Merge tag 'trace-printf-v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/trace...

[drm/drm-misc.git] / kernel / debug / debug_core.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Kernel Debug Core
 *
 * Maintainer: Jason Wessel <jason.wessel@windriver.com>
 *
 * Copyright (C) 2000-2001 VERITAS Software Corporation.
 * Copyright (C) 2002-2004 Timesys Corporation
 * Copyright (C) 2003-2004 Amit S. Kale <amitkale@linsyssoft.com>
 * Copyright (C) 2004 Pavel Machek <pavel@ucw.cz>
 * Copyright (C) 2004-2006 Tom Rini <trini@kernel.crashing.org>
 * Copyright (C) 2004-2006 LinSysSoft Technologies Pvt. Ltd.
 * Copyright (C) 2005-2009 Wind River Systems, Inc.
 * Copyright (C) 2007 MontaVista Software, Inc.
 * Copyright (C) 2008 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
 *
 * Contributors at various stages not listed above:
 *  Jason Wessel ( jason.wessel@windriver.com )
 *  George Anzinger <george@mvista.com>
 *  Anurekh Saxena (anurekh.saxena@timesys.com)
 *  Lake Stevens Instrument Division (Glenn Engel)
 *  Jim Kingdon, Cygnus Support.
 *
 * Original KGDB stub: David Grothe <dave@gcom.com>,
 * Tigran Aivazian <tigran@sco.com>
 */

#define pr_fmt(fmt) "KGDB: " fmt

#include <linux/pid_namespace.h>
#include <linux/clocksource.h>
#include <linux/serial_core.h>
#include <linux/interrupt.h>
#include <linux/spinlock.h>
#include <linux/console.h>
#include <linux/threads.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/string.h>
#include <linux/delay.h>
#include <linux/sched.h>
#include <linux/sysrq.h>
#include <linux/reboot.h>
#include <linux/init.h>
#include <linux/kgdb.h>
#include <linux/kdb.h>
#include <linux/nmi.h>
#include <linux/pid.h>
#include <linux/smp.h>
#include <linux/mm.h>
#include <linux/rcupdate.h>
#include <linux/irq.h>
#include <linux/security.h>

#include <asm/cacheflush.h>
#include <asm/byteorder.h>
#include <linux/atomic.h>

#include "debug_core.h"

static int kgdb_break_asap;

struct debuggerinfo_struct kgdb_info[NR_CPUS];

/* kgdb_connected - Is a host GDB connected to us? */
int                             kgdb_connected;
EXPORT_SYMBOL_GPL(kgdb_connected);

/* All the KGDB handlers are installed */
int                     kgdb_io_module_registered;

/* Guard for recursive entry */
static int                      exception_level;

struct kgdb_io          *dbg_io_ops;
static DEFINE_SPINLOCK(kgdb_registration_lock);

/* Action for the reboot notifier, a global allow kdb to change it */
static int kgdbreboot;
/* kgdb console driver is loaded */
static int kgdb_con_registered;
/* determine if kgdb console output should be used */
static int kgdb_use_con;
/* Flag for alternate operations for early debugging */
bool dbg_is_early = true;
/* Next cpu to become the master debug core */
int dbg_switch_cpu;

/* Use kdb or gdbserver mode */
int dbg_kdb_mode = 1;

module_param(kgdb_use_con, int, 0644);
module_param(kgdbreboot, int, 0644);

/*
 * Holds information about breakpoints in a kernel. These breakpoints are
 * added and removed by gdb.
 */
static struct kgdb_bkpt         kgdb_break[KGDB_MAX_BREAKPOINTS] = {
        [0 ... KGDB_MAX_BREAKPOINTS-1] = { .state = BP_UNDEFINED }
};

/*
 * The CPU# of the active CPU, or -1 if none:
 */
atomic_t                        kgdb_active = ATOMIC_INIT(-1);
EXPORT_SYMBOL_GPL(kgdb_active);
static DEFINE_RAW_SPINLOCK(dbg_master_lock);
static DEFINE_RAW_SPINLOCK(dbg_slave_lock);

/*
 * We use NR_CPUs not PERCPU, in case kgdb is used to debug early
 * bootup code (which might not have percpu set up yet):
 */
static atomic_t                 masters_in_kgdb;
static atomic_t                 slaves_in_kgdb;
atomic_t                        kgdb_setting_breakpoint;

struct task_struct              *kgdb_usethread;
struct task_struct              *kgdb_contthread;

int                             kgdb_single_step;
static pid_t                    kgdb_sstep_pid;

/* to keep track of the CPU which is doing the single stepping*/
atomic_t                        kgdb_cpu_doing_single_step = ATOMIC_INIT(-1);

/*
 * If you are debugging a problem where roundup (the collection of
 * all other CPUs) is a problem [this should be extremely rare],
 * then use the nokgdbroundup option to avoid roundup. In that case
 * the other CPUs might interfere with your debugging context, so
 * use this with care:
 */
static int kgdb_do_roundup = 1;

static int __init opt_nokgdbroundup(char *str)
{
        kgdb_do_roundup = 0;

        return 0;
}

early_param("nokgdbroundup", opt_nokgdbroundup);

/*
 * Finally, some KGDB code :-)
 */

/*
 * Weak aliases for breakpoint management,
 * can be overridden by architectures when needed:
 */
int __weak kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
{
        int err;

        err = copy_from_kernel_nofault(bpt->saved_instr, (char *)bpt->bpt_addr,
                                BREAK_INSTR_SIZE);
        if (err)
                return err;
        err = copy_to_kernel_nofault((char *)bpt->bpt_addr,
                                 arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE);
        return err;
}
NOKPROBE_SYMBOL(kgdb_arch_set_breakpoint);

int __weak kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
{
        return copy_to_kernel_nofault((char *)bpt->bpt_addr,
                                  (char *)bpt->saved_instr, BREAK_INSTR_SIZE);
}
NOKPROBE_SYMBOL(kgdb_arch_remove_breakpoint);

int __weak kgdb_validate_break_address(unsigned long addr)
{
        struct kgdb_bkpt tmp;
        int err;

        if (kgdb_within_blocklist(addr))
                return -EINVAL;

        /* Validate setting the breakpoint and then removing it.  If the
         * remove fails, the kernel needs to emit a bad message because we
         * are deep trouble not being able to put things back the way we
         * found them.
         */
        tmp.bpt_addr = addr;
        err = kgdb_arch_set_breakpoint(&tmp);
        if (err)
                return err;
        err = kgdb_arch_remove_breakpoint(&tmp);
        if (err)
                pr_err("Critical breakpoint error, kernel memory destroyed at: %lx\n",
                       addr);
        return err;
}

unsigned long __weak kgdb_arch_pc(int exception, struct pt_regs *regs)
{
        return instruction_pointer(regs);
}
NOKPROBE_SYMBOL(kgdb_arch_pc);

int __weak kgdb_arch_init(void)
{
        return 0;
}

int __weak kgdb_skipexception(int exception, struct pt_regs *regs)
{
        return 0;
}
NOKPROBE_SYMBOL(kgdb_skipexception);

#ifdef CONFIG_SMP

/*
 * Default (weak) implementation for kgdb_roundup_cpus
 */

void __weak kgdb_call_nmi_hook(void *ignored)
{
        /*
         * NOTE: get_irq_regs() is supposed to get the registers from
         * before the IPI interrupt happened and so is supposed to
         * show where the processor was.  In some situations it's
         * possible we might be called without an IPI, so it might be
         * safer to figure out how to make kgdb_breakpoint() work
         * properly here.
         */
        kgdb_nmicallback(raw_smp_processor_id(), get_irq_regs());
}
NOKPROBE_SYMBOL(kgdb_call_nmi_hook);

static DEFINE_PER_CPU(call_single_data_t, kgdb_roundup_csd) =
        CSD_INIT(kgdb_call_nmi_hook, NULL);

void __weak kgdb_roundup_cpus(void)
{
        call_single_data_t *csd;
        int this_cpu = raw_smp_processor_id();
        int cpu;
        int ret;

        for_each_online_cpu(cpu) {
                /* No need to roundup ourselves */
                if (cpu == this_cpu)
                        continue;

                csd = &per_cpu(kgdb_roundup_csd, cpu);

                /*
                 * If it didn't round up last time, don't try again
                 * since smp_call_function_single_async() will block.
                 *
                 * If rounding_up is false then we know that the
                 * previous call must have at least started and that
                 * means smp_call_function_single_async() won't block.
                 */
                if (kgdb_info[cpu].rounding_up)
                        continue;
                kgdb_info[cpu].rounding_up = true;

                ret = smp_call_function_single_async(cpu, csd);
                if (ret)
                        kgdb_info[cpu].rounding_up = false;
        }
}
NOKPROBE_SYMBOL(kgdb_roundup_cpus);

#endif

/*
 * Some architectures need cache flushes when we set/clear a
 * breakpoint:
 */
static void kgdb_flush_swbreak_addr(unsigned long addr)
{
        if (!CACHE_FLUSH_IS_SAFE)
                return;

        /* Force flush instruction cache if it was outside the mm */
        flush_icache_range(addr, addr + BREAK_INSTR_SIZE);
}
NOKPROBE_SYMBOL(kgdb_flush_swbreak_addr);

/*
 * SW breakpoint management:
 */
int dbg_activate_sw_breakpoints(void)
{
        int error;
        int ret = 0;
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state != BP_SET)
                        continue;

                error = kgdb_arch_set_breakpoint(&kgdb_break[i]);
                if (error) {
                        ret = error;
                        pr_info("BP install failed: %lx\n",
                                kgdb_break[i].bpt_addr);
                        continue;
                }

                kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
                kgdb_break[i].state = BP_ACTIVE;
        }
        return ret;
}
NOKPROBE_SYMBOL(dbg_activate_sw_breakpoints);

int dbg_set_sw_break(unsigned long addr)
{
        int err = kgdb_validate_break_address(addr);
        int breakno = -1;
        int i;

        if (err)
                return err;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if ((kgdb_break[i].state == BP_SET) &&
                                        (kgdb_break[i].bpt_addr == addr))
                        return -EEXIST;
        }
        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state == BP_REMOVED &&
                                        kgdb_break[i].bpt_addr == addr) {
                        breakno = i;
                        break;
                }
        }

        if (breakno == -1) {
                for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                        if (kgdb_break[i].state == BP_UNDEFINED) {
                                breakno = i;
                                break;
                        }
                }
        }

        if (breakno == -1)
                return -E2BIG;

        kgdb_break[breakno].state = BP_SET;
        kgdb_break[breakno].type = BP_BREAKPOINT;
        kgdb_break[breakno].bpt_addr = addr;

        return 0;
}

int dbg_deactivate_sw_breakpoints(void)
{
        int error;
        int ret = 0;
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state != BP_ACTIVE)
                        continue;
                error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
                if (error) {
                        pr_info("BP remove failed: %lx\n",
                                kgdb_break[i].bpt_addr);
                        ret = error;
                }

                kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
                kgdb_break[i].state = BP_SET;
        }
        return ret;
}
NOKPROBE_SYMBOL(dbg_deactivate_sw_breakpoints);

int dbg_remove_sw_break(unsigned long addr)
{
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if ((kgdb_break[i].state == BP_SET) &&
                                (kgdb_break[i].bpt_addr == addr)) {
                        kgdb_break[i].state = BP_REMOVED;
                        return 0;
                }
        }
        return -ENOENT;
}

int kgdb_isremovedbreak(unsigned long addr)
{
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if ((kgdb_break[i].state == BP_REMOVED) &&
                                        (kgdb_break[i].bpt_addr == addr))
                        return 1;
        }
        return 0;
}

int kgdb_has_hit_break(unsigned long addr)
{
        int i;

        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state == BP_ACTIVE &&
                    kgdb_break[i].bpt_addr == addr)
                        return 1;
        }
        return 0;
}

int dbg_remove_all_break(void)
{
        int error;
        int i;

        /* Clear memory breakpoints. */
        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (kgdb_break[i].state != BP_ACTIVE)
                        goto setundefined;
                error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
                if (error)
                        pr_err("breakpoint remove failed: %lx\n",
                               kgdb_break[i].bpt_addr);
setundefined:
                kgdb_break[i].state = BP_UNDEFINED;
        }

        /* Clear hardware breakpoints. */
        if (arch_kgdb_ops.remove_all_hw_break)
                arch_kgdb_ops.remove_all_hw_break();

        return 0;
}

void kgdb_free_init_mem(void)
{
        int i;

        /* Clear init memory breakpoints. */
        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
                if (init_section_contains((void *)kgdb_break[i].bpt_addr, 0))
                        kgdb_break[i].state = BP_UNDEFINED;
        }
}

#ifdef CONFIG_KGDB_KDB
void kdb_dump_stack_on_cpu(int cpu)
{
        if (cpu == raw_smp_processor_id() || !IS_ENABLED(CONFIG_SMP)) {
                dump_stack();
                return;
        }

        if (!(kgdb_info[cpu].exception_state & DCPU_IS_SLAVE)) {
                kdb_printf("ERROR: Task on cpu %d didn't stop in the debugger\n",
                           cpu);
                return;
        }

        /*
         * In general, architectures don't support dumping the stack of a
         * "running" process that's not the current one.  From the point of
         * view of the Linux, kernel processes that are looping in the kgdb
         * slave loop are still "running".  There's also no API (that actually
         * works across all architectures) that can do a stack crawl based
         * on registers passed as a parameter.
         *
         * Solve this conundrum by asking slave CPUs to do the backtrace
         * themselves.
         */
        kgdb_info[cpu].exception_state |= DCPU_WANT_BT;
        while (kgdb_info[cpu].exception_state & DCPU_WANT_BT)
                cpu_relax();
}
#endif

/*
 * Return true if there is a valid kgdb I/O module.  Also if no
 * debugger is attached a message can be printed to the console about
 * waiting for the debugger to attach.
 *
 * The print_wait argument is only to be true when called from inside
 * the core kgdb_handle_exception, because it will wait for the
 * debugger to attach.
 */
static int kgdb_io_ready(int print_wait)
{
        if (!dbg_io_ops)
                return 0;
        if (kgdb_connected)
                return 1;
        if (atomic_read(&kgdb_setting_breakpoint))
                return 1;
        if (print_wait) {
#ifdef CONFIG_KGDB_KDB
                if (!dbg_kdb_mode)
                        pr_crit("waiting... or $3#33 for KDB\n");
#else
                pr_crit("Waiting for remote debugger\n");
#endif
        }
        return 1;
}
NOKPROBE_SYMBOL(kgdb_io_ready);

static int kgdb_reenter_check(struct kgdb_state *ks)
{
        unsigned long addr;

        if (atomic_read(&kgdb_active) != raw_smp_processor_id())
                return 0;

        /* Panic on recursive debugger calls: */
        exception_level++;
        addr = kgdb_arch_pc(ks->ex_vector, ks->linux_regs);
        dbg_deactivate_sw_breakpoints();

        /*
         * If the break point removed ok at the place exception
         * occurred, try to recover and print a warning to the end
         * user because the user planted a breakpoint in a place that
         * KGDB needs in order to function.
         */
        if (dbg_remove_sw_break(addr) == 0) {
                exception_level = 0;
                kgdb_skipexception(ks->ex_vector, ks->linux_regs);
                dbg_activate_sw_breakpoints();
                pr_crit("re-enter error: breakpoint removed %lx\n", addr);
                WARN_ON_ONCE(1);

                return 1;
        }
        dbg_remove_all_break();
        kgdb_skipexception(ks->ex_vector, ks->linux_regs);

        if (exception_level > 1) {
                dump_stack();
                kgdb_io_module_registered = false;
                panic("Recursive entry to debugger");
        }

        pr_crit("re-enter exception: ALL breakpoints killed\n");
#ifdef CONFIG_KGDB_KDB
        /* Allow kdb to debug itself one level */
        return 0;
#endif
        dump_stack();
        panic("Recursive entry to debugger");

        return 1;
}
NOKPROBE_SYMBOL(kgdb_reenter_check);

static void dbg_touch_watchdogs(void)
{
        touch_softlockup_watchdog_sync();
        clocksource_touch_watchdog();
        rcu_cpu_stall_reset();
}
NOKPROBE_SYMBOL(dbg_touch_watchdogs);

static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
                int exception_state)
{
        unsigned long flags;
        int sstep_tries = 100;
        int error;
        int cpu;
        int trace_on = 0;
        int online_cpus = num_online_cpus();
        u64 time_left;

        kgdb_info[ks->cpu].enter_kgdb++;
        kgdb_info[ks->cpu].exception_state |= exception_state;

        if (exception_state == DCPU_WANT_MASTER)
                atomic_inc(&masters_in_kgdb);
        else
                atomic_inc(&slaves_in_kgdb);

        if (arch_kgdb_ops.disable_hw_break)
                arch_kgdb_ops.disable_hw_break(regs);

acquirelock:
        rcu_read_lock();
        /*
         * Interrupts will be restored by the 'trap return' code, except when
         * single stepping.
         */
        local_irq_save(flags);

        cpu = ks->cpu;
        kgdb_info[cpu].debuggerinfo = regs;
        kgdb_info[cpu].task = current;
        kgdb_info[cpu].ret_state = 0;
        kgdb_info[cpu].irq_depth = hardirq_count() >> HARDIRQ_SHIFT;

        /* Make sure the above info reaches the primary CPU */
        smp_mb();

        if (exception_level == 1) {
                if (raw_spin_trylock(&dbg_master_lock))
                        atomic_xchg(&kgdb_active, cpu);
                goto cpu_master_loop;
        }

        /*
         * CPU will loop if it is a slave or request to become a kgdb
         * master cpu and acquire the kgdb_active lock:
         */
        while (1) {
cpu_loop:
                if (kgdb_info[cpu].exception_state & DCPU_NEXT_MASTER) {
                        kgdb_info[cpu].exception_state &= ~DCPU_NEXT_MASTER;
                        goto cpu_master_loop;
                } else if (kgdb_info[cpu].exception_state & DCPU_WANT_MASTER) {
                        if (raw_spin_trylock(&dbg_master_lock)) {
                                atomic_xchg(&kgdb_active, cpu);
                                break;
                        }
                } else if (kgdb_info[cpu].exception_state & DCPU_WANT_BT) {
                        dump_stack();
                        kgdb_info[cpu].exception_state &= ~DCPU_WANT_BT;
                } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
                        if (!raw_spin_is_locked(&dbg_slave_lock))
                                goto return_normal;
                } else {
return_normal:
                        /* Return to normal operation by executing any
                         * hw breakpoint fixup.
                         */
                        if (arch_kgdb_ops.correct_hw_break)
                                arch_kgdb_ops.correct_hw_break();
                        if (trace_on)
                                tracing_on();
                        kgdb_info[cpu].debuggerinfo = NULL;
                        kgdb_info[cpu].task = NULL;
                        kgdb_info[cpu].exception_state &=
                                ~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
                        kgdb_info[cpu].enter_kgdb--;
                        smp_mb__before_atomic();
                        atomic_dec(&slaves_in_kgdb);
                        dbg_touch_watchdogs();
                        local_irq_restore(flags);
                        rcu_read_unlock();
                        return 0;
                }
                cpu_relax();
        }

        /*
         * For single stepping, try to only enter on the processor
         * that was single stepping.  To guard against a deadlock, the
         * kernel will only try for the value of sstep_tries before
         * giving up and continuing on.
         */
        if (atomic_read(&kgdb_cpu_doing_single_step) != -1 &&
            (kgdb_info[cpu].task &&
             kgdb_info[cpu].task->pid != kgdb_sstep_pid) && --sstep_tries) {
                atomic_set(&kgdb_active, -1);
                raw_spin_unlock(&dbg_master_lock);
                dbg_touch_watchdogs();
                local_irq_restore(flags);
                rcu_read_unlock();

                goto acquirelock;
        }

        if (!kgdb_io_ready(1)) {
                kgdb_info[cpu].ret_state = 1;
                goto kgdb_restore; /* No I/O connection, resume the system */
        }

        /*
         * Don't enter if we have hit a removed breakpoint.
         */
        if (kgdb_skipexception(ks->ex_vector, ks->linux_regs))
                goto kgdb_restore;

        atomic_inc(&ignore_console_lock_warning);

        /* Call the I/O driver's pre_exception routine */
        if (dbg_io_ops->pre_exception)
                dbg_io_ops->pre_exception();

        /*
         * Get the passive CPU lock which will hold all the non-primary
         * CPU in a spin state while the debugger is active
         */
        if (!kgdb_single_step)
                raw_spin_lock(&dbg_slave_lock);

#ifdef CONFIG_SMP
        /* If send_ready set, slaves are already waiting */
        if (ks->send_ready)
                atomic_set(ks->send_ready, 1);

        /* Signal the other CPUs to enter kgdb_wait() */
        else if ((!kgdb_single_step) && kgdb_do_roundup)
                kgdb_roundup_cpus();
#endif

        /*
         * Wait for the other CPUs to be notified and be waiting for us:
         */
        time_left = MSEC_PER_SEC;
        while (kgdb_do_roundup && --time_left &&
               (atomic_read(&masters_in_kgdb) + atomic_read(&slaves_in_kgdb)) !=
                   online_cpus)
                udelay(1000);
        if (!time_left)
                pr_crit("Timed out waiting for secondary CPUs.\n");

        /*
         * At this point the primary processor is completely
         * in the debugger and all secondary CPUs are quiescent
         */
        dbg_deactivate_sw_breakpoints();
        kgdb_single_step = 0;
        kgdb_contthread = current;
        exception_level = 0;
        trace_on = tracing_is_on();
        if (trace_on)
                tracing_off();

        while (1) {
cpu_master_loop:
                if (dbg_kdb_mode) {
                        kgdb_connected = 1;
                        error = kdb_stub(ks);
                        if (error == -1)
                                continue;
                        kgdb_connected = 0;
                } else {
                        /*
                         * This is a brutal way to interfere with the debugger
                         * and prevent gdb being used to poke at kernel memory.
                         * This could cause trouble if lockdown is applied when
                         * there is already an active gdb session. For now the
                         * answer is simply "don't do that". Typically lockdown
                         * *will* be applied before the debug core gets started
                         * so only developers using kgdb for fairly advanced
                         * early kernel debug can be biten by this. Hopefully
                         * they are sophisticated enough to take care of
                         * themselves, especially with help from the lockdown
                         * message printed on the console!
                         */
                        if (security_locked_down(LOCKDOWN_DBG_WRITE_KERNEL)) {
                                if (IS_ENABLED(CONFIG_KGDB_KDB)) {
                                        /* Switch back to kdb if possible... */
                                        dbg_kdb_mode = 1;
                                        continue;
                                } else {
                                        /* ... otherwise just bail */
                                        break;
                                }
                        }
                        error = gdb_serial_stub(ks);
                }

                if (error == DBG_PASS_EVENT) {
                        dbg_kdb_mode = !dbg_kdb_mode;
                } else if (error == DBG_SWITCH_CPU_EVENT) {
                        kgdb_info[dbg_switch_cpu].exception_state |=
                                DCPU_NEXT_MASTER;
                        goto cpu_loop;
                } else {
                        kgdb_info[cpu].ret_state = error;
                        break;
                }
        }

        dbg_activate_sw_breakpoints();

        /* Call the I/O driver's post_exception routine */
        if (dbg_io_ops->post_exception)
                dbg_io_ops->post_exception();

        atomic_dec(&ignore_console_lock_warning);

        if (!kgdb_single_step) {
                raw_spin_unlock(&dbg_slave_lock);
                /* Wait till all the CPUs have quit from the debugger. */
                while (kgdb_do_roundup && atomic_read(&slaves_in_kgdb))
                        cpu_relax();
        }

kgdb_restore:
        if (atomic_read(&kgdb_cpu_doing_single_step) != -1) {
                int sstep_cpu = atomic_read(&kgdb_cpu_doing_single_step);
                if (kgdb_info[sstep_cpu].task)
                        kgdb_sstep_pid = kgdb_info[sstep_cpu].task->pid;
                else
                        kgdb_sstep_pid = 0;
        }
        if (arch_kgdb_ops.correct_hw_break)
                arch_kgdb_ops.correct_hw_break();
        if (trace_on)
                tracing_on();

        kgdb_info[cpu].debuggerinfo = NULL;
        kgdb_info[cpu].task = NULL;
        kgdb_info[cpu].exception_state &=
                ~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
        kgdb_info[cpu].enter_kgdb--;
        smp_mb__before_atomic();
        atomic_dec(&masters_in_kgdb);
        /* Free kgdb_active */
        atomic_set(&kgdb_active, -1);
        raw_spin_unlock(&dbg_master_lock);
        dbg_touch_watchdogs();
        local_irq_restore(flags);
        rcu_read_unlock();

        return kgdb_info[cpu].ret_state;
}
NOKPROBE_SYMBOL(kgdb_cpu_enter);

/*
 * kgdb_handle_exception() - main entry point from a kernel exception
 *
 * Locking hierarchy:
 *      interface locks, if any (begin_session)
 *      kgdb lock (kgdb_active)
 */
int
kgdb_handle_exception(int evector, int signo, int ecode, struct pt_regs *regs)
{
        struct kgdb_state kgdb_var;
        struct kgdb_state *ks = &kgdb_var;
        int ret = 0;

        if (arch_kgdb_ops.enable_nmi)
                arch_kgdb_ops.enable_nmi(0);
        /*
         * Avoid entering the debugger if we were triggered due to an oops
         * but panic_timeout indicates the system should automatically
         * reboot on panic. We don't want to get stuck waiting for input
         * on such systems, especially if its "just" an oops.
         */
        if (signo != SIGTRAP && panic_timeout)
                return 1;

        memset(ks, 0, sizeof(struct kgdb_state));
        ks->cpu                 = raw_smp_processor_id();
        ks->ex_vector           = evector;
        ks->signo               = signo;
        ks->err_code            = ecode;
        ks->linux_regs          = regs;

        if (kgdb_reenter_check(ks))
                goto out; /* Ouch, double exception ! */
        if (kgdb_info[ks->cpu].enter_kgdb != 0)
                goto out;

        ret = kgdb_cpu_enter(ks, regs, DCPU_WANT_MASTER);
out:
        if (arch_kgdb_ops.enable_nmi)
                arch_kgdb_ops.enable_nmi(1);
        return ret;
}
NOKPROBE_SYMBOL(kgdb_handle_exception);

/*
 * GDB places a breakpoint at this function to know dynamically loaded objects.
 */
static int module_event(struct notifier_block *self, unsigned long val,
        void *data)
{
        return 0;
}

static struct notifier_block dbg_module_load_nb = {
        .notifier_call  = module_event,
};

int kgdb_nmicallback(int cpu, void *regs)
{
#ifdef CONFIG_SMP
        struct kgdb_state kgdb_var;
        struct kgdb_state *ks = &kgdb_var;

        kgdb_info[cpu].rounding_up = false;

        memset(ks, 0, sizeof(struct kgdb_state));
        ks->cpu                 = cpu;
        ks->linux_regs          = regs;

        if (kgdb_info[ks->cpu].enter_kgdb == 0 &&
                        raw_spin_is_locked(&dbg_master_lock)) {
                kgdb_cpu_enter(ks, regs, DCPU_IS_SLAVE);
                return 0;
        }
#endif
        return 1;
}
NOKPROBE_SYMBOL(kgdb_nmicallback);

int kgdb_nmicallin(int cpu, int trapnr, void *regs, int err_code,
                                                        atomic_t *send_ready)
{
#ifdef CONFIG_SMP
        if (!kgdb_io_ready(0) || !send_ready)
                return 1;

        if (kgdb_info[cpu].enter_kgdb == 0) {
                struct kgdb_state kgdb_var;
                struct kgdb_state *ks = &kgdb_var;

                memset(ks, 0, sizeof(struct kgdb_state));
                ks->cpu                 = cpu;
                ks->ex_vector           = trapnr;
                ks->signo               = SIGTRAP;
                ks->err_code            = err_code;
                ks->linux_regs          = regs;
                ks->send_ready          = send_ready;
                kgdb_cpu_enter(ks, regs, DCPU_WANT_MASTER);
                return 0;
        }
#endif
        return 1;
}
NOKPROBE_SYMBOL(kgdb_nmicallin);

static void kgdb_console_write(struct console *co, const char *s,
   unsigned count)
{
        unsigned long flags;

        /* If we're debugging, or KGDB has not connected, don't try
         * and print. */
        if (!kgdb_connected || atomic_read(&kgdb_active) != -1 || dbg_kdb_mode)
                return;

        local_irq_save(flags);
        gdbstub_msg_write(s, count);
        local_irq_restore(flags);
}

static struct console kgdbcons = {
        .name           = "kgdb",
        .write          = kgdb_console_write,
        .flags          = CON_PRINTBUFFER | CON_ENABLED,
        .index          = -1,
};

static int __init opt_kgdb_con(char *str)
{
        kgdb_use_con = 1;

        if (kgdb_io_module_registered && !kgdb_con_registered) {
                register_console(&kgdbcons);
                kgdb_con_registered = 1;
        }

        return 0;
}

early_param("kgdbcon", opt_kgdb_con);

#ifdef CONFIG_MAGIC_SYSRQ
static void sysrq_handle_dbg(u8 key)
{
        if (!dbg_io_ops) {
                pr_crit("ERROR: No KGDB I/O module available\n");
                return;
        }
        if (!kgdb_connected) {
#ifdef CONFIG_KGDB_KDB
                if (!dbg_kdb_mode)
                        pr_crit("KGDB or $3#33 for KDB\n");
#else
                pr_crit("Entering KGDB\n");
#endif
        }

        kgdb_breakpoint();
}

static const struct sysrq_key_op sysrq_dbg_op = {
        .handler        = sysrq_handle_dbg,
        .help_msg       = "debug(g)",
        .action_msg     = "DEBUG",
};
#endif

void kgdb_panic(const char *msg)
{
        if (!kgdb_io_module_registered)
                return;

        /*
         * We don't want to get stuck waiting for input from user if
         * "panic_timeout" indicates the system should automatically
         * reboot on panic.
         */
        if (panic_timeout)
                return;

        debug_locks_off();
        console_flush_on_panic(CONSOLE_FLUSH_PENDING);

        if (dbg_kdb_mode)
                kdb_printf("PANIC: %s\n", msg);

        kgdb_breakpoint();
}

static void kgdb_initial_breakpoint(void)
{
        kgdb_break_asap = 0;

        pr_crit("Waiting for connection from remote gdb...\n");
        kgdb_breakpoint();
}

void __weak kgdb_arch_late(void)
{
}

void __init dbg_late_init(void)
{
        dbg_is_early = false;
        if (kgdb_io_module_registered)
                kgdb_arch_late();
        kdb_init(KDB_INIT_FULL);

        if (kgdb_io_module_registered && kgdb_break_asap)
                kgdb_initial_breakpoint();
}

static int
dbg_notify_reboot(struct notifier_block *this, unsigned long code, void *x)
{
        /*
         * Take the following action on reboot notify depending on value:
         *    1 == Enter debugger
         *    0 == [the default] detach debug client
         *   -1 == Do nothing... and use this until the board resets
         */
        switch (kgdbreboot) {
        case 1:
                kgdb_breakpoint();
                goto done;
        case -1:
                goto done;
        }
        if (!dbg_kdb_mode)
                gdbstub_exit(code);
done:
        return NOTIFY_DONE;
}

static struct notifier_block dbg_reboot_notifier = {
        .notifier_call          = dbg_notify_reboot,
        .next                   = NULL,
        .priority               = INT_MAX,
};

static void kgdb_register_callbacks(void)
{
        if (!kgdb_io_module_registered) {
                kgdb_io_module_registered = 1;
                kgdb_arch_init();
                if (!dbg_is_early)
                        kgdb_arch_late();
                register_module_notifier(&dbg_module_load_nb);
                register_reboot_notifier(&dbg_reboot_notifier);
#ifdef CONFIG_MAGIC_SYSRQ
                register_sysrq_key('g', &sysrq_dbg_op);
#endif
                if (kgdb_use_con && !kgdb_con_registered) {
                        register_console(&kgdbcons);
                        kgdb_con_registered = 1;
                }
        }
}

static void kgdb_unregister_callbacks(void)
{
        /*
         * When this routine is called KGDB should unregister from
         * handlers and clean up, making sure it is not handling any
         * break exceptions at the time.
         */
        if (kgdb_io_module_registered) {
                kgdb_io_module_registered = 0;
                unregister_reboot_notifier(&dbg_reboot_notifier);
                unregister_module_notifier(&dbg_module_load_nb);
                kgdb_arch_exit();
#ifdef CONFIG_MAGIC_SYSRQ
                unregister_sysrq_key('g', &sysrq_dbg_op);
#endif
                if (kgdb_con_registered) {
                        unregister_console(&kgdbcons);
                        kgdb_con_registered = 0;
                }
        }
}

/**
 *      kgdb_register_io_module - register KGDB IO module
 *      @new_dbg_io_ops: the io ops vector
 *
 *      Register it with the KGDB core.
 */
int kgdb_register_io_module(struct kgdb_io *new_dbg_io_ops)
{
        struct kgdb_io *old_dbg_io_ops;
        int err;

        spin_lock(&kgdb_registration_lock);

        old_dbg_io_ops = dbg_io_ops;
        if (old_dbg_io_ops) {
                if (!old_dbg_io_ops->deinit) {
                        spin_unlock(&kgdb_registration_lock);

                        pr_err("KGDB I/O driver %s can't replace %s.\n",
                                new_dbg_io_ops->name, old_dbg_io_ops->name);
                        return -EBUSY;
                }
                pr_info("Replacing I/O driver %s with %s\n",
                        old_dbg_io_ops->name, new_dbg_io_ops->name);
        }

        if (new_dbg_io_ops->init) {
                err = new_dbg_io_ops->init();
                if (err) {
                        spin_unlock(&kgdb_registration_lock);
                        return err;
                }
        }

        dbg_io_ops = new_dbg_io_ops;

        spin_unlock(&kgdb_registration_lock);

        if (old_dbg_io_ops) {
                old_dbg_io_ops->deinit();
                return 0;
        }

        pr_info("Registered I/O driver %s\n", new_dbg_io_ops->name);

        /* Arm KGDB now. */
        kgdb_register_callbacks();

        if (kgdb_break_asap &&
            (!dbg_is_early || IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG)))
                kgdb_initial_breakpoint();

        return 0;
}
EXPORT_SYMBOL_GPL(kgdb_register_io_module);

/**
 *      kgdb_unregister_io_module - unregister KGDB IO module
 *      @old_dbg_io_ops: the io ops vector
 *
 *      Unregister it with the KGDB core.
 */
void kgdb_unregister_io_module(struct kgdb_io *old_dbg_io_ops)
{
        BUG_ON(kgdb_connected);

        /*
         * KGDB is no longer able to communicate out, so
         * unregister our callbacks and reset state.
         */
        kgdb_unregister_callbacks();

        spin_lock(&kgdb_registration_lock);

        WARN_ON_ONCE(dbg_io_ops != old_dbg_io_ops);
        dbg_io_ops = NULL;

        spin_unlock(&kgdb_registration_lock);

        if (old_dbg_io_ops->deinit)
                old_dbg_io_ops->deinit();

        pr_info("Unregistered I/O driver %s, debugger disabled\n",
                old_dbg_io_ops->name);
}
EXPORT_SYMBOL_GPL(kgdb_unregister_io_module);

int dbg_io_get_char(void)
{
        int ret = dbg_io_ops->read_char();
        if (ret == NO_POLL_CHAR)
                return -1;
        if (!dbg_kdb_mode)
                return ret;
        if (ret == 127)
                return 8;
        return ret;
}

/**
 * kgdb_breakpoint - generate breakpoint exception
 *
 * This function will generate a breakpoint exception.  It is used at the
 * beginning of a program to sync up with a debugger and can be used
 * otherwise as a quick means to stop program execution and "break" into
 * the debugger.
 */
noinline void kgdb_breakpoint(void)
{
        atomic_inc(&kgdb_setting_breakpoint);
        wmb(); /* Sync point before breakpoint */
        arch_kgdb_breakpoint();
        wmb(); /* Sync point after breakpoint */
        atomic_dec(&kgdb_setting_breakpoint);
}
EXPORT_SYMBOL_GPL(kgdb_breakpoint);

static int __init opt_kgdb_wait(char *str)
{
        kgdb_break_asap = 1;

        kdb_init(KDB_INIT_EARLY);
        if (kgdb_io_module_registered &&
            IS_ENABLED(CONFIG_ARCH_HAS_EARLY_DEBUG))
                kgdb_initial_breakpoint();

        return 0;
}

early_param("kgdbwait", opt_kgdb_wait);