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>
21 #include <linux/ftrace.h>
27 #ifdef CONFIG_FUNCTION_RET_TRACER
30 * These functions are picked from those used on
31 * this page for dynamic ftrace. They have been
32 * simplified to ignore all traces in NMI context.
34 static atomic_t in_nmi;
36 void ftrace_nmi_enter(void)
41 void ftrace_nmi_exit(void)
47 * Synchronize accesses to return adresses stack with
50 static raw_spinlock_t ret_stack_lock;
52 /* Add a function return address to the trace stack on thread info.*/
53 static int push_return_trace(unsigned long ret, unsigned long long time,
57 struct thread_info *ti;
61 raw_local_irq_save(flags);
62 __raw_spin_lock(&ret_stack_lock);
64 ti = current_thread_info();
65 /* The return trace stack is full */
66 if (ti->curr_ret_stack == FTRACE_RET_STACK_SIZE - 1) {
71 index = ++ti->curr_ret_stack;
72 ti->ret_stack[index].ret = ret;
73 ti->ret_stack[index].func = func;
74 ti->ret_stack[index].calltime = time;
77 __raw_spin_unlock(&ret_stack_lock);
78 raw_local_irq_restore(flags);
82 /* Retrieve a function return address to the trace stack on thread info.*/
83 static void pop_return_trace(unsigned long *ret, unsigned long long *time,
86 struct thread_info *ti;
90 raw_local_irq_save(flags);
91 __raw_spin_lock(&ret_stack_lock);
93 ti = current_thread_info();
94 index = ti->curr_ret_stack;
95 *ret = ti->ret_stack[index].ret;
96 *func = ti->ret_stack[index].func;
97 *time = ti->ret_stack[index].calltime;
100 __raw_spin_unlock(&ret_stack_lock);
101 raw_local_irq_restore(flags);
105 * Send the trace to the ring-buffer.
106 * @return the original return address.
108 unsigned long ftrace_return_to_handler(void)
110 struct ftrace_retfunc trace;
111 pop_return_trace(&trace.ret, &trace.calltime, &trace.func);
112 trace.rettime = cpu_clock(raw_smp_processor_id());
113 ftrace_function_return(&trace);
119 * Hook the return address and push it in the stack of return addrs
120 * in current thread info.
123 void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
126 unsigned long long calltime;
128 unsigned long return_hooker = (unsigned long)
131 /* Nmi's are currently unsupported */
132 if (atomic_read(&in_nmi))
136 * Protect against fault, even if it shouldn't
137 * happen. This tool is too much intrusive to
138 * ignore such a protection.
141 "1: movl (%[parent_old]), %[old]\n"
142 "2: movl %[return_hooker], (%[parent_replaced])\n"
143 " movl $0, %[faulted]\n"
145 ".section .fixup, \"ax\"\n"
146 "3: movl $1, %[faulted]\n"
149 ".section __ex_table, \"a\"\n"
154 : [parent_replaced] "=rm" (parent), [old] "=r" (old),
155 [faulted] "=r" (faulted)
156 : [parent_old] "0" (parent), [return_hooker] "r" (return_hooker)
160 if (WARN_ON(faulted)) {
161 unregister_ftrace_return();
165 if (WARN_ON(!__kernel_text_address(old))) {
166 unregister_ftrace_return();
171 calltime = cpu_clock(raw_smp_processor_id());
173 if (push_return_trace(old, calltime, self_addr) == -EBUSY)
177 static int __init init_ftrace_function_return(void)
179 ret_stack_lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
182 device_initcall(init_ftrace_function_return);
187 #ifdef CONFIG_DYNAMIC_FTRACE
189 union ftrace_code_union {
190 char code[MCOUNT_INSN_SIZE];
194 } __attribute__((packed));
197 static int ftrace_calc_offset(long ip, long addr)
199 return (int)(addr - ip);
202 unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
204 static union ftrace_code_union calc;
207 calc.offset = ftrace_calc_offset(ip + MCOUNT_INSN_SIZE, addr);
210 * No locking needed, this must be called via kstop_machine
211 * which in essence is like running on a uniprocessor machine.
217 * Modifying code must take extra care. On an SMP machine, if
218 * the code being modified is also being executed on another CPU
219 * that CPU will have undefined results and possibly take a GPF.
220 * We use kstop_machine to stop other CPUS from exectuing code.
221 * But this does not stop NMIs from happening. We still need
222 * to protect against that. We separate out the modification of
223 * the code to take care of this.
225 * Two buffers are added: An IP buffer and a "code" buffer.
227 * 1) Put the instruction pointer into the IP buffer
228 * and the new code into the "code" buffer.
229 * 2) Set a flag that says we are modifying code
230 * 3) Wait for any running NMIs to finish.
233 * 6) Wait for any running NMIs to finish.
235 * If an NMI is executed, the first thing it does is to call
236 * "ftrace_nmi_enter". This will check if the flag is set to write
237 * and if it is, it will write what is in the IP and "code" buffers.
239 * The trick is, it does not matter if everyone is writing the same
240 * content to the code location. Also, if a CPU is executing code
241 * it is OK to write to that code location if the contents being written
242 * are the same as what exists.
245 static atomic_t in_nmi = ATOMIC_INIT(0);
246 static int mod_code_status; /* holds return value of text write */
247 static int mod_code_write; /* set when NMI should do the write */
248 static void *mod_code_ip; /* holds the IP to write to */
249 static void *mod_code_newcode; /* holds the text to write to the IP */
251 static unsigned nmi_wait_count;
252 static atomic_t nmi_update_count = ATOMIC_INIT(0);
254 int ftrace_arch_read_dyn_info(char *buf, int size)
258 r = snprintf(buf, size, "%u %u",
260 atomic_read(&nmi_update_count));
264 static void ftrace_mod_code(void)
267 * Yes, more than one CPU process can be writing to mod_code_status.
268 * (and the code itself)
269 * But if one were to fail, then they all should, and if one were
270 * to succeed, then they all should.
272 mod_code_status = probe_kernel_write(mod_code_ip, mod_code_newcode,
277 void ftrace_nmi_enter(void)
280 /* Must have in_nmi seen before reading write flag */
282 if (mod_code_write) {
284 atomic_inc(&nmi_update_count);
288 void ftrace_nmi_exit(void)
290 /* Finish all executions before clearing in_nmi */
295 static void wait_for_nmi(void)
299 while (atomic_read(&in_nmi)) {
309 do_ftrace_mod_code(unsigned long ip, void *new_code)
311 mod_code_ip = (void *)ip;
312 mod_code_newcode = new_code;
314 /* The buffers need to be visible before we let NMIs write them */
319 /* Make sure write bit is visible before we wait on NMIs */
324 /* Make sure all running NMIs have finished before we write the code */
329 /* Make sure the write happens before clearing the bit */
334 /* make sure NMIs see the cleared bit */
339 return mod_code_status;
345 static unsigned char ftrace_nop[MCOUNT_INSN_SIZE];
347 unsigned char *ftrace_nop_replace(void)
353 ftrace_modify_code(unsigned long ip, unsigned char *old_code,
354 unsigned char *new_code)
356 unsigned char replaced[MCOUNT_INSN_SIZE];
359 * Note: Due to modules and __init, code can
360 * disappear and change, we need to protect against faulting
361 * as well as code changing. We do this by using the
362 * probe_kernel_* functions.
364 * No real locking needed, this code is run through
365 * kstop_machine, or before SMP starts.
368 /* read the text we want to modify */
369 if (probe_kernel_read(replaced, (void *)ip, MCOUNT_INSN_SIZE))
372 /* Make sure it is what we expect it to be */
373 if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
376 /* replace the text with the new text */
377 if (do_ftrace_mod_code(ip, new_code))
385 int ftrace_update_ftrace_func(ftrace_func_t func)
387 unsigned long ip = (unsigned long)(&ftrace_call);
388 unsigned char old[MCOUNT_INSN_SIZE], *new;
391 memcpy(old, &ftrace_call, MCOUNT_INSN_SIZE);
392 new = ftrace_call_replace(ip, (unsigned long)func);
393 ret = ftrace_modify_code(ip, old, new);
398 int __init ftrace_dyn_arch_init(void *data)
400 extern const unsigned char ftrace_test_p6nop[];
401 extern const unsigned char ftrace_test_nop5[];
402 extern const unsigned char ftrace_test_jmp[];
406 * There is no good nop for all x86 archs.
407 * We will default to using the P6_NOP5, but first we
408 * will test to make sure that the nop will actually
409 * work on this CPU. If it faults, we will then
410 * go to a lesser efficient 5 byte nop. If that fails
411 * we then just use a jmp as our nop. This isn't the most
412 * efficient nop, but we can not use a multi part nop
413 * since we would then risk being preempted in the middle
414 * of that nop, and if we enabled tracing then, it might
415 * cause a system crash.
417 * TODO: check the cpuid to determine the best nop.
421 "jmp ftrace_test_p6nop\n"
424 "nop\n" /* 2 byte jmp + 3 bytes */
429 ".byte 0x66,0x66,0x66,0x66,0x90\n"
431 ".section .fixup, \"ax\"\n"
433 " jmp ftrace_test_nop5\n"
437 _ASM_EXTABLE(ftrace_test_p6nop, 2b)
438 _ASM_EXTABLE(ftrace_test_nop5, 3b)
439 : "=r"(faulted) : "0" (faulted));
443 pr_info("ftrace: converting mcount calls to 0f 1f 44 00 00\n");
444 memcpy(ftrace_nop, ftrace_test_p6nop, MCOUNT_INSN_SIZE);
447 pr_info("ftrace: converting mcount calls to 66 66 66 66 90\n");
448 memcpy(ftrace_nop, ftrace_test_nop5, MCOUNT_INSN_SIZE);
451 pr_info("ftrace: converting mcount calls to jmp . + 5\n");
452 memcpy(ftrace_nop, ftrace_test_jmp, MCOUNT_INSN_SIZE);
456 /* The return code is retured via data */
457 *(unsigned long *)data = 0;