2 * Code for replacing ftrace calls with jumps.
4 * Copyright (C) 2007-2008 Steven Rostedt <srostedt@redhat.com>
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.
12 #include <linux/spinlock.h>
13 #include <linux/hardirq.h>
14 #include <linux/uaccess.h>
15 #include <linux/ftrace.h>
16 #include <linux/percpu.h>
17 #include <linux/init.h>
18 #include <linux/list.h>
20 #include <asm/ftrace.h>
24 static unsigned char ftrace_nop[MCOUNT_INSN_SIZE];
26 union ftrace_code_union {
27 char code[MCOUNT_INSN_SIZE];
31 } __attribute__((packed));
35 static int ftrace_calc_offset(long ip, long addr)
37 return (int)(addr - ip);
40 unsigned char *ftrace_nop_replace(void)
45 unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
47 static union ftrace_code_union calc;
50 calc.offset = ftrace_calc_offset(ip + MCOUNT_INSN_SIZE, addr);
53 * No locking needed, this must be called via kstop_machine
54 * which in essence is like running on a uniprocessor machine.
60 * Modifying code must take extra care. On an SMP machine, if
61 * the code being modified is also being executed on another CPU
62 * that CPU will have undefined results and possibly take a GPF.
63 * We use kstop_machine to stop other CPUS from exectuing code.
64 * But this does not stop NMIs from happening. We still need
65 * to protect against that. We separate out the modification of
66 * the code to take care of this.
68 * Two buffers are added: An IP buffer and a "code" buffer.
70 * 1) Put in the instruction pointer into the IP buffer
71 * and the new code into the "code" buffer.
72 * 2) Set a flag that says we are modifying code
73 * 3) Wait for any running NMIs to finish.
76 * 6) Wait for any running NMIs to finish.
78 * If an NMI is executed, the first thing it does is to call
79 * "ftrace_nmi_enter". This will check if the flag is set to write
80 * and if it is, it will write what is in the IP and "code" buffers.
82 * The trick is, it does not matter if everyone is writing the same
83 * content to the code location. Also, if a CPU is executing code
84 * it is OK to write to that code location if the contents being written
85 * are the same as what exists.
88 static atomic_t in_nmi;
89 static int mod_code_status;
90 static int mod_code_write;
91 static void *mod_code_ip;
92 static void *mod_code_newcode;
94 static void ftrace_mod_code(void)
97 * Yes, more than one CPU process can be writing to mod_code_status.
98 * (and the code itself)
99 * But if one were to fail, then they all should, and if one were
100 * to succeed, then they all should.
102 mod_code_status = probe_kernel_write(mod_code_ip, mod_code_newcode,
107 void ftrace_nmi_enter(void)
110 /* Must have in_nmi seen before reading write flag */
116 void ftrace_nmi_exit(void)
118 /* Finish all executions before clearing in_nmi */
123 static void wait_for_nmi(void)
125 while (atomic_read(&in_nmi))
130 do_ftrace_mod_code(unsigned long ip, void *new_code)
132 mod_code_ip = (void *)ip;
133 mod_code_newcode = new_code;
135 /* The buffers need to be visible before we let NMIs write them */
140 /* Make sure write bit is visible before we wait on NMIs */
145 /* Make sure all running NMIs have finished before we write the code */
150 /* Make sure the write happens before clearing the bit */
155 /* make sure NMIs see the cleared bit */
160 return mod_code_status;
165 ftrace_modify_code(unsigned long ip, unsigned char *old_code,
166 unsigned char *new_code)
168 unsigned char replaced[MCOUNT_INSN_SIZE];
171 * Note: Due to modules and __init, code can
172 * disappear and change, we need to protect against faulting
173 * as well as code changing. We do this by using the
174 * probe_kernel_* functions.
176 * No real locking needed, this code is run through
177 * kstop_machine, or before SMP starts.
180 /* read the text we want to modify */
181 if (probe_kernel_read(replaced, (void *)ip, MCOUNT_INSN_SIZE))
184 /* Make sure it is what we expect it to be */
185 if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
188 /* replace the text with the new text */
189 if (do_ftrace_mod_code(ip, new_code))
197 int ftrace_update_ftrace_func(ftrace_func_t func)
199 unsigned long ip = (unsigned long)(&ftrace_call);
200 unsigned char old[MCOUNT_INSN_SIZE], *new;
203 memcpy(old, &ftrace_call, MCOUNT_INSN_SIZE);
204 new = ftrace_call_replace(ip, (unsigned long)func);
205 ret = ftrace_modify_code(ip, old, new);
210 int __init ftrace_dyn_arch_init(void *data)
212 extern const unsigned char ftrace_test_p6nop[];
213 extern const unsigned char ftrace_test_nop5[];
214 extern const unsigned char ftrace_test_jmp[];
218 * There is no good nop for all x86 archs.
219 * We will default to using the P6_NOP5, but first we
220 * will test to make sure that the nop will actually
221 * work on this CPU. If it faults, we will then
222 * go to a lesser efficient 5 byte nop. If that fails
223 * we then just use a jmp as our nop. This isn't the most
224 * efficient nop, but we can not use a multi part nop
225 * since we would then risk being preempted in the middle
226 * of that nop, and if we enabled tracing then, it might
227 * cause a system crash.
229 * TODO: check the cpuid to determine the best nop.
233 "jmp ftrace_test_p6nop\n"
236 "nop\n" /* 2 byte jmp + 3 bytes */
241 ".byte 0x66,0x66,0x66,0x66,0x90\n"
243 ".section .fixup, \"ax\"\n"
245 " jmp ftrace_test_nop5\n"
249 _ASM_EXTABLE(ftrace_test_p6nop, 2b)
250 _ASM_EXTABLE(ftrace_test_nop5, 3b)
251 : "=r"(faulted) : "0" (faulted));
255 pr_info("ftrace: converting mcount calls to 0f 1f 44 00 00\n");
256 memcpy(ftrace_nop, ftrace_test_p6nop, MCOUNT_INSN_SIZE);
259 pr_info("ftrace: converting mcount calls to 66 66 66 66 90\n");
260 memcpy(ftrace_nop, ftrace_test_nop5, MCOUNT_INSN_SIZE);
263 pr_info("ftrace: converting mcount calls to jmp . + 5\n");
264 memcpy(ftrace_nop, ftrace_test_jmp, MCOUNT_INSN_SIZE);
268 /* The return code is retured via data */
269 *(unsigned long *)data = 0;