arch/x86/kernel/ftrace.c

   1 /*
   2  * Code for replacing ftrace calls with jumps.
   3  *
   4  * Copyright (C) 2007-2008 Steven Rostedt <srostedt@redhat.com>
   5  *
   6  * Thanks goes to Ingo Molnar, for suggesting the idea.
   7  * Mathieu Desnoyers, for suggesting postponing the modifications.
   8  * Arjan van de Ven, for keeping me straight, and explaining to me
   9  * the dangers of modifying code on the run.
  10  */
  11
  12 #include <linux/spinlock.h>
  13 #include <linux/hardirq.h>
  14 #include <linux/ftrace.h>
  15 #include <linux/percpu.h>
  16 #include <linux/init.h>
  17 #include <linux/list.h>
  18
  19 #include <asm/alternative.h>
  20 #include <asm/ftrace.h>
  21
  22
  23 /* Long is fine, even if it is only 4 bytes ;-) */
  24 static long *ftrace_nop;
  25
  26 union ftrace_code_union {
  27         char code[MCOUNT_INSN_SIZE];
  28         struct {
  29                 char e8;
  30                 int offset;
  31         } __attribute__((packed));
  32 };
  33
  34
  35 static int notrace ftrace_calc_offset(long ip, long addr)
  36 {
  37         return (int)(addr - ip);
  38 }
  39
  40 notrace unsigned char *ftrace_nop_replace(void)
  41 {
  42         return (char *)ftrace_nop;
  43 }
  44
  45 notrace unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
  46 {
  47         static union ftrace_code_union calc;
  48
  49         calc.e8         = 0xe8;
  50         calc.offset     = ftrace_calc_offset(ip + MCOUNT_INSN_SIZE, addr);
  51
  52         /*
  53          * No locking needed, this must be called via kstop_machine
  54          * which in essence is like running on a uniprocessor machine.
  55          */
  56         return calc.code;
  57 }
  58
  59 notrace int
  60 ftrace_modify_code(unsigned long ip, unsigned char *old_code,
  61                    unsigned char *new_code)
  62 {
  63         unsigned replaced;
  64         unsigned old = *(unsigned *)old_code; /* 4 bytes */
  65         unsigned new = *(unsigned *)new_code; /* 4 bytes */
  66         unsigned char newch = new_code[4];
  67         int faulted = 0;
  68
  69         /*
  70          * Note: Due to modules and __init, code can
  71          *  disappear and change, we need to protect against faulting
  72          *  as well as code changing.
  73          *
  74          * No real locking needed, this code is run through
  75          * kstop_machine.
  76          */
  77         asm volatile (
  78                 "1: lock\n"
  79                 "   cmpxchg %3, (%2)\n"
  80                 "   jnz 2f\n"
  81                 "   movb %b4, 4(%2)\n"
  82                 "2:\n"
  83                 ".section .fixup, \"ax\"\n"
  84                 "3:     movl $1, %0\n"
  85                 "       jmp 2b\n"
  86                 ".previous\n"
  87                 _ASM_EXTABLE(1b, 3b)
  88                 : "=r"(faulted), "=a"(replaced)
  89                 : "r"(ip), "r"(new), "c"(newch),
  90                   "0"(faulted), "a"(old)
  91                 : "memory");
  92         sync_core();
  93
  94         if (replaced != old && replaced != new)
  95                 faulted = 2;
  96
  97         return faulted;
  98 }
  99
 100 notrace int ftrace_update_ftrace_func(ftrace_func_t func)
 101 {
 102         unsigned long ip = (unsigned long)(&ftrace_call);
 103         unsigned char old[MCOUNT_INSN_SIZE], *new;
 104         int ret;
 105
 106         memcpy(old, &ftrace_call, MCOUNT_INSN_SIZE);
 107         new = ftrace_call_replace(ip, (unsigned long)func);
 108         ret = ftrace_modify_code(ip, old, new);
 109
 110         return ret;
 111 }
 112
 113 notrace int ftrace_mcount_set(unsigned long *data)
 114 {
 115         unsigned long ip = (long)(&mcount_call);
 116         unsigned long *addr = data;
 117         unsigned char old[MCOUNT_INSN_SIZE], *new;
 118
 119         /*
 120          * Replace the mcount stub with a pointer to the
 121          * ip recorder function.
 122          */
 123         memcpy(old, &mcount_call, MCOUNT_INSN_SIZE);
 124         new = ftrace_call_replace(ip, *addr);
 125         *addr = ftrace_modify_code(ip, old, new);
 126
 127         return 0;
 128 }
 129
 130 int __init ftrace_dyn_arch_init(void *data)
 131 {
 132         const unsigned char *const *noptable = find_nop_table();
 133
 134         /* This is running in kstop_machine */
 135
 136         ftrace_mcount_set(data);
 137
 138         ftrace_nop = (unsigned long *)noptable[MCOUNT_INSN_SIZE];
 139
 140         return 0;
 141 }