sysrq.c

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/*
 *  Linux Magic System Request Key Hacks
 *
 *  (c) 1997 Martin Mares <[email protected]>
 *  based on ideas by Pavel Machek <[email protected]>
 *
 *  (c) 2000 Crutcher Dunnavant <[email protected]>
 *  overhauled to use key registration
 *  based upon discusions in irc://irc.openprojects.net/#kernelnewbies
 *
 *  Copyright (c) 2010 Dmitry Torokhov
 *  Input handler conversion
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/sched.h>
#include <linux/interrupt.h>
#include <linux/mm.h>
#include <linux/fs.h>
#include <linux/mount.h>
#include <linux/kdev_t.h>
#include <linux/major.h>
#include <linux/reboot.h>
#include <linux/sysrq.h>
#include <linux/kbd_kern.h>
#include <linux/proc_fs.h>
#include <linux/nmi.h>
#include <linux/quotaops.h>
#include <linux/perf_event.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/suspend.h>
#include <linux/writeback.h>
#include <linux/swap.h>
#include <linux/spinlock.h>
#include <linux/vt_kern.h>
#include <linux/workqueue.h>
#include <linux/hrtimer.h>
#include <linux/oom.h>
#include <linux/slab.h>
#include <linux/input.h>
#include <linux/uaccess.h>

#include <asm/ptrace.h>
#include <asm/irq_regs.h>

/* Whether we react on sysrq keys or just ignore them */
static int __read_mostly sysrq_enabled = SYSRQ_DEFAULT_ENABLE;
static bool __read_mostly sysrq_always_enabled;

static bool sysrq_on(void)
{
    return sysrq_enabled || sysrq_always_enabled;
}

/*
 * A value of 1 means 'all', other nonzero values are an op mask:
 */
static bool sysrq_on_mask(int mask)
{
    return sysrq_always_enabled ||
           sysrq_enabled == 1 ||
           (sysrq_enabled & mask);
}

static int __init sysrq_always_enabled_setup(char *str)
{
    sysrq_always_enabled = true;
    pr_info("sysrq always enabled.\n");

    return 1;
}

__setup("sysrq_always_enabled", sysrq_always_enabled_setup);


static void sysrq_handle_loglevel(int key)
{
    int i;

    i = key - '0';
    console_loglevel = 7;
    printk("Loglevel set to %d\n", i);
    console_loglevel = i;
}
static struct sysrq_key_op sysrq_loglevel_op = {
    .handler    = sysrq_handle_loglevel,
    .help_msg   = "loglevel(0-9)",
    .action_msg = "Changing Loglevel",
    .enable_mask    = SYSRQ_ENABLE_LOG,
};

#ifdef CONFIG_VT
static void sysrq_handle_SAK(int key)
{
    struct work_struct *SAK_work = &vc_cons[fg_console].SAK_work;
    schedule_work(SAK_work);
}
static struct sysrq_key_op sysrq_SAK_op = {
    .handler    = sysrq_handle_SAK,
    .help_msg   = "saK",
    .action_msg = "SAK",
    .enable_mask    = SYSRQ_ENABLE_KEYBOARD,
};
#else
#define sysrq_SAK_op (*(struct sysrq_key_op *)NULL)
#endif

#ifdef CONFIG_VT
static void sysrq_handle_unraw(int key)
{
    vt_reset_unicode(fg_console);
}

static struct sysrq_key_op sysrq_unraw_op = {
    .handler    = sysrq_handle_unraw,
    .help_msg   = "unRaw",
    .action_msg = "Keyboard mode set to system default",
    .enable_mask    = SYSRQ_ENABLE_KEYBOARD,
};
#else
#define sysrq_unraw_op (*(struct sysrq_key_op *)NULL)
#endif /* CONFIG_VT */

static void sysrq_handle_crash(int key)
{
    char *killer = NULL;

    panic_on_oops = 1;  /* force panic */
    wmb();
    *killer = 1;
}
static struct sysrq_key_op sysrq_crash_op = {
    .handler    = sysrq_handle_crash,
    .help_msg   = "Crash",
    .action_msg = "Trigger a crash",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};

static void sysrq_handle_reboot(int key)
{
    lockdep_off();
    local_irq_enable();
    emergency_restart();
}
static struct sysrq_key_op sysrq_reboot_op = {
    .handler    = sysrq_handle_reboot,
    .help_msg   = "reBoot",
    .action_msg = "Resetting",
    .enable_mask    = SYSRQ_ENABLE_BOOT,
};

static void sysrq_handle_sync(int key)
{
    emergency_sync();
}
static struct sysrq_key_op sysrq_sync_op = {
    .handler    = sysrq_handle_sync,
    .help_msg   = "Sync",
    .action_msg = "Emergency Sync",
    .enable_mask    = SYSRQ_ENABLE_SYNC,
};

static void sysrq_handle_show_timers(int key)
{
    sysrq_timer_list_show();
}

static struct sysrq_key_op sysrq_show_timers_op = {
    .handler    = sysrq_handle_show_timers,
    .help_msg   = "show-all-timers(Q)",
    .action_msg = "Show clockevent devices & pending hrtimers (no others)",
};

static void sysrq_handle_mountro(int key)
{
    emergency_remount();
}
static struct sysrq_key_op sysrq_mountro_op = {
    .handler    = sysrq_handle_mountro,
    .help_msg   = "Unmount",
    .action_msg = "Emergency Remount R/O",
    .enable_mask    = SYSRQ_ENABLE_REMOUNT,
};

#ifdef CONFIG_LOCKDEP
static void sysrq_handle_showlocks(int key)
{
    debug_show_all_locks();
}

static struct sysrq_key_op sysrq_showlocks_op = {
    .handler    = sysrq_handle_showlocks,
    .help_msg   = "show-all-locks(D)",
    .action_msg = "Show Locks Held",
};
#else
#define sysrq_showlocks_op (*(struct sysrq_key_op *)NULL)
#endif

#ifdef CONFIG_SMP
static DEFINE_SPINLOCK(show_lock);

static void showacpu(void *dummy)
{
    unsigned long flags;

    /* Idle CPUs have no interesting backtrace. */
    if (idle_cpu(smp_processor_id()))
        return;

    spin_lock_irqsave(&show_lock, flags);
    printk(KERN_INFO "CPU%d:\n", smp_processor_id());
    show_stack(NULL, NULL);
    spin_unlock_irqrestore(&show_lock, flags);
}

static void sysrq_showregs_othercpus(struct work_struct *dummy)
{
    smp_call_function(showacpu, NULL, 0);
}

static DECLARE_WORK(sysrq_showallcpus, sysrq_showregs_othercpus);

static void sysrq_handle_showallcpus(int key)
{
    /*
     * Fall back to the workqueue based printing if the
     * backtrace printing did not succeed or the
     * architecture has no support for it:
     */
    if (!trigger_all_cpu_backtrace()) {
        struct pt_regs *regs = get_irq_regs();

        if (regs) {
            printk(KERN_INFO "CPU%d:\n", smp_processor_id());
            show_regs(regs);
        }
        schedule_work(&sysrq_showallcpus);
    }
}

static struct sysrq_key_op sysrq_showallcpus_op = {
    .handler    = sysrq_handle_showallcpus,
    .help_msg   = "show-backtrace-all-active-cpus(L)",
    .action_msg = "Show backtrace of all active CPUs",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};
#endif

static void sysrq_handle_showregs(int key)
{
    struct pt_regs *regs = get_irq_regs();
    if (regs)
        show_regs(regs);
    perf_event_print_debug();
}
static struct sysrq_key_op sysrq_showregs_op = {
    .handler    = sysrq_handle_showregs,
    .help_msg   = "show-registers(P)",
    .action_msg = "Show Regs",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};

static void sysrq_handle_showstate(int key)
{
    show_state();
}
static struct sysrq_key_op sysrq_showstate_op = {
    .handler    = sysrq_handle_showstate,
    .help_msg   = "show-task-states(T)",
    .action_msg = "Show State",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};

static void sysrq_handle_showstate_blocked(int key)
{
    show_state_filter(TASK_UNINTERRUPTIBLE);
}
static struct sysrq_key_op sysrq_showstate_blocked_op = {
    .handler    = sysrq_handle_showstate_blocked,
    .help_msg   = "show-blocked-tasks(W)",
    .action_msg = "Show Blocked State",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};

#ifdef CONFIG_TRACING
#include <linux/ftrace.h>

static void sysrq_ftrace_dump(int key)
{
    ftrace_dump(DUMP_ALL);
}
static struct sysrq_key_op sysrq_ftrace_dump_op = {
    .handler    = sysrq_ftrace_dump,
    .help_msg   = "dump-ftrace-buffer(Z)",
    .action_msg = "Dump ftrace buffer",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};
#else
#define sysrq_ftrace_dump_op (*(struct sysrq_key_op *)NULL)
#endif

static void sysrq_handle_showmem(int key)
{
    show_mem(0);
}
static struct sysrq_key_op sysrq_showmem_op = {
    .handler    = sysrq_handle_showmem,
    .help_msg   = "show-memory-usage(M)",
    .action_msg = "Show Memory",
    .enable_mask    = SYSRQ_ENABLE_DUMP,
};

/*
 * Signal sysrq helper function.  Sends a signal to all user processes.
 */
static void send_sig_all(int sig)
{
    struct task_struct *p;

    read_lock(&tasklist_lock);
    for_each_process(p) {
        if (p->flags & PF_KTHREAD)
            continue;
        if (is_global_init(p))
            continue;

        do_send_sig_info(sig, SEND_SIG_FORCED, p, true);
    }
    read_unlock(&tasklist_lock);
}

static void sysrq_handle_term(int key)
{
    send_sig_all(SIGTERM);
    console_loglevel = 8;
}
static struct sysrq_key_op sysrq_term_op = {
    .handler    = sysrq_handle_term,
    .help_msg   = "terminate-all-tasks(E)",
    .action_msg = "Terminate All Tasks",
    .enable_mask    = SYSRQ_ENABLE_SIGNAL,
};

static void moom_callback(struct work_struct *ignored)
{
    out_of_memory(node_zonelist(0, GFP_KERNEL), GFP_KERNEL, 0, NULL, true);
}

static DECLARE_WORK(moom_work, moom_callback);

static void sysrq_handle_moom(int key)
{
    schedule_work(&moom_work);
}
static struct sysrq_key_op sysrq_moom_op = {
    .handler    = sysrq_handle_moom,
    .help_msg   = "memory-full-oom-kill(F)",
    .action_msg = "Manual OOM execution",
    .enable_mask    = SYSRQ_ENABLE_SIGNAL,
};

#ifdef CONFIG_BLOCK
static void sysrq_handle_thaw(int key)
{
    emergency_thaw_all();
}
static struct sysrq_key_op sysrq_thaw_op = {
    .handler    = sysrq_handle_thaw,
    .help_msg   = "thaw-filesystems(J)",
    .action_msg = "Emergency Thaw of all frozen filesystems",
    .enable_mask    = SYSRQ_ENABLE_SIGNAL,
};
#endif

static void sysrq_handle_kill(int key)
{
    send_sig_all(SIGKILL);
    console_loglevel = 8;
}
static struct sysrq_key_op sysrq_kill_op = {
    .handler    = sysrq_handle_kill,
    .help_msg   = "kill-all-tasks(I)",
    .action_msg = "Kill All Tasks",
    .enable_mask    = SYSRQ_ENABLE_SIGNAL,
};

static void sysrq_handle_unrt(int key)
{
    normalize_rt_tasks();
}
static struct sysrq_key_op sysrq_unrt_op = {
    .handler    = sysrq_handle_unrt,
    .help_msg   = "nice-all-RT-tasks(N)",
    .action_msg = "Nice All RT Tasks",
    .enable_mask    = SYSRQ_ENABLE_RTNICE,
};

/* Key Operations table and lock */
static DEFINE_SPINLOCK(sysrq_key_table_lock);

static struct sysrq_key_op *sysrq_key_table[36] = {
    &sysrq_loglevel_op,     /* 0 */
    &sysrq_loglevel_op,     /* 1 */
    &sysrq_loglevel_op,     /* 2 */
    &sysrq_loglevel_op,     /* 3 */
    &sysrq_loglevel_op,     /* 4 */
    &sysrq_loglevel_op,     /* 5 */
    &sysrq_loglevel_op,     /* 6 */
    &sysrq_loglevel_op,     /* 7 */
    &sysrq_loglevel_op,     /* 8 */
    &sysrq_loglevel_op,     /* 9 */

    /*
     * a: Don't use for system provided sysrqs, it is handled specially on
     * sparc and will never arrive.
     */
    NULL,               /* a */
    &sysrq_reboot_op,       /* b */
    &sysrq_crash_op,        /* c & ibm_emac driver debug */
    &sysrq_showlocks_op,        /* d */
    &sysrq_term_op,         /* e */
    &sysrq_moom_op,         /* f */
    /* g: May be registered for the kernel debugger */
    NULL,               /* g */
    NULL,               /* h - reserved for help */
    &sysrq_kill_op,         /* i */
#ifdef CONFIG_BLOCK
    &sysrq_thaw_op,         /* j */
#else
    NULL,               /* j */
#endif
    &sysrq_SAK_op,          /* k */
#ifdef CONFIG_SMP
    &sysrq_showallcpus_op,      /* l */
#else
    NULL,               /* l */
#endif
    &sysrq_showmem_op,      /* m */
    &sysrq_unrt_op,         /* n */
    /* o: This will often be registered as 'Off' at init time */
    NULL,               /* o */
    &sysrq_showregs_op,     /* p */
    &sysrq_show_timers_op,      /* q */
    &sysrq_unraw_op,        /* r */
    &sysrq_sync_op,         /* s */
    &sysrq_showstate_op,        /* t */
    &sysrq_mountro_op,      /* u */
    /* v: May be registered for frame buffer console restore */
    NULL,               /* v */
    &sysrq_showstate_blocked_op,    /* w */
    /* x: May be registered on ppc/powerpc for xmon */
    /* x: May be registered on sparc64 for global PMU dump */
    NULL,               /* x */
    /* y: May be registered on sparc64 for global register dump */
    NULL,               /* y */
    &sysrq_ftrace_dump_op,      /* z */
};

/* key2index calculation, -1 on invalid index */
static int sysrq_key_table_key2index(int key)
{
    int retval;

    if ((key >= '0') && (key <= '9'))
        retval = key - '0';
    else if ((key >= 'a') && (key <= 'z'))
        retval = key + 10 - 'a';
    else
        retval = -1;
    return retval;
}

/*
 * get and put functions for the table, exposed to modules.
 */
struct sysrq_key_op *__sysrq_get_key_op(int key)
{
        struct sysrq_key_op *op_p = NULL;
        int i;

    i = sysrq_key_table_key2index(key);
    if (i != -1)
            op_p = sysrq_key_table[i];

        return op_p;
}

static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
{
        int i = sysrq_key_table_key2index(key);

        if (i != -1)
                sysrq_key_table[i] = op_p;
}

void __handle_sysrq(int key, bool check_mask)
{
    struct sysrq_key_op *op_p;
    int orig_log_level;
    int i;
    unsigned long flags;

    spin_lock_irqsave(&sysrq_key_table_lock, flags);
    /*
     * Raise the apparent loglevel to maximum so that the sysrq header
     * is shown to provide the user with positive feedback.  We do not
     * simply emit this at KERN_EMERG as that would change message
     * routing in the consumers of /proc/kmsg.
     */
    orig_log_level = console_loglevel;
    console_loglevel = 7;
    printk(KERN_INFO "SysRq : ");

        op_p = __sysrq_get_key_op(key);
        if (op_p) {
        /*
         * Should we check for enabled operations (/proc/sysrq-trigger
         * should not) and is the invoked operation enabled?
         */
        if (!check_mask || sysrq_on_mask(op_p->enable_mask)) {
            printk("%s\n", op_p->action_msg);
            console_loglevel = orig_log_level;
            op_p->handler(key);
        } else {
            printk("This sysrq operation is disabled.\n");
        }
    } else {
        printk("HELP : ");
        /* Only print the help msg once per handler */
        for (i = 0; i < ARRAY_SIZE(sysrq_key_table); i++) {
            if (sysrq_key_table[i]) {
                int j;

                for (j = 0; sysrq_key_table[i] !=
                        sysrq_key_table[j]; j++)
                    ;
                if (j != i)
                    continue;
                printk("%s ", sysrq_key_table[i]->help_msg);
            }
        }
        printk("\n");
        console_loglevel = orig_log_level;
    }
    spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
}

void handle_sysrq(int key)
{
    if (sysrq_on())
        __handle_sysrq(key, true);
}
EXPORT_SYMBOL(handle_sysrq);

#ifdef CONFIG_INPUT

/* Simple translation table for the SysRq keys */
static const unsigned char sysrq_xlate[KEY_CNT] =
        "\000\0331234567890-=\177\t"                    /* 0x00 - 0x0f */
        "qwertyuiop[]\r\000as"                          /* 0x10 - 0x1f */
        "dfghjkl;'`\000\\zxcv"                          /* 0x20 - 0x2f */
        "bnm,./\000*\000 \000\201\202\203\204\205"      /* 0x30 - 0x3f */
        "\206\207\210\211\212\000\000789-456+1"         /* 0x40 - 0x4f */
        "230\177\000\000\213\214\000\000\000\000\000\000\000\000\000\000" /* 0x50 - 0x5f */
        "\r\000/";                                      /* 0x60 - 0x6f */

struct sysrq_state {
    struct input_handle handle;
    struct work_struct reinject_work;
    unsigned long key_down[BITS_TO_LONGS(KEY_CNT)];
    unsigned int alt;
    unsigned int alt_use;
    bool active;
    bool need_reinject;
    bool reinjecting;
};

static void sysrq_reinject_alt_sysrq(struct work_struct *work)
{
    struct sysrq_state *sysrq =
            container_of(work, struct sysrq_state, reinject_work);
    struct input_handle *handle = &sysrq->handle;
    unsigned int alt_code = sysrq->alt_use;

    if (sysrq->need_reinject) {
        /* we do not want the assignment to be reordered */
        sysrq->reinjecting = true;
        mb();

        /* Simulate press and release of Alt + SysRq */
        input_inject_event(handle, EV_KEY, alt_code, 1);
        input_inject_event(handle, EV_KEY, KEY_SYSRQ, 1);
        input_inject_event(handle, EV_SYN, SYN_REPORT, 1);

        input_inject_event(handle, EV_KEY, KEY_SYSRQ, 0);
        input_inject_event(handle, EV_KEY, alt_code, 0);
        input_inject_event(handle, EV_SYN, SYN_REPORT, 1);

        mb();
        sysrq->reinjecting = false;
    }
}

static bool sysrq_filter(struct input_handle *handle,
             unsigned int type, unsigned int code, int value)
{
    struct sysrq_state *sysrq = handle->private;
    bool was_active = sysrq->active;
    bool suppress;

    /*
     * Do not filter anything if we are in the process of re-injecting
     * Alt+SysRq combination.
     */
    if (sysrq->reinjecting)
        return false;

    switch (type) {

    case EV_SYN:
        suppress = false;
        break;

    case EV_KEY:
        switch (code) {

        case KEY_LEFTALT:
        case KEY_RIGHTALT:
            if (!value) {
                /* One of ALTs is being released */
                if (sysrq->active && code == sysrq->alt_use)
                    sysrq->active = false;

                sysrq->alt = KEY_RESERVED;

            } else if (value != 2) {
                sysrq->alt = code;
                sysrq->need_reinject = false;
            }
            break;

        case KEY_SYSRQ:
            if (value == 1 && sysrq->alt != KEY_RESERVED) {
                sysrq->active = true;
                sysrq->alt_use = sysrq->alt;
                /*
                 * If nothing else will be pressed we'll need
                 * to re-inject Alt-SysRq keysroke.
                 */
                sysrq->need_reinject = true;
            }

            /*
             * Pretend that sysrq was never pressed at all. This
             * is needed to properly handle KGDB which will try
             * to release all keys after exiting debugger. If we
             * do not clear key bit it KGDB will end up sending
             * release events for Alt and SysRq, potentially
             * triggering print screen function.
             */
            if (sysrq->active)
                clear_bit(KEY_SYSRQ, handle->dev->key);

            break;

        default:
            if (sysrq->active && value && value != 2) {
                sysrq->need_reinject = false;
                __handle_sysrq(sysrq_xlate[code], true);
            }
            break;
        }

        suppress = sysrq->active;

        if (!sysrq->active) {
            /*
             * If we are not suppressing key presses keep track of
             * keyboard state so we can release keys that have been
             * pressed before entering SysRq mode.
             */
            if (value)
                set_bit(code, sysrq->key_down);
            else
                clear_bit(code, sysrq->key_down);

            if (was_active)
                schedule_work(&sysrq->reinject_work);

        } else if (value == 0 &&
               test_and_clear_bit(code, sysrq->key_down)) {
            /*
             * Pass on release events for keys that was pressed before
             * entering SysRq mode.
             */
            suppress = false;
        }
        break;

    default:
        suppress = sysrq->active;
        break;
    }

    return suppress;
}

static int sysrq_connect(struct input_handler *handler,
             struct input_dev *dev,
             const struct input_device_id *id)
{
    struct sysrq_state *sysrq;
    int error;

    sysrq = kzalloc(sizeof(struct sysrq_state), GFP_KERNEL);
    if (!sysrq)
        return -ENOMEM;

    INIT_WORK(&sysrq->reinject_work, sysrq_reinject_alt_sysrq);

    sysrq->handle.dev = dev;
    sysrq->handle.handler = handler;
    sysrq->handle.name = "sysrq";
    sysrq->handle.private = sysrq;

    error = input_register_handle(&sysrq->handle);
    if (error) {
        pr_err("Failed to register input sysrq handler, error %d\n",
            error);
        goto err_free;
    }

    error = input_open_device(&sysrq->handle);
    if (error) {
        pr_err("Failed to open input device, error %d\n", error);
        goto err_unregister;
    }

    return 0;

 err_unregister:
    input_unregister_handle(&sysrq->handle);
 err_free:
    kfree(sysrq);
    return error;
}

static void sysrq_disconnect(struct input_handle *handle)
{
    struct sysrq_state *sysrq = handle->private;

    input_close_device(handle);
    cancel_work_sync(&sysrq->reinject_work);
    input_unregister_handle(handle);
    kfree(sysrq);
}

/*
 * We are matching on KEY_LEFTALT instead of KEY_SYSRQ because not all
 * keyboards have SysRq key predefined and so user may add it to keymap
 * later, but we expect all such keyboards to have left alt.
 */
static const struct input_device_id sysrq_ids[] = {
    {
        .flags = INPUT_DEVICE_ID_MATCH_EVBIT |
                INPUT_DEVICE_ID_MATCH_KEYBIT,
        .evbit = { BIT_MASK(EV_KEY) },
        .keybit = { BIT_MASK(KEY_LEFTALT) },
    },
    { },
};

static struct input_handler sysrq_handler = {
    .filter     = sysrq_filter,
    .connect    = sysrq_connect,
    .disconnect = sysrq_disconnect,
    .name       = "sysrq",
    .id_table   = sysrq_ids,
};

static bool sysrq_handler_registered;

static inline void sysrq_register_handler(void)
{
    int error;

    error = input_register_handler(&sysrq_handler);
    if (error)
        pr_err("Failed to register input handler, error %d", error);
    else
        sysrq_handler_registered = true;
}

static inline void sysrq_unregister_handler(void)
{
    if (sysrq_handler_registered) {
        input_unregister_handler(&sysrq_handler);
        sysrq_handler_registered = false;
    }
}

#else

static inline void sysrq_register_handler(void)
{
}

static inline void sysrq_unregister_handler(void)
{
}

#endif /* CONFIG_INPUT */

int sysrq_toggle_support(int enable_mask)
{
    bool was_enabled = sysrq_on();

    sysrq_enabled = enable_mask;

    if (was_enabled != sysrq_on()) {
        if (sysrq_on())
            sysrq_register_handler();
        else
            sysrq_unregister_handler();
    }

    return 0;
}

static int __sysrq_swap_key_ops(int key, struct sysrq_key_op *insert_op_p,
                                struct sysrq_key_op *remove_op_p)
{
    int retval;
    unsigned long flags;

    spin_lock_irqsave(&sysrq_key_table_lock, flags);
    if (__sysrq_get_key_op(key) == remove_op_p) {
        __sysrq_put_key_op(key, insert_op_p);
        retval = 0;
    } else {
        retval = -1;
    }
    spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
    return retval;
}

int register_sysrq_key(int key, struct sysrq_key_op *op_p)
{
    return __sysrq_swap_key_ops(key, op_p, NULL);
}
EXPORT_SYMBOL(register_sysrq_key);

int unregister_sysrq_key(int key, struct sysrq_key_op *op_p)
{
    return __sysrq_swap_key_ops(key, NULL, op_p);
}
EXPORT_SYMBOL(unregister_sysrq_key);

#ifdef CONFIG_PROC_FS
/*
 * writing 'C' to /proc/sysrq-trigger is like sysrq-C
 */
static ssize_t write_sysrq_trigger(struct file *file, const char __user *buf,
                   size_t count, loff_t *ppos)
{
    if (count) {
        char c;

        if (get_user(c, buf))
            return -EFAULT;
        __handle_sysrq(c, false);
    }

    return count;
}

static const struct file_operations proc_sysrq_trigger_operations = {
    .write      = write_sysrq_trigger,
    .llseek     = noop_llseek,
};

static void sysrq_init_procfs(void)
{
    if (!proc_create("sysrq-trigger", S_IWUSR, NULL,
             &proc_sysrq_trigger_operations))
        pr_err("Failed to register proc interface\n");
}

#else

static inline void sysrq_init_procfs(void)
{
}

#endif /* CONFIG_PROC_FS */

static int __init sysrq_init(void)
{
    sysrq_init_procfs();

    if (sysrq_on())
        sysrq_register_handler();

    return 0;
}
module_init(sysrq_init);