xref: /linux-6.15/kernel/debug/debug_core.c (revision fe557319)

/*
 * Kernel Debug Core
 *
 * Maintainer: Jason Wessel <[email protected]>
 *
 * Copyright (C) 2000-2001 VERITAS Software Corporation.
 * Copyright (C) 2002-2004 Timesys Corporation
 * Copyright (C) 2003-2004 Amit S. Kale <[email protected]>
 * Copyright (C) 2004 Pavel Machek <[email protected]>
 * Copyright (C) 2004-2006 Tom Rini <[email protected]>
 * 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 <[email protected]>
 *
 * Contributors at various stages not listed above:
 *  Jason Wessel ( [email protected] )
 *  George Anzinger <[email protected]>
 *  Anurekh Saxena ([email protected])
 *  Lake Stevens Instrument Division (Glenn Engel)
 *  Jim Kingdon, Cygnus Support.
 *
 * Original KGDB stub: David Grothe <[email protected]>,
 * Tigran Aivazian <[email protected]>
 *
 * This file is licensed under the terms of the GNU General Public License
 * version 2. This program is licensed "as is" without any warranty of any
 * kind, whether express or implied.
 */

#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/vmacache.h>
#include <linux/rcupdate.h>
#include <linux/irq.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 notifiter, 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;

static int __init opt_kgdb_con(char *str)
{
	kgdb_use_con = 1;
	return 0;
}

early_param("kgdbcon", opt_kgdb_con);

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;
static atomic_t			kgdb_break_tasklet_var;
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 overriden 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;
}

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);
}

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

int __weak kgdb_arch_init(void)
{
	return 0;
}

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

#ifdef CONFIG_SMP

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

static DEFINE_PER_CPU(call_single_data_t, kgdb_roundup_csd);

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());
}

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;

		csd->func = kgdb_call_nmi_hook;
		ret = smp_call_function_single_async(cpu, csd);
		if (ret)
			kgdb_info[cpu].rounding_up = false;
	}
}

#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;

	if (current->mm) {
		int i;

		for (i = 0; i < VMACACHE_SIZE; i++) {
			if (!current->vmacache.vmas[i])
				continue;
			flush_cache_range(current->vmacache.vmas[i],
					  addr, addr + BREAK_INSTR_SIZE);
		}
	}

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

/*
 * 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;
}

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;
}

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;
}

#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;
}

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;
}

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

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:
	/*
	 * 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);
			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);

		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 {
			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;
		}
	}

	/* 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);

	return kgdb_info[cpu].ret_state;
}

/*
 * 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;
}

/*
 * 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;
}

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;
}

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,
};

#ifdef CONFIG_MAGIC_SYSRQ
static void sysrq_handle_dbg(int 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;

	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] detatch debug client
	 *   -1 == Do nothing... and use this until the board resets
	 */
	switch (kgdbreboot) {
	case 1:
		kgdb_breakpoint();
	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;
		}
	}
}

/*
 * There are times a tasklet needs to be used vs a compiled in
 * break point so as to cause an exception outside a kgdb I/O module,
 * such as is the case with kgdboe, where calling a breakpoint in the
 * I/O driver itself would be fatal.
 */
static void kgdb_tasklet_bpt(unsigned long ing)
{
	kgdb_breakpoint();
	atomic_set(&kgdb_break_tasklet_var, 0);
}

static DECLARE_TASKLET(kgdb_tasklet_breakpoint, kgdb_tasklet_bpt, 0);

void kgdb_schedule_breakpoint(void)
{
	if (atomic_read(&kgdb_break_tasklet_var) ||
		atomic_read(&kgdb_active) != -1 ||
		atomic_read(&kgdb_setting_breakpoint))
		return;
	atomic_inc(&kgdb_break_tasklet_var);
	tasklet_schedule(&kgdb_tasklet_breakpoint);
}
EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint);

/**
 *	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);

/**
 *	kkgdb_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);