ftrace.c

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/*
 * Copyright (C) 2008 Matt Fleming <[email protected]>
 * Copyright (C) 2008 Paul Mundt <[email protected]>
 *
 * Code for replacing ftrace calls with jumps.
 *
 * Copyright (C) 2007-2008 Steven Rostedt <[email protected]>
 *
 * 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/uaccess.h>
#include <linux/ftrace.h>
#include <linux/string.h>
#include <linux/init.h>
#include <linux/io.h>
#include <linux/kernel.h>
#include <asm/ftrace.h>
#include <asm/cacheflush.h>
#include <asm/unistd.h>
#include <trace/syscall.h>

#ifdef CONFIG_DYNAMIC_FTRACE
static unsigned char ftrace_replaced_code[MCOUNT_INSN_SIZE];

static unsigned char ftrace_nop[4];
/*
 * If we're trying to nop out a call to a function, we instead
 * place a call to the address after the memory table.
 *
 * 8c011060 <a>:
 * 8c011060:       02 d1           mov.l   8c01106c <a+0xc>,r1
 * 8c011062:       22 4f           sts.l   pr,@-r15
 * 8c011064:       02 c7           mova    8c011070 <a+0x10>,r0
 * 8c011066:       2b 41           jmp     @r1
 * 8c011068:       2a 40           lds     r0,pr
 * 8c01106a:       09 00           nop
 * 8c01106c:       68 24           .word 0x2468     <--- ip
 * 8c01106e:       1d 8c           .word 0x8c1d
 * 8c011070:       26 4f           lds.l   @r15+,pr <--- ip + MCOUNT_INSN_SIZE
 *
 * We write 0x8c011070 to 0x8c01106c so that on entry to a() we branch
 * past the _mcount call and continue executing code like normal.
 */
static unsigned char *ftrace_nop_replace(unsigned long ip)
{
    __raw_writel(ip + MCOUNT_INSN_SIZE, ftrace_nop);
    return ftrace_nop;
}

static unsigned char *ftrace_call_replace(unsigned long ip, unsigned long addr)
{
    /* Place the address in the memory table. */
    __raw_writel(addr, ftrace_replaced_code);

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

    flush_icache_range(ip, ip + MCOUNT_INSN_SIZE);

    return 0;
}

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

    memcpy(old, (unsigned char *)ip, MCOUNT_INSN_SIZE);
    new = ftrace_call_replace(ip, (unsigned long)func);

    return ftrace_modify_code(ip, old, new);
}

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(ip);

    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(ip);
    new = ftrace_call_replace(ip, addr);

    return ftrace_modify_code(rec->ip, old, new);
}

int __init ftrace_dyn_arch_init(void *data)
{
    /* The return code is retured via data */
    __raw_writel(0, (unsigned long)data);

    return 0;
}
#endif /* CONFIG_DYNAMIC_FTRACE */

#ifdef CONFIG_FUNCTION_GRAPH_TRACER
#ifdef CONFIG_DYNAMIC_FTRACE
extern void ftrace_graph_call(void);

static int ftrace_mod(unsigned long ip, unsigned long old_addr,
              unsigned long new_addr)
{
    unsigned char code[MCOUNT_INSN_SIZE];

    if (probe_kernel_read(code, (void *)ip, MCOUNT_INSN_SIZE))
        return -EFAULT;

    if (old_addr != __raw_readl((unsigned long *)code))
        return -EINVAL;

    __raw_writel(new_addr, ip);
    return 0;
}

int ftrace_enable_ftrace_graph_caller(void)
{
    unsigned long ip, old_addr, new_addr;

    ip = (unsigned long)(&ftrace_graph_call) + GRAPH_INSN_OFFSET;
    old_addr = (unsigned long)(&skip_trace);
    new_addr = (unsigned long)(&ftrace_graph_caller);

    return ftrace_mod(ip, old_addr, new_addr);
}

int ftrace_disable_ftrace_graph_caller(void)
{
    unsigned long ip, old_addr, new_addr;

    ip = (unsigned long)(&ftrace_graph_call) + GRAPH_INSN_OFFSET;
    old_addr = (unsigned long)(&ftrace_graph_caller);
    new_addr = (unsigned long)(&skip_trace);

    return ftrace_mod(ip, old_addr, new_addr);
}
#endif /* CONFIG_DYNAMIC_FTRACE */

/*
 * Hook the return address and push it in the stack of return addrs
 * in the current thread info.
 *
 * This is the main routine for the function graph tracer. The function
 * graph tracer essentially works like this:
 *
 * parent is the stack address containing self_addr's return address.
 * We pull the real return address out of parent and store it in
 * current's ret_stack. Then, we replace the return address on the stack
 * with the address of return_to_handler. self_addr is the function that
 * called mcount.
 *
 * When self_addr returns, it will jump to return_to_handler which calls
 * ftrace_return_to_handler. ftrace_return_to_handler will pull the real
 * return address off of current's ret_stack and jump to it.
 */
void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr)
{
    unsigned long old;
    int faulted, err;
    struct ftrace_graph_ent trace;
    unsigned long return_hooker = (unsigned long)&return_to_handler;

    if (unlikely(atomic_read(&current->tracing_graph_pause)))
        return;

    /*
     * Protect against fault, even if it shouldn't
     * happen. This tool is too much intrusive to
     * ignore such a protection.
     */
    __asm__ __volatile__(
        "1:                     \n\t"
        "mov.l      @%2, %0             \n\t"
        "2:                     \n\t"
        "mov.l      %3, @%2             \n\t"
        "mov        #0, %1              \n\t"
        "3:                     \n\t"
        ".section .fixup, \"ax\"            \n\t"
        "4:                     \n\t"
        "mov.l      5f, %0              \n\t"
        "jmp        @%0             \n\t"
        " mov       #1, %1              \n\t"
        ".balign 4                  \n\t"
        "5: .long 3b                \n\t"
        ".previous                  \n\t"
        ".section __ex_table,\"a\"          \n\t"
        ".long 1b, 4b                   \n\t"
        ".long 2b, 4b                   \n\t"
        ".previous                  \n\t"
        : "=&r" (old), "=r" (faulted)
        : "r" (parent), "r" (return_hooker)
    );

    if (unlikely(faulted)) {
        ftrace_graph_stop();
        WARN_ON(1);
        return;
    }

    err = ftrace_push_return_trace(old, self_addr, &trace.depth, 0);
    if (err == -EBUSY) {
        __raw_writel(old, parent);
        return;
    }

    trace.func = self_addr;

    /* Only trace if the calling function expects to */
    if (!ftrace_graph_entry(&trace)) {
        current->curr_ret_stack--;
        __raw_writel(old, parent);
    }
}
#endif /* CONFIG_FUNCTION_GRAPH_TRACER */