root/arch/x86/kernel/ftrace.c

DEFINITIONS

1. ftrace_calc_offset
2. ftrace_nop_replace
3. ftrace_call_replace
4. ftrace_modify_code
5. ftrace_update_ftrace_func
6. ftrace_mcount_set
7. ftrace_dyn_arch_init

/*
 * Code for replacing ftrace calls with jumps.
 *
 * Copyright (C) 2007-2008 Steven Rostedt <srostedt@redhat.com>
 *
 * Thanks goes to Ingo Molnar, for suggesting the idea.
 * Mathieu Desnoyers, for suggesting postponing the modifications.
 * Arjan van de Ven, for keeping me straight, and explaining to me
 * the dangers of modifying code on the run.
 */

#include <linux/spinlock.h>
#include <linux/hardirq.h>
#include <linux/ftrace.h>
#include <linux/percpu.h>
#include <linux/init.h>
#include <linux/list.h>

#include <asm/alternative.h>
#include <asm/ftrace.h>


/* Long is fine, even if it is only 4 bytes ;-) */
static long *ftrace_nop;

union ftrace_code_union {
        char code[MCOUNT_INSN_SIZE];
        struct {
                char e8;
                int offset;
        } __attribute__((packed));
};


static int notrace ftrace_calc_offset(long ip, long addr)
{
        return (int)(addr - ip);
}

notrace unsigned char *ftrace_nop_replace(void)
{
        return (char *)ftrace_nop;
}

notrace unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
{
        static union ftrace_code_union calc;

        calc.e8         = 0xe8;
        calc.offset     = ftrace_calc_offset(ip + MCOUNT_INSN_SIZE, addr);

        /*
         * No locking needed, this must be called via kstop_machine
         * which in essence is like running on a uniprocessor machine.
         */
        return calc.code;
}

notrace int
ftrace_modify_code(unsigned long ip, unsigned char *old_code,
                   unsigned char *new_code)
{
        unsigned replaced;
        unsigned old = *(unsigned *)old_code; /* 4 bytes */
        unsigned new = *(unsigned *)new_code; /* 4 bytes */
        unsigned char newch = new_code[4];
        int faulted = 0;

        /*
         * Note: Due to modules and __init, code can
         *  disappear and change, we need to protect against faulting
         *  as well as code changing.
         *
         * No real locking needed, this code is run through
         * kstop_machine.
         */
        asm volatile (
                "1: lock\n"
                "   cmpxchg %3, (%2)\n"
                "   jnz 2f\n"
                "   movb %b4, 4(%2)\n"
                "2:\n"
                ".section .fixup, \"ax\"\n"
                "3:     movl $1, %0\n"
                "       jmp 2b\n"
                ".previous\n"
                _ASM_EXTABLE(1b, 3b)
                : "=r"(faulted), "=a"(replaced)
                : "r"(ip), "r"(new), "c"(newch),
                  "0"(faulted), "a"(old)
                : "memory");
        sync_core();

        if (replaced != old && replaced != new)
                faulted = 2;

        return faulted;
}

notrace int ftrace_update_ftrace_func(ftrace_func_t func)
{
        unsigned long ip = (unsigned long)(&ftrace_call);
        unsigned char old[MCOUNT_INSN_SIZE], *new;
        int ret;

        memcpy(old, &ftrace_call, MCOUNT_INSN_SIZE);
        new = ftrace_call_replace(ip, (unsigned long)func);
        ret = ftrace_modify_code(ip, old, new);

        return ret;
}

notrace int ftrace_mcount_set(unsigned long *data)
{
        unsigned long ip = (long)(&mcount_call);
        unsigned long *addr = data;
        unsigned char old[MCOUNT_INSN_SIZE], *new;

        /*
         * Replace the mcount stub with a pointer to the
         * ip recorder function.
         */
        memcpy(old, &mcount_call, MCOUNT_INSN_SIZE);
        new = ftrace_call_replace(ip, *addr);
        *addr = ftrace_modify_code(ip, old, new);

        return 0;
}

int __init ftrace_dyn_arch_init(void *data)
{
        const unsigned char *const *noptable = find_nop_table();

        /* This is running in kstop_machine */

        ftrace_mcount_set(data);

        ftrace_nop = (unsigned long *)noptable[MCOUNT_INSN_SIZE];

        return 0;
}