sh: Set the cfa_offset to 0 if we see a DW_CFA_def_cfa_register op

[linux-2.6.git] / arch / sh / kernel / dwarf.c

/*
 * Copyright (C) 2009 Matt Fleming <matt@console-pimps.org>
 *
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file "COPYING" in the main directory of this archive
 * for more details.
 *
 * This is an implementation of a DWARF unwinder. Its main purpose is
 * for generating stacktrace information. Based on the DWARF 3
 * specification from http://www.dwarfstd.org.
 *
 * TODO:
 *      - DWARF64 doesn't work.
 */

/* #define DEBUG */
#include <linux/kernel.h>
#include <linux/io.h>
#include <linux/list.h>
#include <linux/mm.h>
#include <asm/dwarf.h>
#include <asm/unwinder.h>
#include <asm/sections.h>
#include <asm/unaligned.h>
#include <asm/dwarf.h>
#include <asm/stacktrace.h>

static LIST_HEAD(dwarf_cie_list);
DEFINE_SPINLOCK(dwarf_cie_lock);

static LIST_HEAD(dwarf_fde_list);
DEFINE_SPINLOCK(dwarf_fde_lock);

static struct dwarf_cie *cached_cie;

/*
 * Figure out whether we need to allocate some dwarf registers. If dwarf
 * registers have already been allocated then we may need to realloc
 * them. "reg" is a register number that we need to be able to access
 * after this call.
 *
 * Register numbers start at zero, therefore we need to allocate space
 * for "reg" + 1 registers.
 */
static void dwarf_frame_alloc_regs(struct dwarf_frame *frame,
                                   unsigned int reg)
{
        struct dwarf_reg *regs;
        unsigned int num_regs = reg + 1;
        size_t new_size;
        size_t old_size;

        new_size = num_regs * sizeof(*regs);
        old_size = frame->num_regs * sizeof(*regs);

        /* Fast path: don't allocate any regs if we've already got enough. */
        if (frame->num_regs >= num_regs)
                return;

        regs = kzalloc(new_size, GFP_ATOMIC);
        if (!regs) {
                printk(KERN_WARNING "Unable to allocate DWARF registers\n");
                /*
                 * Let's just bomb hard here, we have no way to
                 * gracefully recover.
                 */
                BUG();
        }

        if (frame->regs) {
                memcpy(regs, frame->regs, old_size);
                kfree(frame->regs);
        }

        frame->regs = regs;
        frame->num_regs = num_regs;
}

/**
 *      dwarf_read_addr - read dwarf data
 *      @src: source address of data
 *      @dst: destination address to store the data to
 *
 *      Read 'n' bytes from @src, where 'n' is the size of an address on
 *      the native machine. We return the number of bytes read, which
 *      should always be 'n'. We also have to be careful when reading
 *      from @src and writing to @dst, because they can be arbitrarily
 *      aligned. Return 'n' - the number of bytes read.
 */
static inline int dwarf_read_addr(unsigned long *src, unsigned long *dst)
{
        u32 val = get_unaligned(src);
        put_unaligned(val, dst);
        return sizeof(unsigned long *);
}

/**
 *      dwarf_read_uleb128 - read unsigned LEB128 data
 *      @addr: the address where the ULEB128 data is stored
 *      @ret: address to store the result
 *
 *      Decode an unsigned LEB128 encoded datum. The algorithm is taken
 *      from Appendix C of the DWARF 3 spec. For information on the
 *      encodings refer to section "7.6 - Variable Length Data". Return
 *      the number of bytes read.
 */
static inline unsigned long dwarf_read_uleb128(char *addr, unsigned int *ret)
{
        unsigned int result;
        unsigned char byte;
        int shift, count;

        result = 0;
        shift = 0;
        count = 0;

        while (1) {
                byte = __raw_readb(addr);
                addr++;
                count++;

                result |= (byte & 0x7f) << shift;
                shift += 7;

                if (!(byte & 0x80))
                        break;
        }

        *ret = result;

        return count;
}

/**
 *      dwarf_read_leb128 - read signed LEB128 data
 *      @addr: the address of the LEB128 encoded data
 *      @ret: address to store the result
 *
 *      Decode signed LEB128 data. The algorithm is taken from Appendix
 *      C of the DWARF 3 spec. Return the number of bytes read.
 */
static inline unsigned long dwarf_read_leb128(char *addr, int *ret)
{
        unsigned char byte;
        int result, shift;
        int num_bits;
        int count;

        result = 0;
        shift = 0;
        count = 0;

        while (1) {
                byte = __raw_readb(addr);
                addr++;
                result |= (byte & 0x7f) << shift;
                shift += 7;
                count++;

                if (!(byte & 0x80))
                        break;
        }

        /* The number of bits in a signed integer. */
        num_bits = 8 * sizeof(result);

        if ((shift < num_bits) && (byte & 0x40))
                result |= (-1 << shift);

        *ret = result;

        return count;
}

/**
 *      dwarf_read_encoded_value - return the decoded value at @addr
 *      @addr: the address of the encoded value
 *      @val: where to write the decoded value
 *      @encoding: the encoding with which we can decode @addr
 *
 *      GCC emits encoded address in the .eh_frame FDE entries. Decode
 *      the value at @addr using @encoding. The decoded value is written
 *      to @val and the number of bytes read is returned.
 */
static int dwarf_read_encoded_value(char *addr, unsigned long *val,
                                    char encoding)
{
        unsigned long decoded_addr = 0;
        int count = 0;

        switch (encoding & 0x70) {
        case DW_EH_PE_absptr:
                break;
        case DW_EH_PE_pcrel:
                decoded_addr = (unsigned long)addr;
                break;
        default:
                pr_debug("encoding=0x%x\n", (encoding & 0x70));
                BUG();
        }

        if ((encoding & 0x07) == 0x00)
                encoding |= DW_EH_PE_udata4;

        switch (encoding & 0x0f) {
        case DW_EH_PE_sdata4:
        case DW_EH_PE_udata4:
                count += 4;
                decoded_addr += get_unaligned((u32 *)addr);
                __raw_writel(decoded_addr, val);
                break;
        default:
                pr_debug("encoding=0x%x\n", encoding);
                BUG();
        }

        return count;
}

/**
 *      dwarf_entry_len - return the length of an FDE or CIE
 *      @addr: the address of the entry
 *      @len: the length of the entry
 *
 *      Read the initial_length field of the entry and store the size of
 *      the entry in @len. We return the number of bytes read. Return a
 *      count of 0 on error.
 */
static inline int dwarf_entry_len(char *addr, unsigned long *len)
{
        u32 initial_len;
        int count;

        initial_len = get_unaligned((u32 *)addr);
        count = 4;

        /*
         * An initial length field value in the range DW_LEN_EXT_LO -
         * DW_LEN_EXT_HI indicates an extension, and should not be
         * interpreted as a length. The only extension that we currently
         * understand is the use of DWARF64 addresses.
         */
        if (initial_len >= DW_EXT_LO && initial_len <= DW_EXT_HI) {
                /*
                 * The 64-bit length field immediately follows the
                 * compulsory 32-bit length field.
                 */
                if (initial_len == DW_EXT_DWARF64) {
                        *len = get_unaligned((u64 *)addr + 4);
                        count = 12;
                } else {
                        printk(KERN_WARNING "Unknown DWARF extension\n");
                        count = 0;
                }
        } else
                *len = initial_len;

        return count;
}

/**
 *      dwarf_lookup_cie - locate the cie
 *      @cie_ptr: pointer to help with lookup
 */
static struct dwarf_cie *dwarf_lookup_cie(unsigned long cie_ptr)
{
        struct dwarf_cie *cie, *n;
        unsigned long flags;

        spin_lock_irqsave(&dwarf_cie_lock, flags);

        /*
         * We've cached the last CIE we looked up because chances are
         * that the FDE wants this CIE.
         */
        if (cached_cie && cached_cie->cie_pointer == cie_ptr) {
                cie = cached_cie;
                goto out;
        }

        list_for_each_entry_safe(cie, n, &dwarf_cie_list, link) {
                if (cie->cie_pointer == cie_ptr) {
                        cached_cie = cie;
                        break;
                }
        }

        /* Couldn't find the entry in the list. */
        if (&cie->link == &dwarf_cie_list)
                cie = NULL;
out:
        spin_unlock_irqrestore(&dwarf_cie_lock, flags);
        return cie;
}

/**
 *      dwarf_lookup_fde - locate the FDE that covers pc
 *      @pc: the program counter
 */
struct dwarf_fde *dwarf_lookup_fde(unsigned long pc)
{
        unsigned long flags;
        struct dwarf_fde *fde, *n;

        spin_lock_irqsave(&dwarf_fde_lock, flags);
        list_for_each_entry_safe(fde, n, &dwarf_fde_list, link) {
                unsigned long start, end;

                start = fde->initial_location;
                end = fde->initial_location + fde->address_range;

                if (pc >= start && pc < end)
                        break;
        }

        /* Couldn't find the entry in the list. */
        if (&fde->link == &dwarf_fde_list)
                fde = NULL;

        spin_unlock_irqrestore(&dwarf_fde_lock, flags);

        return fde;
}

/**
 *      dwarf_cfa_execute_insns - execute instructions to calculate a CFA
 *      @insn_start: address of the first instruction
 *      @insn_end: address of the last instruction
 *      @cie: the CIE for this function
 *      @fde: the FDE for this function
 *      @frame: the instructions calculate the CFA for this frame
 *      @pc: the program counter of the address we're interested in
 *      @define_ra: keep executing insns until the return addr reg is defined?
 *
 *      Execute the Call Frame instruction sequence starting at
 *      @insn_start and ending at @insn_end. The instructions describe
 *      how to calculate the Canonical Frame Address of a stackframe.
 *      Store the results in @frame.
 */
static int dwarf_cfa_execute_insns(unsigned char *insn_start,
                                   unsigned char *insn_end,
                                   struct dwarf_cie *cie,
                                   struct dwarf_fde *fde,
                                   struct dwarf_frame *frame,
                                   unsigned long pc,
                                   bool define_ra)
{
        unsigned char insn;
        unsigned char *current_insn;
        unsigned int count, delta, reg, expr_len, offset;
        bool seen_ra_reg;

        current_insn = insn_start;

        /*
         * If we're executing instructions for the dwarf_unwind_stack()
         * FDE we need to keep executing instructions until the value of
         * DWARF_ARCH_RA_REG is defined. See the comment in
         * dwarf_unwind_stack() for more details.
         */
        if (define_ra)
                seen_ra_reg = false;
        else
                seen_ra_reg = true;

        while (current_insn < insn_end && (frame->pc <= pc || !seen_ra_reg) ) {
                insn = __raw_readb(current_insn++);

                if (!seen_ra_reg) {
                        if (frame->num_regs >= DWARF_ARCH_RA_REG &&
                            frame->regs[DWARF_ARCH_RA_REG].flags)
                                seen_ra_reg = true;
                }

                /*
                 * Firstly, handle the opcodes that embed their operands
                 * in the instructions.
                 */
                switch (DW_CFA_opcode(insn)) {
                case DW_CFA_advance_loc:
                        delta = DW_CFA_operand(insn);
                        delta *= cie->code_alignment_factor;
                        frame->pc += delta;
                        continue;
                        /* NOTREACHED */
                case DW_CFA_offset:
                        reg = DW_CFA_operand(insn);
                        count = dwarf_read_uleb128(current_insn, &offset);
                        current_insn += count;
                        offset *= cie->data_alignment_factor;
                        dwarf_frame_alloc_regs(frame, reg);
                        frame->regs[reg].addr = offset;
                        frame->regs[reg].flags |= DWARF_REG_OFFSET;
                        continue;
                        /* NOTREACHED */
                case DW_CFA_restore:
                        reg = DW_CFA_operand(insn);
                        continue;
                        /* NOTREACHED */
                }

                /*
                 * Secondly, handle the opcodes that don't embed their
                 * operands in the instruction.
                 */
                switch (insn) {
                case DW_CFA_nop:
                        continue;
                case DW_CFA_advance_loc1:
                        delta = *current_insn++;
                        frame->pc += delta * cie->code_alignment_factor;
                        break;
                case DW_CFA_advance_loc2:
                        delta = get_unaligned((u16 *)current_insn);
                        current_insn += 2;
                        frame->pc += delta * cie->code_alignment_factor;
                        break;
                case DW_CFA_advance_loc4:
                        delta = get_unaligned((u32 *)current_insn);
                        current_insn += 4;
                        frame->pc += delta * cie->code_alignment_factor;
                        break;
                case DW_CFA_offset_extended:
                        count = dwarf_read_uleb128(current_insn, &reg);
                        current_insn += count;
                        count = dwarf_read_uleb128(current_insn, &offset);
                        current_insn += count;
                        offset *= cie->data_alignment_factor;
                        break;
                case DW_CFA_restore_extended:
                        count = dwarf_read_uleb128(current_insn, &reg);
                        current_insn += count;
                        break;
                case DW_CFA_undefined:
                        count = dwarf_read_uleb128(current_insn, &reg);
                        current_insn += count;
                        break;
                case DW_CFA_def_cfa:
                        count = dwarf_read_uleb128(current_insn,
                                                   &frame->cfa_register);
                        current_insn += count;
                        count = dwarf_read_uleb128(current_insn,
                                                   &frame->cfa_offset);
                        current_insn += count;

                        frame->flags |= DWARF_FRAME_CFA_REG_OFFSET;
                        break;
                case DW_CFA_def_cfa_register:
                        count = dwarf_read_uleb128(current_insn,
                                                   &frame->cfa_register);
                        current_insn += count;
                        frame->cfa_offset = 0;
                        frame->flags |= DWARF_FRAME_CFA_REG_OFFSET;
                        break;
                case DW_CFA_def_cfa_offset:
                        count = dwarf_read_uleb128(current_insn, &offset);
                        current_insn += count;
                        frame->cfa_offset = offset;
                        break;
                case DW_CFA_def_cfa_expression:
                        count = dwarf_read_uleb128(current_insn, &expr_len);
                        current_insn += count;

                        frame->cfa_expr = current_insn;
                        frame->cfa_expr_len = expr_len;
                        current_insn += expr_len;

                        frame->flags |= DWARF_FRAME_CFA_REG_EXP;
                        break;
                case DW_CFA_offset_extended_sf:
                        count = dwarf_read_uleb128(current_insn, &reg);
                        current_insn += count;
                        count = dwarf_read_leb128(current_insn, &offset);
                        current_insn += count;
                        offset *= cie->data_alignment_factor;
                        dwarf_frame_alloc_regs(frame, reg);
                        frame->regs[reg].flags |= DWARF_REG_OFFSET;
                        frame->regs[reg].addr = offset;
                        break;
                case DW_CFA_val_offset:
                        count = dwarf_read_uleb128(current_insn, &reg);
                        current_insn += count;
                        count = dwarf_read_leb128(current_insn, &offset);
                        offset *= cie->data_alignment_factor;
                        frame->regs[reg].flags |= DWARF_REG_OFFSET;
                        frame->regs[reg].addr = offset;
                        break;
                default:
                        pr_debug("unhandled DWARF instruction 0x%x\n", insn);
                        break;
                }
        }

        return 0;
}

/**
 *      dwarf_unwind_stack - recursively unwind the stack
 *      @pc: address of the function to unwind
 *      @prev: struct dwarf_frame of the previous stackframe on the callstack
 *
 *      Return a struct dwarf_frame representing the most recent frame
 *      on the callstack. Each of the lower (older) stack frames are
 *      linked via the "prev" member.
 */
struct dwarf_frame *dwarf_unwind_stack(unsigned long pc,
                                       struct dwarf_frame *prev)
{
        struct dwarf_frame *frame;
        struct dwarf_cie *cie;
        struct dwarf_fde *fde;
        unsigned long addr;
        int i, offset;
        bool define_ra = false;

        /*
         * If this is the first invocation of this recursive function we
         * need get the contents of a physical register to get the CFA
         * in order to begin the virtual unwinding of the stack.
         *
         * Setting "define_ra" to true indictates that we want
         * dwarf_cfa_execute_insns() to continue executing instructions
         * until we know how to calculate the value of DWARF_ARCH_RA_REG
         * (which we need in order to kick off the whole unwinding
         * process).
         *
         * NOTE: the return address is guaranteed to be setup by the
         * time this function makes its first function call.
         */
        if (!pc && !prev) {
                pc = (unsigned long)&dwarf_unwind_stack;
                define_ra = true;
        }

        frame = kzalloc(sizeof(*frame), GFP_ATOMIC);
        if (!frame)
                return NULL;

        frame->prev = prev;

        fde = dwarf_lookup_fde(pc);
        if (!fde) {
                /*
                 * This is our normal exit path - the one that stops the
                 * recursion. There's two reasons why we might exit
                 * here,
                 *
                 *      a) pc has no asscociated DWARF frame info and so
                 *      we don't know how to unwind this frame. This is
                 *      usually the case when we're trying to unwind a
                 *      frame that was called from some assembly code
                 *      that has no DWARF info, e.g. syscalls.
                 *
                 *      b) the DEBUG info for pc is bogus. There's
                 *      really no way to distinguish this case from the
                 *      case above, which sucks because we could print a
                 *      warning here.
                 */
                return NULL;
        }

        cie = dwarf_lookup_cie(fde->cie_pointer);

        frame->pc = fde->initial_location;

        /* CIE initial instructions */
        dwarf_cfa_execute_insns(cie->initial_instructions,
                                cie->instructions_end, cie, fde,
                                frame, pc, false);

        /* FDE instructions */
        dwarf_cfa_execute_insns(fde->instructions, fde->end, cie,
                                fde, frame, pc, define_ra);

        /* Calculate the CFA */
        switch (frame->flags) {
        case DWARF_FRAME_CFA_REG_OFFSET:
                if (prev) {
                        BUG_ON(!prev->regs[frame->cfa_register].flags);

                        addr = prev->cfa;
                        addr += prev->regs[frame->cfa_register].addr;
                        frame->cfa = __raw_readl(addr);

                } else {
                        /*
                         * Again, this is the first invocation of this
                         * recurisve function. We need to physically
                         * read the contents of a register in order to
                         * get the Canonical Frame Address for this
                         * function.
                         */
                        frame->cfa = dwarf_read_arch_reg(frame->cfa_register);
                }

                frame->cfa += frame->cfa_offset;
                break;
        default:
                BUG();
        }

        /* If we haven't seen the return address reg, we're screwed. */
        BUG_ON(!frame->regs[DWARF_ARCH_RA_REG].flags);

        for (i = 0; i <= frame->num_regs; i++) {
                struct dwarf_reg *reg = &frame->regs[i];

                if (!reg->flags)
                        continue;

                offset = reg->addr;
                offset += frame->cfa;
        }

        addr = frame->cfa + frame->regs[DWARF_ARCH_RA_REG].addr;
        frame->return_addr = __raw_readl(addr);

        frame->next = dwarf_unwind_stack(frame->return_addr, frame);
        return frame;
}

static int dwarf_parse_cie(void *entry, void *p, unsigned long len,
                           unsigned char *end)
{
        struct dwarf_cie *cie;
        unsigned long flags;
        int count;

        cie = kzalloc(sizeof(*cie), GFP_KERNEL);
        if (!cie)
                return -ENOMEM;

        cie->length = len;

        /*
         * Record the offset into the .eh_frame section
         * for this CIE. It allows this CIE to be
         * quickly and easily looked up from the
         * corresponding FDE.
         */
        cie->cie_pointer = (unsigned long)entry;

        cie->version = *(char *)p++;
        BUG_ON(cie->version != 1);

        cie->augmentation = p;
        p += strlen(cie->augmentation) + 1;

        count = dwarf_read_uleb128(p, &cie->code_alignment_factor);
        p += count;

        count = dwarf_read_leb128(p, &cie->data_alignment_factor);
        p += count;

        /*
         * Which column in the rule table contains the
         * return address?
         */
        if (cie->version == 1) {
                cie->return_address_reg = __raw_readb(p);
                p++;
        } else {
                count = dwarf_read_uleb128(p, &cie->return_address_reg);
                p += count;
        }

        if (cie->augmentation[0] == 'z') {
                unsigned int length, count;
                cie->flags |= DWARF_CIE_Z_AUGMENTATION;

                count = dwarf_read_uleb128(p, &length);
                p += count;

                BUG_ON((unsigned char *)p > end);

                cie->initial_instructions = p + length;
                cie->augmentation++;
        }

        while (*cie->augmentation) {
                /*
                 * "L" indicates a byte showing how the
                 * LSDA pointer is encoded. Skip it.
                 */
                if (*cie->augmentation == 'L') {
                        p++;
                        cie->augmentation++;
                } else if (*cie->augmentation == 'R') {
                        /*
                         * "R" indicates a byte showing
                         * how FDE addresses are
                         * encoded.
                         */
                        cie->encoding = *(char *)p++;
                        cie->augmentation++;
                } else if (*cie->augmentation == 'P') {
                        /*
                         * "R" indicates a personality
                         * routine in the CIE
                         * augmentation.
                         */
                        BUG();
                } else if (*cie->augmentation == 'S') {
                        BUG();
                } else {
                        /*
                         * Unknown augmentation. Assume
                         * 'z' augmentation.
                         */
                        p = cie->initial_instructions;
                        BUG_ON(!p);
                        break;
                }
        }

        cie->initial_instructions = p;
        cie->instructions_end = end;

        /* Add to list */
        spin_lock_irqsave(&dwarf_cie_lock, flags);
        list_add_tail(&cie->link, &dwarf_cie_list);
        spin_unlock_irqrestore(&dwarf_cie_lock, flags);

        return 0;
}

static int dwarf_parse_fde(void *entry, u32 entry_type,
                           void *start, unsigned long len)
{
        struct dwarf_fde *fde;
        struct dwarf_cie *cie;
        unsigned long flags;
        int count;
        void *p = start;

        fde = kzalloc(sizeof(*fde), GFP_KERNEL);
        if (!fde)
                return -ENOMEM;

        fde->length = len;

        /*
         * In a .eh_frame section the CIE pointer is the
         * delta between the address within the FDE
         */
        fde->cie_pointer = (unsigned long)(p - entry_type - 4);

        cie = dwarf_lookup_cie(fde->cie_pointer);
        fde->cie = cie;

        if (cie->encoding)
                count = dwarf_read_encoded_value(p, &fde->initial_location,
                                                 cie->encoding);
        else
                count = dwarf_read_addr(p, &fde->initial_location);

        p += count;

        if (cie->encoding)
                count = dwarf_read_encoded_value(p, &fde->address_range,
                                                 cie->encoding & 0x0f);
        else
                count = dwarf_read_addr(p, &fde->address_range);

        p += count;

        if (fde->cie->flags & DWARF_CIE_Z_AUGMENTATION) {
                unsigned int length;
                count = dwarf_read_uleb128(p, &length);
                p += count + length;
        }

        /* Call frame instructions. */
        fde->instructions = p;
        fde->end = start + len;

        /* Add to list. */
        spin_lock_irqsave(&dwarf_fde_lock, flags);
        list_add_tail(&fde->link, &dwarf_fde_list);
        spin_unlock_irqrestore(&dwarf_fde_lock, flags);

        return 0;
}

static void dwarf_unwinder_dump(struct task_struct *task, struct pt_regs *regs,
                                unsigned long *sp,
                                const struct stacktrace_ops *ops, void *data)
{
        struct dwarf_frame *frame;

        frame = dwarf_unwind_stack(0, NULL);

        while (frame && frame->return_addr) {
                ops->address(data, frame->return_addr, 1);
                frame = frame->next;
        }
}

static struct unwinder dwarf_unwinder = {
        .name = "dwarf-unwinder",
        .dump = dwarf_unwinder_dump,
        .rating = 150,
};

static void dwarf_unwinder_cleanup(void)
{
        struct dwarf_cie *cie, *m;
        struct dwarf_fde *fde, *n;
        unsigned long flags;

        /*
         * Deallocate all the memory allocated for the DWARF unwinder.
         * Traverse all the FDE/CIE lists and remove and free all the
         * memory associated with those data structures.
         */
        spin_lock_irqsave(&dwarf_cie_lock, flags);
        list_for_each_entry_safe(cie, m, &dwarf_cie_list, link)
                kfree(cie);
        spin_unlock_irqrestore(&dwarf_cie_lock, flags);

        spin_lock_irqsave(&dwarf_fde_lock, flags);
        list_for_each_entry_safe(fde, n, &dwarf_fde_list, link)
                kfree(fde);
        spin_unlock_irqrestore(&dwarf_fde_lock, flags);
}

/**
 *      dwarf_unwinder_init - initialise the dwarf unwinder
 *
 *      Build the data structures describing the .dwarf_frame section to
 *      make it easier to lookup CIE and FDE entries. Because the
 *      .eh_frame section is packed as tightly as possible it is not
 *      easy to lookup the FDE for a given PC, so we build a list of FDE
 *      and CIE entries that make it easier.
 */
void dwarf_unwinder_init(void)
{
        u32 entry_type;
        void *p, *entry;
        int count, err;
        unsigned long len;
        unsigned int c_entries, f_entries;
        unsigned char *end;
        INIT_LIST_HEAD(&dwarf_cie_list);
        INIT_LIST_HEAD(&dwarf_fde_list);

        c_entries = 0;
        f_entries = 0;
        entry = &__start_eh_frame;

        while ((char *)entry < __stop_eh_frame) {
                p = entry;

                count = dwarf_entry_len(p, &len);
                if (count == 0) {
                        /*
                         * We read a bogus length field value. There is
                         * nothing we can do here apart from disabling
                         * the DWARF unwinder. We can't even skip this
                         * entry and move to the next one because 'len'
                         * tells us where our next entry is.
                         */
                        goto out;
                } else
                        p += count;

                /* initial length does not include itself */
                end = p + len;

                entry_type = get_unaligned((u32 *)p);
                p += 4;

                if (entry_type == DW_EH_FRAME_CIE) {
                        err = dwarf_parse_cie(entry, p, len, end);
                        if (err < 0)
                                goto out;
                        else
                                c_entries++;
                } else {
                        err = dwarf_parse_fde(entry, entry_type, p, len);
                        if (err < 0)
                                goto out;
                        else
                                f_entries++;
                }

                entry = (char *)entry + len + 4;
        }

        printk(KERN_INFO "DWARF unwinder initialised: read %u CIEs, %u FDEs\n",
               c_entries, f_entries);

        err = unwinder_register(&dwarf_unwinder);
        if (err)
                goto out;

        return;

out:
        printk(KERN_ERR "Failed to initialise DWARF unwinder: %d\n", err);
        dwarf_unwinder_cleanup();
}