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/*
* 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/uaccess.h>
#include <linux/ftrace.h>
#include <linux/percpu.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/list.h>
#include <trace/syscall.h>
#include <asm/cacheflush.h>
#include <asm/ftrace.h>
#include <asm/nops.h>
#include <asm/nmi.h>
#ifdef CONFIG_DYNAMIC_FTRACE
int ftrace_arch_code_modify_prepare(void)
{
set_kernel_text_rw();
return 0;
}
int ftrace_arch_code_modify_post_process(void)
{
set_kernel_text_ro();
return 0;
}
union ftrace_code_union {
char code[MCOUNT_INSN_SIZE];
struct {
char e8;
int offset;
} __attribute__((packed));
};
static int ftrace_calc_offset(long ip, long addr)
{
return (int)(addr - ip);
}
static 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;
}
/*
* Modifying code must take extra care. On an SMP machine, if
* the code being modified is also being executed on another CPU
* that CPU will have undefined results and possibly take a GPF.
* We use kstop_machine to stop other CPUS from exectuing code.
* But this does not stop NMIs from happening. We still need
* to protect against that. We separate out the modification of
* the code to take care of this.
*
* Two buffers are added: An IP buffer and a "code" buffer.
*
* 1) Put the instruction pointer into the IP buffer
* and the new code into the "code" buffer.
* 2) Wait for any running NMIs to finish and set a flag that says
* we are modifying code, it is done in an atomic operation.
* 3) Write the code
* 4) clear the flag.
* 5) Wait for any running NMIs to finish.
*
* If an NMI is executed, the first thing it does is to call
* "ftrace_nmi_enter". This will check if the flag is set to write
* and if it is, it will write what is in the IP and "code" buffers.
*
* The trick is, it does not matter if everyone is writing the same
* content to the code location. Also, if a CPU is executing code
* it is OK to write to that code location if the contents being written
* are the same as what exists.
*/
#define MOD_CODE_WRITE_FLAG (1 << 31) /* set when NMI should do the write */
static atomic_t nmi_running = ATOMIC_INIT(0);
static int mod_code_status; /* holds return value of text write */
static void *mod_code_ip; /* holds the IP to write to */
static void *mod_code_newcode; /* holds the text to write to the IP */
static unsigned nmi_wait_count;
static atomic_t nmi_update_count = ATOMIC_INIT(0);
int ftrace_arch_read_dyn_info(char *buf, int size)
{
int r;
r = snprintf(buf, size, "%u %u",
nmi_wait_count,
atomic_read(&nmi_update_count));
return r;
}
static void clear_mod_flag(void)
{
int old = atomic_read(&nmi_running);
for (;;) {
int new = old & ~MOD_CODE_WRITE_FLAG;
if (old == new)
break;
old = atomic_cmpxchg(&nmi_running, old, new);
}
}
static void ftrace_mod_code(void)
{
/*
* Yes, more than one CPU process can be writing to mod_code_status.
* (and the code itself)
* But if one were to fail, then they all should, and if one were
* to succeed, then they all should.
*/
mod_code_status = probe_kernel_write(mod_code_ip, mod_code_newcode,
MCOUNT_INSN_SIZE);
/* if we fail, then kill any new writers */
if (mod_code_status)
clear_mod_flag();
}
void ftrace_nmi_enter(void)
{
if (atomic_inc_return(&nmi_running) & MOD_CODE_WRITE_FLAG) {
smp_rmb();
ftrace_mod_code();
atomic_inc(&nmi_update_count);
}
/* Must have previous changes seen before executions */
smp_mb();
}
void ftrace_nmi_exit(void)
{
/* Finish all executions before clearing nmi_running */
smp_mb();
atomic_dec(&nmi_running);
}
static void wait_for_nmi_and_set_mod_flag(void)
{
if (!atomic_cmpxchg(&nmi_running, 0, MOD_CODE_WRITE_FLAG))
return;
do {
cpu_relax();
} while (atomic_cmpxchg(&nmi_running, 0, MOD_CODE_WRITE_FLAG));
nmi_wait_count++;
}
static void wait_for_nmi(void)
{
if (!atomic_read(&nmi_running))
return;
do {
cpu_relax();
} while (atomic_read(&nmi_running));
nmi_wait_count++;
}
static int
do_ftrace_mod_code(unsigned long ip, void *new_code)
{
mod_code_ip = (void *)ip;
mod_code_newcode = new_code;
/* The buffers need to be visible before we let NMIs write them */
smp_mb();
wait_for_nmi_and_set_mod_flag();
/* Make sure all running NMIs have finished before we write the code */
smp_mb();
ftrace_mod_code();
/* Make sure the write happens before clearing the bit */
smp_mb();
clear_mod_flag();
wait_for_nmi();
return mod_code_status;
}
static unsigned char ftrace_nop[MCOUNT_INSN_SIZE];
static unsigned char *ftrace_nop_replace(void)
{
return ftrace_nop;
}
static int
ftrace_modify_code(unsigned long ip, unsigned char *old_code,
unsigned char *new_code)
{
unsigned char replaced[MCOUNT_INSN_SIZE];
/*
* Note: Due to modules and __init, code can
* disappear and change, we need to protect against faulting
* as well as code changing. We do this by using the
* probe_kernel_* functions.
*
* No real locking needed, this code is run through
* kstop_machine, or before SMP starts.
*/
/* read the text we want to modify */
if (probe_kernel_read(replaced, (void *)ip, MCOUNT_INSN_SIZE))
return -EFAULT;
/* Make sure it is what we expect it to be */
if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
return -EINVAL;
/* replace the text with the new text */
if (do_ftrace_mod_code(ip, new_code))
return -EPERM;
sync_core();
return 0;
}
int ftrace_make_nop(struct module *mod,
struct dyn_ftrace *rec, unsigned long addr)
{
unsigned char *new, *old;
unsigned long ip = rec->ip;
old = ftrace_call_replace(ip, addr);
new = ftrace_nop_replace();
return ftrace_modify_code(rec->ip, old, new);
}
int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
{
unsigned char *new, *old;
unsigned long ip = rec->ip;
old = ftrace_nop_replace();
new = ftrace_call_replace(ip, addr);
return ftrace_modify_code(rec->ip, old, new);
}
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;
}
int __init ftrace_dyn_arch_init(void *data)
{
extern const unsigned char ftrace_test_p6nop[];
extern const unsigned char ftrace_test_nop5[];
extern const unsigned char ftrace_test_jmp[];
int faulted = 0;
/*
* There is no good nop for all x86 archs.
* We will default to using the P6_NOP5, but first we
* will test to make sure that the nop will actually
* work on this CPU. If it faults, we will then
* go to a lesser efficient 5 byte nop. If that fails
* we then just use a jmp as our nop. This isn't the most
* efficient nop, but we can not use a multi part nop
* since we would then risk being preempted in the middle
* of that nop, and if we enabled tracing then, it might
* cause a system crash.
*
* TODO: check the cpuid to determine the best nop.
*/
asm volatile (
"ftrace_test_jmp:"
"jmp ftrace_test_p6nop\n"
"nop\n"
"nop\n"
"nop\n" /* 2 byte jmp + 3 bytes */
"ftrace_test_p6nop:"
P6_NOP5
"jmp 1f\n"
"ftrace_test_nop5:"
".byte 0x66,0x66,0x66,0x66,0x90\n"
"1:"
".section .fixup, \"ax\"\n"
"2: movl $1, %0\n"
" jmp ftrace_test_nop5\n"
"3: movl $2, %0\n"
" jmp 1b\n"
".previous\n"
_ASM_EXTABLE(ftrace_test_p6nop, 2b)
_ASM_EXTABLE(ftrace_test_nop5, 3b)
: "=r"(faulted) : "0" (faulted));
switch (faulted) {
case
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