backtrace.c

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#include <linux/oprofile.h>
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/uaccess.h>
#include <asm/ptrace.h>
#include <asm/stacktrace.h>
#include <linux/stacktrace.h>
#include <linux/kernel.h>
#include <asm/sections.h>
#include <asm/inst.h>

struct stackframe {
    unsigned long sp;
    unsigned long pc;
    unsigned long ra;
};

static inline int get_mem(unsigned long addr, unsigned long *result)
{
    unsigned long *address = (unsigned long *) addr;
    if (!access_ok(VERIFY_READ, addr, sizeof(unsigned long)))
        return -1;
    if (__copy_from_user_inatomic(result, address, sizeof(unsigned long)))
        return -3;
    return 0;
}

/*
 * These two instruction helpers were taken from process.c
 */
static inline int is_ra_save_ins(union mips_instruction *ip)
{
    /* sw / sd $ra, offset($sp) */
    return (ip->i_format.opcode == sw_op || ip->i_format.opcode == sd_op)
        && ip->i_format.rs == 29 && ip->i_format.rt == 31;
}

static inline int is_sp_move_ins(union mips_instruction *ip)
{
    /* addiu/daddiu sp,sp,-imm */
    if (ip->i_format.rs != 29 || ip->i_format.rt != 29)
        return 0;
    if (ip->i_format.opcode == addiu_op || ip->i_format.opcode == daddiu_op)
        return 1;
    return 0;
}

/*
 * Looks for specific instructions that mark the end of a function.
 * This usually means we ran into the code area of the previous function.
 */
static inline int is_end_of_function_marker(union mips_instruction *ip)
{
    /* jr ra */
    if (ip->r_format.func == jr_op && ip->r_format.rs == 31)
        return 1;
    /* lui gp */
    if (ip->i_format.opcode == lui_op && ip->i_format.rt == 28)
        return 1;
    return 0;
}

/*
 * TODO for userspace stack unwinding:
 * - handle cases where the stack is adjusted inside a function
 *     (generally doesn't happen)
 * - find optimal value for max_instr_check
 * - try to find a way to handle leaf functions
 */

static inline int unwind_user_frame(struct stackframe *old_frame,
                    const unsigned int max_instr_check)
{
    struct stackframe new_frame = *old_frame;
    off_t ra_offset = 0;
    size_t stack_size = 0;
    unsigned long addr;

    if (old_frame->pc == 0 || old_frame->sp == 0 || old_frame->ra == 0)
        return -9;

    for (addr = new_frame.pc; (addr + max_instr_check > new_frame.pc)
        && (!ra_offset || !stack_size); --addr) {
        union mips_instruction ip;

        if (get_mem(addr, (unsigned long *) &ip))
            return -11;

        if (is_sp_move_ins(&ip)) {
            int stack_adjustment = ip.i_format.simmediate;
            if (stack_adjustment > 0)
                /* This marks the end of the previous function,
                   which means we overran. */
                break;
            stack_size = (unsigned) stack_adjustment;
        } else if (is_ra_save_ins(&ip)) {
            int ra_slot = ip.i_format.simmediate;
            if (ra_slot < 0)
                /* This shouldn't happen. */
                break;
            ra_offset = ra_slot;
        } else if (is_end_of_function_marker(&ip))
            break;
    }

    if (!ra_offset || !stack_size)
        return -1;

    if (ra_offset) {
        new_frame.ra = old_frame->sp + ra_offset;
        if (get_mem(new_frame.ra, &(new_frame.ra)))
            return -13;
    }

    if (stack_size) {
        new_frame.sp = old_frame->sp + stack_size;
        if (get_mem(new_frame.sp, &(new_frame.sp)))
            return -14;
    }

    if (new_frame.sp > old_frame->sp)
        return -2;

    new_frame.pc = old_frame->ra;
    *old_frame = new_frame;

    return 0;
}

static inline void do_user_backtrace(unsigned long low_addr,
                     struct stackframe *frame,
                     unsigned int depth)
{
    const unsigned int max_instr_check = 512;
    const unsigned long high_addr = low_addr + THREAD_SIZE;

    while (depth-- && !unwind_user_frame(frame, max_instr_check)) {
        oprofile_add_trace(frame->ra);
        if (frame->sp < low_addr || frame->sp > high_addr)
            break;
    }
}

#ifndef CONFIG_KALLSYMS
static inline void do_kernel_backtrace(unsigned long low_addr,
                       struct stackframe *frame,
                       unsigned int depth) { }
#else
static inline void do_kernel_backtrace(unsigned long low_addr,
                       struct stackframe *frame,
                       unsigned int depth)
{
    while (depth-- && frame->pc) {
        frame->pc = unwind_stack_by_address(low_addr,
                            &(frame->sp),
                            frame->pc,
                            &(frame->ra));
        oprofile_add_trace(frame->ra);
    }
}
#endif

void notrace op_mips_backtrace(struct pt_regs *const regs, unsigned int depth)
{
    struct stackframe frame = { .sp = regs->regs[29],
                    .pc = regs->cp0_epc,
                    .ra = regs->regs[31] };
    const int userspace = user_mode(regs);
    const unsigned long low_addr = ALIGN(frame.sp, THREAD_SIZE);

    if (userspace)
        do_user_backtrace(low_addr, &frame, depth);
    else
        do_kernel_backtrace(low_addr, &frame, depth);
}