);
        }

        /*
         * Decode the next bundle.
         * TILE always stores instruction bundles in little-endian
         * mode, even when the chip is running in big-endian mode.
         */
        bundle.bits = le64_to_cpu(prefetched_bundles[next_bundle++]);
        bundle.num_insns =
            parse_insn_tile(bundle.bits, pc, bundle.insns);
        num_info_ops = bt_get_info_ops(&bundle, info_operands);

        /* First look at any one_ago info ops if they are interesting,
         * since they should shadow any non-one-ago info ops.
         */
        for (one_ago = (pc != start_pc) ? 1 : 0;
             one_ago >= 0; one_ago--) {
            int i;
            for (i = 0; i < num_info_ops; i++) {
                int info_operand = info_operands[i];
                if (info_operand < CALLER_UNKNOWN_BASE) {
                    /* Weird; reserved value, ignore it. */
                    continue;
                }

                /* Skip info ops which are not in the
                 * "one_ago" mode we want right now.
                 */
                if (((info_operand & ONE_BUNDLE_AGO_FLAG) != 0)
                    != (one_ago != 0))
                    continue;

                /* Clear the flag to make later checking
                 * easier. */
                info_operand &= ~ONE_BUNDLE_AGO_FLAG;

                /* Default to looking at PC_IN_LR_FLAG. */
                if (info_operand & PC_IN_LR_FLAG)
                    location->pc_location =
                        PC_LOC_IN_LR;
                else
                    location->pc_location =
                        PC_LOC_ON_STACK;

                switch (info_operand) {
                case CALLER_UNKNOWN_BASE:
                    location->pc_location = PC_LOC_UNKNOWN;
                    location->sp_location = SP_LOC_UNKNOWN;
                    return;

                case CALLER_SP_IN_R52_BASE:
                case CALLER_SP_IN_R52_BASE | PC_IN_LR_FLAG:
                    location->sp_location = SP_LOC_IN_R52;
                    return;

                default:
                {
                    const unsigned int val = info_operand
                        - CALLER_SP_OFFSET_BASE;
                    const unsigned int sp_offset =
                        (val >> NUM_INFO_OP_FLAGS) * 8;
                    if (sp_offset < 32768) {
                        /* This is a properly encoded
                         * SP offset. */
                        location->sp_location =
                            SP_LOC_OFFSET;
                        location->sp_offset =
                            sp_offset;
                        return;
                    } else {
                        /* This looked like an SP
                         * offset, but it's outside
                         * the legal range, so this
                         * must be an unrecognized
                         * info operand.  Ignore it.
                         */
                    }
                }
                break;
                }
            }
        }

        if (seen_terminating_bundle) {
            /* We saw a terminating bundle during the previous
             * iteration, so we were only looking for an info op.
             */
            break;
        }

        if (bundle.bits == 0) {
            /* Wacky terminating bundle. Stop looping, and hope
             * we've already seen enough to find the caller.
             */
            break;
        }

        /*
         * Try to determine caller's SP.
         */

        if (!sp_determined) {
            int adjust;
            if (bt_has_addi_sp(&bundle, &adjust)
#ifdef __tilegx__
                || bt_has_add_sp(&bundle, &adjust, moveli_args)
#endif
                ) {
                location->sp_location = SP_LOC_OFFSET;

                if (adjust <= 0) {
                    /* We are in prolog about to adjust
                     * SP. */
                    location->sp_offset = 0;
                } else {
                    /* We are in epilog restoring SP. */
                    location->sp_offset = adjust;
                }

                sp_determined = true;
            } else {
                if (bt_has_move_r52_sp(&bundle)) {
                    /* Maybe in prolog, creating an
                     * alloca-style frame.  But maybe in
                     * the middle of a fixed-size frame
                     * clobbering r52 with SP.
                     */
                    sp_moved_to_r52 = true;
                }

                if (bt_modifies_sp(&bundle)) {
                    if (sp_moved_to_r52) {
                        /* We saw SP get saved into
                         * r52 earlier (or now), which
                         * must have been in the
                         * prolog, so we now know that
                         * SP is still holding the
                         * caller's sp value.
                         */
                        location->sp_location =
                            SP_LOC_OFFSET;
                        location->sp_offset = 0;
                    } else {
                        /* Someone must have saved
                         * aside the caller's SP value
                         * into r52, so r52 holds the
                         * current value.
                         */
                        location->sp_location =
                            SP_LOC_IN_R52;
                    }
                    sp_determined = true;
                }
            }

#ifdef __tilegx__
            /* Track moveli arguments for -m32 mode. */
            bt_update_moveli(&bundle, moveli_args);
#endif
        }

        if (bt_has_iret(&bundle)) {
            /* This is a terminating bundle. */
            seen_terminating_bundle = true;
            continue;
        }

        /*
         * Try to determine caller's PC.
         */

        jrp_reg = -1;
        has_jrp = bt_has_jrp(&bundle, &jrp_reg);
        if (has_jrp)
            seen_terminating_bundle = true;

        if (location->pc_location == PC_LOC_UNKNOWN) {
            if (has_jrp) {
                if (jrp_reg == TREG_LR && !lr_modified) {
                    /* Looks like a leaf function, or else
                     * lr is already restored. */
                    location->pc_location =
                        PC_LOC_IN_LR;
                } else {
                    location->pc_location =
                        PC_LOC_ON_STACK;
                }
            } else if (bt_has_sw_sp_lr(&bundle)) {
                /* In prolog, spilling initial lr to stack. */
                location->pc_location = PC_LOC_IN_LR;
            } else if (bt_modifies_lr(&bundle)) {
                lr_modified = true;
            }
        }
    }
}

/* Initializes a backtracer to start from the given location.
 *
 * If the frame pointer cannot be determined it is set to -1.
 *
 * state: The state to be filled in.
 * read_memory_func: A callback that reads memory.
 * read_memory_func_extra: An arbitrary argument to read_memory_func.
 * pc: The current PC.
 * lr: The current value of the 'lr' register.
 * sp: The current value of the 'sp' register.
 * r52: The current value of the 'r52' register.
 */
void backtrace_init(BacktraceIterator *state,
            BacktraceMemoryReader read_memory_func,
            void *read_memory_func_extra,
            unsigned long pc, unsigned long lr,
            unsigned long sp, unsigned long r52)
{
    CallerLocation location;
    unsigned long fp, initial_frame_caller_pc;

    /* Find out where we are in the initial frame. */
    find_caller_pc_and_caller_sp(&location, pc,
                     read_memory_func, read_memory_func_extra);

    switch (location.sp_location) {
    case SP_LOC_UNKNOWN:
        /* Give up. */
        fp = -1;
        break;

    case SP_LOC_IN_R52:
        fp = r52;
        break;

    case SP_LOC_OFFSET:
        fp = sp + location.sp_offset;
        break;

    default:
        /* Give up. */
        fp = -1;
        break;
    }

    /* If the frame pointer is not aligned to the basic word size
     * something terrible happened and we should mark it as invalid.
     */
    if (fp % sizeof(bt_int_reg_t) != 0)
        fp = -1;

    /* -1 means "don't know initial_frame_caller_pc". */
    initial_frame_caller_pc = -1;

    switch (location.pc_location) {
    case PC_LOC_UNKNOWN:
        /* Give up. */
        fp = -1;
        break;

    case PC_LOC_IN_LR:
        if (lr == 0 || lr % TILE_BUNDLE_ALIGNMENT_IN_BYTES != 0) {
            /* Give up. */
            fp = -1;
        } else {
            initial_frame_caller_pc = lr;
        }
        break;

    case PC_LOC_ON_STACK:
        /* Leave initial_frame_caller_pc as -1,
         * meaning check the stack.
         */
        break;

    default:
        /* Give up. */
        fp = -1;
        break;
    }

    state->pc = pc;
    state->sp = sp;
    state->fp = fp;
    state->initial_frame_caller_pc = initial_frame_caller_pc;
    state->read_memory_func = read_memory_func;
    state->read_memory_func_extra = read_memory_func_extra;
}

/* Handle the case where the register holds more bits than the VA. */
static bool valid_addr_reg(bt_int_reg_t reg)
{
    return ((unsigned long)reg == reg);
}

/* Advances the backtracing state to the calling frame, returning
 * true iff successful.
 */
bool backtrace_next(BacktraceIterator *state)
{
    unsigned long next_fp, next_pc;
    bt_int_reg_t next_frame[2];

    if (state->fp == -1) {
        /* No parent frame. */
        return false;
    }

    /* Try to read the frame linkage data chaining to the next function. */
    if (!state->read_memory_func(&next_frame, state->fp, sizeof next_frame,
                     state->read_memory_func_extra)) {
        return false;
    }

    next_fp = next_frame[1];
    if (!valid_addr_reg(next_frame[1]) ||
        next_fp % sizeof(bt_int_reg_t) != 0) {
        /* Caller's frame pointer is suspect, so give up. */
        return false;
    }

    if (state->initial_frame_caller_pc != -1) {
        /* We must be in the initial stack frame and already know the
         * caller PC.
         */
        next_pc = state->initial_frame_caller_pc;

        /* Force reading stack next time, in case we were in the
         * initial frame.  We don't do this above just to paranoidly
         * avoid changing the struct at all when we return false.
         */
        state->initial_frame_caller_pc = -1;
    } else {
        /* Get the caller PC from the frame linkage area. */
        next_pc = next_frame[0];
        if (!valid_addr_reg(next_frame[0]) || next_pc == 0 ||
            next_pc % TILE_BUNDLE_ALIGNMENT_IN_BYTES != 0) {
            /* The PC is suspect, so give up. */
            return false;
        }
    }

    /* Update state to become the caller's stack frame. */
    state->pc = next_pc;
    state->sp = state->fp;
    state->fp = next_fp;

    return true;
}