gdbstub initial code, v7

gdbstub initial code, v7

[Oleg Nesterov]

[-- Attachment #1: Type: text/plain, Size: 207 bytes --]

Changes:

	- report signals. A bit more code changes than I expected.

	- implement QPassSignals, trivial.

Note: $CSIG is not supported yet, and I am not sure I understand
how it should work. Next time...


[-- Attachment #2: ugdb.c --]
[-- Type: text/plain, Size: 46528 bytes --]

#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/utrace.h>
#include <linux/poll.h>
#include <linux/mm.h>
#include <linux/regset.h>
#include <asm/uaccess.h>

static int o_remote_debug;
module_param_named(echo, o_remote_debug, bool, 0);

#define BUFFER_SIZE		1024
#define PACKET_SIZE		1024

struct pbuf {
	char	*cur, *pkt;
	char	buf[BUFFER_SIZE];
};

static inline void pb_init(struct pbuf *pb)
{
	pb->cur = pb->buf;
	pb->pkt = NULL;
}

enum {
	U_STOP_IDLE = 0,
	U_STOP_PENDING,
	U_STOP_SENT,
};

struct ugdb {
	struct list_head	u_processes;
	struct list_head	u_stopped;

	int			u_stop_state;

	struct mutex		u_mutex;
	spinlock_t		u_slock;

	struct ugdb_thread
				*u_cur_tinfo,
				*u_cur_hg,
				*u_cur_hc;

	wait_queue_head_t	u_wait;

	int			u_err;

	struct pbuf		u_pbuf;
	char			u_cbuf[PACKET_SIZE];
	int			u_clen;

	sigset_t		u_sig_ign;

	unsigned int
				u_no_ack:1,
				u_allstop:1;
};

static inline void ugdb_ck_stopped(struct ugdb *ugdb)
{
	spin_lock(&ugdb->u_slock);
	WARN_ON(!list_empty(&ugdb->u_stopped) &&
				ugdb->u_stop_state == U_STOP_IDLE);
	WARN_ON(list_empty(&ugdb->u_stopped) &&
				ugdb->u_stop_state == U_STOP_PENDING);
	spin_unlock(&ugdb->u_slock);
}

static struct ugdb *ugdb_create(void)
{
	struct ugdb *ugdb;
	int err;

	err = -ENODEV;
	// XXX: ugly. proc_reg_open() should take care.
	if (!try_module_get(THIS_MODULE))
		goto out;

	err = -ENOMEM;
	ugdb = kzalloc(sizeof(*ugdb), GFP_KERNEL);
	if (!ugdb)
		goto put_module;

	INIT_LIST_HEAD(&ugdb->u_processes);
	INIT_LIST_HEAD(&ugdb->u_stopped);

	mutex_init(&ugdb->u_mutex);
	spin_lock_init(&ugdb->u_slock);

	init_waitqueue_head(&ugdb->u_wait);

	pb_init(&ugdb->u_pbuf);

	return ugdb;

put_module:
	module_put(THIS_MODULE);
out:
	return ERR_PTR(err);
}

#define P_DETACHING	(1 << 1)
#define P_ZOMBIE	(1 << 2)

struct ugdb_process {
	int			p_pid;
	int			p_state;

	struct list_head	p_threads;

	struct ugdb		*p_ugdb;
	struct list_head	p_processes;
};

static inline bool process_alive(struct ugdb_process *process)
{
	return !(process->p_state & P_ZOMBIE);
}

static inline void mark_process_dead(struct ugdb_process *process)
{
	process->p_state |= P_ZOMBIE;
}

static struct ugdb_process *ugdb_create_process(struct ugdb *ugdb, int pid_nr)
{
	struct ugdb_process *process;

	process = kzalloc(sizeof(*process), GFP_KERNEL);
	if (!process)
		return NULL;

	process->p_pid = pid_nr;
	process->p_ugdb = ugdb;
	INIT_LIST_HEAD(&process->p_threads);
	list_add_tail(&process->p_processes, &ugdb->u_processes);

	return process;
}

#define T_STOP_RUN	0
#define T_STOP_REQ	(1 << 0)	/* requested by gdb */
#define T_STOP_ALL	(1 << 1)	/* vCont;c:pX.-1, for report_clone */
#define T_STOP_ACK	(1 << 2)	/* visible to vStopped */
#define T_STOP_STOPPED	(1 << 3)	/* reported as stopped to gdb */
					/* TASK_TRACED + deactivated ? */

#define T_EV_NONE		0
#define T_EV_EXIT		(1 << 24)
#define T_EV_SIGN		(2 << 24)

#define T_EV_TYPE(event)	((0xff << 24) & (event))
#define T_EV_DATA(event)	(~(0xff << 24) & (event))

struct ugdb_thread {
	int			t_tid;
	int			t_stop_state;
	int			t_stop_event;

	struct ugdb		*t_ugdb;
	struct ugdb_process	*t_process;

	struct list_head	t_threads;
	struct list_head	t_stopped;

	struct pid		*t_spid;

	struct utrace_engine	*t_engine;
};

static inline bool thread_alive(struct ugdb_thread *thread)
{
	WARN_ON((thread->t_tid != 0) != process_alive(thread->t_process));

	return thread->t_tid != 0;
}

static inline void mark_thread_dead(struct ugdb_thread *thread)
{
	mark_process_dead(thread->t_process);
	thread->t_tid = 0;
}

/*
 * The thread should be alive, and it can't pass ugdb_report_death()
 * if the caller holds ugdb->u_mutex. However the tracee can be
 * reaped anyway, pid_task() can return NULL after detach_pid().
 */
static inline struct task_struct *thread_to_task(struct ugdb_thread *thread)
{
	BUG_ON(!thread_alive(thread));

	return pid_task(thread->t_spid, PIDTYPE_PID);
}

static struct ugdb_thread *ugdb_create_thread(struct ugdb_process *process,
						struct pid *spid)
{
	struct ugdb_thread *thread;

	thread = kzalloc(sizeof(*thread), GFP_KERNEL);
	if (!thread)
		return NULL;

	thread->t_tid = pid_vnr(spid);
	thread->t_spid = get_pid(spid);
	thread->t_process = process;
	thread->t_ugdb = process->p_ugdb;
	INIT_LIST_HEAD(&thread->t_stopped);
	list_add_tail(&thread->t_threads, &process->p_threads);

	return thread;
}

static void ugdb_del_stopped(struct ugdb *ugdb, struct ugdb_thread *thread)
{
	if (!list_empty(&thread->t_stopped)) {
		WARN_ON(!(thread->t_stop_state & T_STOP_ACK));

		spin_lock(&ugdb->u_slock);
		list_del_init(&thread->t_stopped);

		if (!(thread->t_stop_state & T_STOP_STOPPED)) {
			if (ugdb->u_stop_state == U_STOP_PENDING &&
					list_empty(&ugdb->u_stopped))
				ugdb->u_stop_state = U_STOP_IDLE;
		}
		spin_unlock(&ugdb->u_slock);
	}

	thread->t_stop_state = T_STOP_RUN;
}

static void ugdb_destroy_thread(struct ugdb_thread *thread)
{
	struct ugdb *ugdb = thread->t_ugdb;

	ugdb_ck_stopped(ugdb);

	ugdb_del_stopped(ugdb, thread);

	/* NULL if attach fails */
	if (thread->t_engine)
		utrace_engine_put(thread->t_engine);

	list_del(&thread->t_threads);
	put_pid(thread->t_spid);
	kfree(thread);
}

static int ugdb_set_events(struct ugdb_thread *thread,
				unsigned long events)
{
	WARN_ON(!thread_alive(thread));

	events |= (UTRACE_EVENT(CLONE) | UTRACE_EVENT(DEATH) |
			UTRACE_EVENT_SIGNAL_ALL);

	return utrace_set_events_pid(thread->t_spid, thread->t_engine,
					events);
}

static int ugdb_control(struct ugdb_thread *thread,
				enum utrace_resume_action action)
{
	// XXX: temporary racy check
	WARN_ON(!thread_alive(thread) && action != UTRACE_DETACH);

	return utrace_control_pid(thread->t_spid, thread->t_engine,
					action);
}

static const struct utrace_engine_ops ugdb_utrace_ops;

static void ugdb_detach_thread(struct ugdb_thread *thread, bool running)
{
	int ret;

	ret = ugdb_control(thread, UTRACE_DETACH);

	/* engine->flags == 0, it can't run a callback */
	if (!running)
		return;

	/* callbacks are no longer possible */
	if (!ret)
		return;

	if (ret == -EINPROGRESS) {
		/*
		 * We raced with a callback, it can't be ->report_death().
		 * However, we can not use utrace_barrier_pid(), it can
		 * hang "forever" until the next utrace_resume() if it
		 * sees ->ops == &utrace_detached_ops set by us, but the
		 * tracee is no longer running.
		 *
		 * But: we have no choice.
		 */
		do {
			ret = utrace_barrier_pid(thread->t_spid,
						thread->t_engine);
		} while (ret == -ERESTARTSYS);
	} else {
		/*
		 * Nothing can help us to synchronize with ->report_death.
		 * We do not know if it was already called or not, we can't
		 * know if it is running. utrace_barrier_pid() can't help,
		 * it can return zero and then later ->report_death() will
		 * be called. Or it can return -ESRCH just because the task
		 * was alredy released and pid_task() == NULL, but this
		 * doesn't mean ->report_death() can't be called later.
		 *
		 * Fortunately, we know that the tracee is dying or dead,
		 * engine->ops should be changed after ugdb_report_death()
		 * returns UTRACE_DETACH.
		 */
		 while (thread->t_engine->ops == &ugdb_utrace_ops) {
		 	schedule_timeout_uninterruptible(1);
		 }
	}
}

/*
 * returns NULL if raced with exit(), or ERR_PTR().
 */
static struct ugdb_thread *ugdb_attach_thread(struct ugdb_process *process,
						struct pid *spid)
{
	struct ugdb_thread *thread;
	struct utrace_engine *engine;
	struct task_struct *task;

	thread = ugdb_create_thread(process, spid);
	if (!thread)
		goto err;

	engine = utrace_attach_pid(thread->t_spid, UTRACE_ATTACH_CREATE,
					&ugdb_utrace_ops, thread);
	if (IS_ERR(engine))
		goto free_thread;

	thread->t_engine = engine;

	if (ugdb_set_events(thread, 0))
		goto detach_thread;

	return thread;

detach_thread:
	ugdb_detach_thread(thread, false);
free_thread:
	ugdb_destroy_thread(thread);
err:
	rcu_read_lock();
	task = pid_task(spid, PIDTYPE_PID);
	if (task && task->exit_state)
		task = NULL;
	rcu_read_unlock();

	return task ? ERR_PTR(-ENOMEM) : NULL;
}

static inline bool is_subthread(struct ugdb_process *process,
				struct ugdb_thread *thread)
{
	return thread && thread->t_process == process;
}

static inline void ugdb_reset_tinfo(struct ugdb *ugdb)
{
	ugdb->u_cur_tinfo = NULL;
}

static void ugdb_destroy_process(struct ugdb_process *process)
{
	struct ugdb *ugdb = process->p_ugdb;
	struct ugdb_thread *thread;

	mutex_lock(&ugdb->u_mutex);
	process->p_state |= P_DETACHING;
	list_del(&process->p_processes);

	if (is_subthread(process, ugdb->u_cur_hg))
		ugdb->u_cur_hg = NULL;
	if (is_subthread(process, ugdb->u_cur_hc))
		ugdb->u_cur_hc = NULL;

	/* I hope gdb won't do detach from under qfThreadInfo */
	if (ugdb->u_cur_tinfo) {
		printk(KERN_WARNING "ugdb: detach from under qfThreadInfo\n");
		ugdb_reset_tinfo(ugdb);
	}
	mutex_unlock(&ugdb->u_mutex);

	while (!list_empty(&process->p_threads)) {
		thread = list_first_entry(&process->p_threads,
				struct ugdb_thread, t_threads);
		ugdb_detach_thread(thread, true);
		ugdb_destroy_thread(thread);
	}

	BUG_ON(!list_empty(&process->p_threads));

	kfree(process);
}

static void ugdb_destroy(struct ugdb *ugdb)
{
	struct ugdb_process *process;

	while (!list_empty(&ugdb->u_processes)) {
		process = list_first_entry(&ugdb->u_processes,
				struct ugdb_process, p_processes);
		ugdb_destroy_process(process);
	}

	BUG_ON(!list_empty(&ugdb->u_processes));
	BUG_ON(!list_empty(&ugdb->u_stopped));

	module_put(THIS_MODULE);
	kfree(ugdb);
}

static struct pid *get_next_pid(struct pid *main, struct pid *curr)
{
	struct task_struct *task;
	struct sighand_struct *sighand;
	struct pid *next = NULL;

	rcu_read_lock();
	/*
	 * If task/sighand is NULL we return NULL. This is fine if
	 * the caller is get_first_pid(), we should abort attaching.
	 *
	 * But this can also happen if curr was already attached,
	 * and this is wrong. Fortunately, this is very unlikely
	 * case. The attached sub-thread can't pass ->report_death,
	 * if it was reaped the caller of release_task() must be
	 * ptracer who re-parented this thread.
	 */
	task = pid_task(curr, PIDTYPE_PID);
	if (!task)
		goto unlock_rcu;

	// XXX: we need lock_task_sighand() but it is not exported,
	// so we ran race with de_thread().
	sighand = rcu_dereference(task->sighand);
	if (!sighand)
		goto unlock_rcu;

	spin_lock_irq(&sighand->siglock);
	for (;;) {
		task = next_thread(task);

		// XXX: if main is not leader we can race with exec.
		if (task_pid(task) == main)
			break;

		if (!task->exit_state) {
			next = get_pid(task_pid(task));
			break;
		}
	}
	spin_unlock_irq(&sighand->siglock);
unlock_rcu:
	rcu_read_unlock();

	return next;
}

static struct pid *get_first_pid(struct pid *main)
{
	struct task_struct *leader;

	rcu_read_lock();
	leader = pid_task(main, PIDTYPE_PID);
	if (leader && leader->exit_state)
		leader = NULL;
	rcu_read_unlock();

	/*
	 * The group-leader is alive, try to attach. If it exits
	 * before utrace_set_events(), get_first_pid() will be
	 * called again and it will notice ->exit_state != 0.
	 */
	if (leader)
		return get_pid(main);

	/*
	 * Try to find the live sub-thread. If the whole group
	 * is dead it returns NULL and the caller aborts.
	 */
	return get_next_pid(main, main);
}

static int ugdb_attach_all_threads(struct ugdb *ugdb,
				struct ugdb_process *process,
				struct pid *main_pid)
{
	struct ugdb_thread *thread;
	struct pid *curr_pid;

	mutex_lock(&ugdb->u_mutex);

	for (;;) {
		curr_pid = get_first_pid(main_pid);
		if (!curr_pid)
			goto abort;

		thread = ugdb_attach_thread(process, curr_pid);
		put_pid(curr_pid);

		if (IS_ERR(thread))
			goto abort;

		if (thread)
			break;
	}

	for (;;) {
		struct pid *next_pid;

		next_pid = get_next_pid(main_pid, curr_pid);
		if (!next_pid)
			break;

		thread = ugdb_attach_thread(process, next_pid);
		put_pid(next_pid);

		if (IS_ERR(thread))
			goto abort;

		if (!thread)
			continue;

		curr_pid = next_pid;
	}

	mutex_unlock(&ugdb->u_mutex);
	return 0;

abort:
	mutex_unlock(&ugdb->u_mutex);
	return -1;
}

static int ugdb_attach(struct ugdb *ugdb, int pid_nr)
{
	struct pid *main_pid;
	struct ugdb_process *process;
	int err;

	// XXX: check if exists
	// XXX: check if group leader ?

	err = -ESRCH;
	main_pid = find_get_pid(pid_nr);
	if (!main_pid)
		goto out;

	err = -ENOMEM;
	process = ugdb_create_process(ugdb, pid_nr);
	if (!process)
		goto free_pid;

	err = ugdb_attach_all_threads(ugdb, process, main_pid);
	if (err)
		ugdb_destroy_process(process);

free_pid:
	put_pid(main_pid);
out:
	return err;
}

static struct ugdb_process *ugdb_find_process(struct ugdb *ugdb, int pid)
{
	struct ugdb_process *process;

	list_for_each_entry(process, &ugdb->u_processes, p_processes) {
		if (process->p_pid == pid)
			return process;
	}

	return NULL;
}

static struct ugdb_thread *ugdb_find_thread(struct ugdb *ugdb, int pid, int tid)
{
	struct ugdb_process *process;
	struct ugdb_thread *thread;

	list_for_each_entry(process, &ugdb->u_processes, p_processes) {
		if (unlikely(!process_alive(process)))
			continue;
		if (pid && process->p_pid != pid)
			continue;

		list_for_each_entry(thread, &process->p_threads, t_threads) {
			if (WARN_ON(!thread_alive(thread)))
				continue;
			if (!tid || thread->t_tid == tid)
				return thread;
		}

		if (pid)
			break;
	}

	return NULL;
}

static int ugdb_detach(struct ugdb *ugdb, int pid)
{
	struct ugdb_process *process = ugdb_find_process(ugdb, pid);

	if (!process)
		return -1;

	ugdb_destroy_process(process);
	return 0;
}

#define CUR_TINFO_END	((struct ugdb_thread *)1)

static struct ugdb_thread *ugdb_advance_tinfo(struct ugdb *ugdb)
{
	struct ugdb_thread *cur, *nxt;
	struct ugdb_process *process;

	cur = ugdb->u_cur_tinfo;

	if (cur == CUR_TINFO_END) {
		ugdb->u_cur_tinfo = NULL;
		return NULL;
	}

	if (!cur) {
		list_for_each_entry(process, &ugdb->u_processes, p_processes) {
			if (unlikely(!process_alive(process)))
				continue;

			if (!list_empty(&process->p_threads)) {
				cur = list_first_entry(&process->p_threads,
						struct ugdb_thread, t_threads);
				break;
			}
		}

		if (!cur)
			return NULL;
	}

	process = cur->t_process;

	if (list_is_last(&cur->t_threads, &process->p_threads)) {
		nxt = CUR_TINFO_END;

		list_for_each_entry_continue(process, &ugdb->u_processes, p_processes) {
			if (unlikely(!process_alive(process)))
				continue;

			if (!list_empty(&process->p_threads)) {
				nxt = list_first_entry(&process->p_threads,
						struct ugdb_thread, t_threads);
				break;
			}
		}
	} else {
		nxt = list_first_entry(&cur->t_threads,
				struct ugdb_thread, t_threads);
	}

	ugdb->u_cur_tinfo = nxt;
	return cur;
}

// -----------------------------------------------------------------------------
static bool ugdb_add_stopped(struct ugdb_thread *thread, int stop_event)
{
	struct ugdb *ugdb = thread->t_ugdb;
	bool ret = false;

	ugdb_ck_stopped(ugdb);

	spin_lock(&ugdb->u_slock);
	if (stop_event == T_EV_NONE) {
		if (WARN_ON(thread->t_stop_state & T_STOP_ACK))
			goto unlock;
		if (WARN_ON(!list_empty(&thread->t_stopped)))
			goto unlock;

		/* raced with ugdb_cont_thread() */
		if (!(thread->t_stop_state & T_STOP_REQ))
			goto unlock;
	}

	if (thread->t_stop_state & T_STOP_ACK) {
		if (thread->t_stop_state & T_STOP_STOPPED)
			/*
			 * Alas, we can't report this event. We already
			 * reported T00 and there is no way to inform gdb
			 * the state of tracee was changed.
			 */
			goto unlock;
	} else {
		WARN_ON(thread->t_stop_state & T_STOP_STOPPED);

		thread->t_stop_state |= T_STOP_ACK;
		list_add_tail(&thread->t_stopped, &ugdb->u_stopped);

		if (ugdb->u_stop_state == U_STOP_IDLE) {
			ugdb->u_stop_state = U_STOP_PENDING;
			wake_up_all(&ugdb->u_wait);
		}
	}

	thread->t_stop_event = stop_event;
	ret = true;
unlock:
	spin_unlock(&ugdb->u_slock);

	return ret;
}

static void ugdb_process_exit(struct ugdb_thread *thread)
{
	struct ugdb *ugdb = thread->t_ugdb;
	int status;

	BUG_ON(!thread_alive(thread));

	ugdb_del_stopped(ugdb, thread);
	mark_thread_dead(thread);

	// XXX: OOPS, we can't read ->signal->group_exit_code !!!
	status = current->exit_code;
	if (ugdb_add_stopped(thread, T_EV_EXIT | status))
		return;

	WARN_ON(1);
}

static int ugdb_stop_thread(struct ugdb_thread *thread, bool all)
{
	struct ugdb *ugdb = thread->t_ugdb;
	int err;

	WARN_ON(!thread_alive(thread));

	ugdb_ck_stopped(ugdb);

	if (thread->t_stop_state != T_STOP_RUN) {
		if (!all || (thread->t_stop_state & T_STOP_ALL))
			return 0;
		/*
		 * Otherwise we should set T_STOP_ALL anyway,
		 *
		 *	(gdb) interrupt &
		 *	(gbd) interrupt -a &
		 *
		 * to ensure -a actually works if it races with clone.
		 */
	}

	err = -EALREADY;
	spin_lock(&ugdb->u_slock);
	if (thread->t_stop_state == T_STOP_RUN) {
		thread->t_stop_state = T_STOP_REQ;
		err = 0;
	}

	/*
	 * We hold ugdb->u_mutex, we can't race with ugdb_report_clone().
	 * ugdb->u_slock protects us against ugdb_add_stopped(). We can
	 * change ->t_stop_state even if we did not initiate this stop.
	 */
	if (all)
		thread->t_stop_state |= T_STOP_ALL;
	spin_unlock(&ugdb->u_slock);

	if (err)
		return 0;

	// XXX: we don't do UTRACE_STOP! this means we can't
	// stop TASK_STOPEED task. Need to discuss jctl issues.
	// if we do UTRACE_STOP we should call ugdb_add_stopped().

	ugdb_set_events(thread, UTRACE_EVENT(QUIESCE));
	err = ugdb_control(thread, UTRACE_INTERRUPT);
	if (err && err != -EINPROGRESS)
		return err;
	return 1;
}

static int ugdb_cont_thread(struct ugdb_thread *thread, bool all)
{
	struct ugdb *ugdb = thread->t_ugdb;
	int ret;

	WARN_ON(!thread_alive(thread));

	ugdb_ck_stopped(ugdb);

	// XXX: gdb shouldn't explicitly cont an unreported thread
	WARN_ON(!all && !(thread->t_stop_state & T_STOP_STOPPED));

	if (thread->t_stop_state == T_STOP_RUN)
		return 0;

	spin_lock(&ugdb->u_slock);
	/*
	 * Nothing to do except clear the pending T_STOP_REQ.
	 */
	ret = 0;
	if (!(thread->t_stop_state & T_STOP_ACK))
		goto set_run;

	/*
	 * Alas. Thanks to remote protocol, we can't cont this
	 * thread. We probably already sent the notification, we
	 * can do nothing except ack that %Stop later in response
	 * to vStopped.
	 *
	 * OTOH, gdb shouldn't send 'c' if this thread was not
	 * reported as stopped. However, this means that gdb can
	 * see the new %Stop:T00 notification after vCont;c:pX.-1,
	 * it should handle this case correctly anyway. I hope.
	 *
	 * If this stop was not initiated by gdb we should not
	 * cancel it too, this event should be reported first.
	 */
	ret = -1;
	if (!(thread->t_stop_state & T_STOP_STOPPED))
		goto unlock;

	ret = 1;
	list_del_init(&thread->t_stopped);
set_run:
	thread->t_stop_state = T_STOP_RUN;
unlock:
	spin_unlock(&ugdb->u_slock);

	if (ret >= 0) {
		// XXX: OK, this all is racy, and I do not see any
		// solution except: implement UTRACE_STOP_STICKY and
		// move this code up under the lock, or add
		// utrace_engine_ops->notify_stopped().

		// 1. UTRACE_RESUME is racy, this is fixeable.
		// 2. we need utrace_barrier() to close the race
		//    with the callback which is going to return
		//    UTRACE_STOP, but:
		//    	a) we can deadlock (solveable)
		//	b) in this case UTRACE_RESUME can race with
		//	   another stop initiated by tracee itself.

		ugdb_set_events(thread, 0);
		ugdb_control(thread, UTRACE_RESUME);
	}

	return ret;
}

static struct ugdb_thread *ugdb_next_stopped(struct ugdb *ugdb)
{
	struct ugdb_thread *thread = NULL;

	// XXX: temporary racy check
	WARN_ON(ugdb->u_stop_state == U_STOP_IDLE);

	spin_lock(&ugdb->u_slock);
	if (list_empty(&ugdb->u_stopped)) {
		ugdb->u_stop_state = U_STOP_IDLE;
	} else {
		ugdb->u_stop_state = U_STOP_SENT;

		thread = list_first_entry(&ugdb->u_stopped,
					struct ugdb_thread, t_stopped);

		thread->t_stop_state |= T_STOP_STOPPED;
		list_del_init(&thread->t_stopped);
	}
	spin_unlock(&ugdb->u_slock);

	return thread;
}

// -----------------------------------------------------------------------------
static bool ugdb_stop_pending(struct ugdb_thread *thread)
{
	if (!(thread->t_stop_state & T_STOP_REQ))
		return false;

	if (!(thread->t_stop_state & T_STOP_ACK))
		return ugdb_add_stopped(thread, T_EV_NONE);

	return true;
}

static u32 ugdb_report_quiesce(u32 action, struct utrace_engine *engine,
					unsigned long event)
{
	struct ugdb_thread *thread = engine->data;

	WARN_ON(!thread_alive(thread));

	/* ensure SIGKILL can't race with stop/cont in progress */
	if (event != UTRACE_EVENT(DEATH)) {
		if (ugdb_stop_pending(thread))
			return UTRACE_STOP;
	}

	return utrace_resume_action(action);
}

static u32 ugdb_report_signal(u32 action, struct utrace_engine *engine,
				struct pt_regs *regs,
				siginfo_t *info,
				const struct k_sigaction *orig_ka,
				struct k_sigaction *return_ka)
{
	struct ugdb_thread *thread = engine->data;
	struct ugdb *ugdb = thread->t_ugdb;
	int signr;

	WARN_ON(!thread_alive(thread));

	switch (utrace_signal_action(action)) {
	case UTRACE_SIGNAL_HANDLER:
	case UTRACE_SIGNAL_REPORT:
		if (ugdb_stop_pending(thread))
			return UTRACE_STOP | UTRACE_SIGNAL_IGN;

		return action;

	default:
		break;
	}

	signr = info->si_signo;
	if (sigismember(&ugdb->u_sig_ign, signr))
		return action;

	if (ugdb_add_stopped(thread, T_EV_SIGN | signr))
		return UTRACE_STOP | UTRACE_SIGNAL_IGN;

	/*
	 * We already reported T00 to gdb. We can't change our state,
	 * we are already stopped from gdb pov. Push back this signal
	 * to report it later, after "continue".
	 */
	return UTRACE_STOP | UTRACE_SIGNAL_HOLD;
	// XXX: is this correct ??? check utrace_get_signal...
}

static bool is_already_attached(struct ugdb_process *process,
				struct task_struct *task)
{
	struct ugdb_thread *thread;

	if (likely(!task_utrace_flags(task)))
		return false;

	/*
	 * Currently there is no way to know if it was attached by us.
	 * We can't trust utrace_attach_task(UTRACE_ATTACH_MATCH_OPS),
	 * ugdb attaches without UTRACE_ATTACH_EXCLUSIVE. We have to
	 * check every attached thread.
	 *
	 * This is really bad, but without multitracing this can only
	 * happen in unlikely case right after ugdb_attach_all_threads().
	 */
	list_for_each_entry(thread, &process->p_threads, t_threads) {
		if (thread->t_spid == task_pid(task))
			return true;
	}

	return false;
}

static u32 ugdb_report_clone(u32 action, struct utrace_engine *engine,
			       unsigned long clone_flags,
			       struct task_struct *task)
{
	struct ugdb_thread *thread = engine->data;
	struct ugdb_process *process = thread->t_process;
	struct ugdb *ugdb = thread->t_ugdb;
	struct ugdb_thread *new_thread;

	WARN_ON(!thread_alive(thread));

	if (!(clone_flags & CLONE_THREAD))
		goto out;

	mutex_lock(&ugdb->u_mutex);
	if (process->p_state & P_DETACHING)
		goto unlock;
	/*
	 * This can only happen if we raced with ugdb_attach() which
	 * could attach both current and the new PF_STARTING child.
	 */
	if (unlikely(is_already_attached(process, task)))
		goto unlock;

	new_thread = ugdb_attach_thread(process, task_pid(task));
	BUG_ON(!new_thread);

	if (WARN_ON(IS_ERR(new_thread)))
		goto unlock;

	if (thread->t_stop_state & T_STOP_ALL)
		ugdb_stop_thread(new_thread, false);

unlock:
	mutex_unlock(&ugdb->u_mutex);
out:
	return utrace_resume_action(action);
}

static u32 ugdb_report_death(struct utrace_engine *engine,
				bool group_dead, int signal)
{
	struct ugdb_thread *thread = engine->data;
	struct ugdb_process *process = thread->t_process;
	struct ugdb *ugdb = thread->t_ugdb;

	WARN_ON(!thread_alive(thread));

	mutex_lock(&ugdb->u_mutex);
	if (process->p_state & P_DETACHING)
		goto unlock;

	if (ugdb->u_cur_hg == thread)
		ugdb->u_cur_hg = NULL;
	if (ugdb->u_cur_hc == thread)
		ugdb->u_cur_hc = NULL;

	if (ugdb->u_cur_tinfo == thread)
		ugdb_advance_tinfo(ugdb);

	if (list_is_singular(&process->p_threads))
		ugdb_process_exit(thread);
	else
		ugdb_destroy_thread(thread);

unlock:
	mutex_unlock(&ugdb->u_mutex);

	return UTRACE_DETACH;
}

static const struct utrace_engine_ops ugdb_utrace_ops = {
	.report_quiesce	= ugdb_report_quiesce,
	.report_signal	= ugdb_report_signal,
	.report_clone	= ugdb_report_clone,
	.report_death	= ugdb_report_death,
};

// -----------------------------------------------------------------------------
static inline int pb_size(struct pbuf *pb)
{
	return pb->cur - pb->buf;
}

static inline int pb_room(struct pbuf *pb)
{
	return pb->buf + BUFFER_SIZE - pb->cur;
}

static inline void pb_putc(struct pbuf *pb, char c)
{
	if (WARN_ON(pb->cur >= pb->buf + BUFFER_SIZE-1))
		return;
	*pb->cur++ = c;
}

static void pb_memcpy(struct pbuf *pb, const void *data, int size)
{
	if (WARN_ON(size > pb_room(pb)))
		return;
	memcpy(pb->cur, data, size);
	pb->cur += size;
}

static inline void pb_puts(struct pbuf *pb, const char *s)
{
	pb_memcpy(pb, s, strlen(s));
}

static inline void pb_putb(struct pbuf *pb, unsigned char val)
{
	static char hex[] = "0123456789abcdef";
	pb_putc(pb, hex[(val & 0xf0) >> 4]);
	pb_putc(pb, hex[(val & 0x0f) >> 0]);
}

static void pb_putbs(struct pbuf *pb, const char *data, int size)
{
	while (size--)
		pb_putb(pb, *data++);
}

static inline void __pb_start(struct pbuf *pb, char pref)
{
	WARN_ON(pb->pkt);
	pb_putc(pb, pref);
	pb->pkt = pb->cur;
}

static inline void pb_start(struct pbuf *pb)
{
	return __pb_start(pb, '$');
}

static inline void pb_cancel(struct pbuf *pb)
{
	if (WARN_ON(!pb->pkt))
		return;

	pb->cur = pb->pkt - 1;
	pb->pkt = NULL;
}

static void pb_end(struct pbuf *pb)
{
	unsigned char csm = 0;
	char *pkt = pb->pkt;

	pb->pkt = NULL;
	if (WARN_ON(!pkt))
		return;

	while (pkt < pb->cur) {
		WARN_ON(*pkt == '$' || *pkt == '#' || *pkt == '%');
		csm += (unsigned char)*pkt++;
	}

	pb_putc(pb, '#');
	pb_putb(pb, csm);
}

static inline void pb_packs(struct pbuf *pb, const char *s)
{
	pb_start(pb);
	pb_puts(pb, s);
	pb_end(pb);
}

static void __attribute__ ((format(printf, 3, 4)))
__pb_format(struct pbuf *pb, bool whole_pkt, const char *fmt, ...)
{
	int room = pb_room(pb), size;
	va_list args;

	if (whole_pkt)
		pb_start(pb);

	va_start(args, fmt);
	size = vsnprintf(pb->cur, room, fmt, args);
	va_end(args);

	if (WARN_ON(size > room))
		return;

	pb->cur += size;

	if (whole_pkt)
		pb_end(pb);
}

#define pb_printf(pb, args...)	__pb_format((pb), false, args)
#define pb_packf(pb, args...)	__pb_format((pb), true,  args)

static inline void *pb_alloc_bs(struct pbuf *pb, int size)
{
	if (unlikely(pb_room(pb) < 2 * size + 4))
		return NULL;
	return pb->cur + size + 1;
}

static inline void *pb_alloc_tmp(struct pbuf *pb, int size)
{
	if (unlikely(pb_room(pb) < size))
		return NULL;
	return pb->cur + BUFFER_SIZE - size;
}

static inline void pb_flush(struct pbuf *pb, int size)
{
	int keep = pb_size(pb) - size;
	if (keep)
		memmove(pb->buf, pb->buf + size, keep);
	pb->cur -= size;
}

static int pb_copy_to_user(struct pbuf *pb, char __user *ubuf, int size)
{
	int copy = min(size, pb_size(pb));

	if (!copy)
		return -EAGAIN;

	if (o_remote_debug)
		printk(KERN_INFO "<= %.*s\n", min(copy, 64), pb->buf);

	if (copy_to_user(ubuf, pb->buf, copy))
		return -EFAULT;

	pb_flush(pb, copy);
	return copy;
}

// -----------------------------------------------------------------------------
// XXX: include/gdb/signals.h:target_signal
// incomplete: 7, 29, rt?
static int to_gdb_sigmap[] = {
	[SIGHUP]	= 1,
	[SIGINT]	= 2,
	[SIGQUIT]	= 3,
	[SIGILL]	= 4,
	[SIGTRAP]	= 5,
	[SIGABRT]	= 6,
	[SIGIOT]	= 0,	/* ??? */
	[SIGBUS]	= 10,
	[SIGFPE]	= 8,
	[SIGKILL]	= 9,
	[SIGUSR1]	= 30,
	[SIGSEGV]	= 11,
	[SIGUSR2]	= 31,
	[SIGPIPE]	= 13,
	[SIGALRM]	= 14,
	[SIGTERM]	= 15,
	[SIGSTKFLT]	= 0,	/* ??? */
	[SIGCHLD]	= 20,
	[SIGCONT]	= 19,
	[SIGSTOP]	= 17,
	[SIGTSTP]	= 18,
	[SIGTTIN]	= 21,
	[SIGTTOU]	= 22,
	[SIGURG]	= 16,
	[SIGXCPU]	= 24,
	[SIGXFSZ]	= 25,
	[SIGVTALRM]	= 26,
	[SIGPROF]	= 27,
	[SIGWINCH]	= 28,
	[SIGIO]		= 23,
	[SIGPWR]	= 32,
	[SIGSYS]	= 12,
};

static int sig_to_gdb(unsigned sig)
{
	if (sig < ARRAY_SIZE(to_gdb_sigmap) && to_gdb_sigmap[sig])
		return to_gdb_sigmap[sig];
	return sig;

}

static int sig_from_gdb(unsigned sig)
{
	int i;

	if (!sig || !valid_signal(sig))
		return 0;

	for (i = 0; i < ARRAY_SIZE(to_gdb_sigmap); i++) {
		if (to_gdb_sigmap[i] == sig)
			return i;
	}

	return sig;
}

static int ugdb_report_stopped(struct ugdb *ugdb, bool async)
{
	struct ugdb_thread *thread;
	int pid, tid, event, data;
	struct pbuf *pb;
	char ex_r;

	mutex_lock(&ugdb->u_mutex);
	thread = ugdb_next_stopped(ugdb);
	if (!thread)
		goto unlock;

	event = thread->t_stop_event;

	WARN_ON(thread_alive(thread) != (T_EV_TYPE(event) != T_EV_EXIT));

	pid = thread->t_process->p_pid;
	tid = thread->t_tid;
unlock:
	mutex_unlock(&ugdb->u_mutex);

	if (!thread)
		return false;

	pb = &ugdb->u_pbuf;

	// XXX: damn, cleanup me...
	if (async) {
		__pb_start(pb, '%');
		pb_puts(pb, "Stop:");
	} else {
		pb_start(pb);
	}

	data = T_EV_DATA(event);
	switch (T_EV_TYPE(event)) {
	case T_EV_EXIT:
		if (data & 0xff) {
			data = sig_to_gdb(data & 0xff);
			ex_r = 'X';
		} else {
			data >>= 8;
			ex_r = 'W';
		}

		pb_printf(pb, "%c%x;process:%x", ex_r, data, pid);
		ugdb_destroy_process(thread->t_process);
		break;

	case T_EV_SIGN:
	case T_EV_NONE:
		pb_printf(pb, "T%02xthread:p%x.%x;",
					sig_to_gdb(data), pid, tid);
		break;

	default:
		printk(KERN_INFO "ugdb: bad stop event %x\n", event);
	}

	pb_end(pb);

	return true;
}

const char *handle_vstopped(struct ugdb *ugdb)
{
	if (ugdb->u_stop_state != U_STOP_SENT)
		return "E01";

	if (ugdb_report_stopped(ugdb, false))
		return NULL;

	return "OK";
}

static const char *handle_thread_info(struct ugdb *ugdb, bool start)
{
	struct ugdb_thread *thread;
	int pid = 0, tid;

	mutex_lock(&ugdb->u_mutex);
	if (start)
		ugdb_reset_tinfo(ugdb);
	else if (!ugdb->u_cur_tinfo)
		printk(KERN_INFO "ugdb: unexpected qsThreadInfo\n");

	thread = ugdb_advance_tinfo(ugdb);
	if (thread) {
		pid = thread->t_process->p_pid;
		tid = thread->t_tid;
	}
	mutex_unlock(&ugdb->u_mutex);

	if (!pid)
		return start ? "E01" : "l";

	pb_packf(&ugdb->u_pbuf, "mp%x.%x", pid, tid);
	return NULL;
}

static char *parse_xid(char *str, int *ppid, bool multi)
{
	if (*str == '-') {
		str++;

		if (multi && *str++ == '1')
			*ppid = -1;
		else
			str = NULL;
	} else {
		char *cur = str;

		*ppid = simple_strtoul(cur, &str, 16);
		if (str == cur)
			str = NULL;
	}

	return str;
}

static char *parse_pid_tid(char *str, int *ppid, int *ptid, bool multi)
{
	if (*str++ != 'p')
		return NULL;

	str = parse_xid(str, ppid, multi);
	if (!str)
		return NULL;

	if (*str++ != '.')
		return NULL;

	str = parse_xid(str, ptid, multi);
	if (!str)
		return NULL;

	return str;
}

static const char *handle_set_cur(struct ugdb *ugdb, char *cmd)
{

	struct ugdb_thread **pthread;
	int pid, tid;

	switch (*cmd++) {
	case 'g':
		pthread = &ugdb->u_cur_hg;
		break;

	case 'c':
		pthread = &ugdb->u_cur_hc;
		break;

	default:
		goto err;
	}

	if (!parse_pid_tid(cmd, &pid, &tid, false))
		goto err;

	mutex_lock(&ugdb->u_mutex);
	*pthread = ugdb_find_thread(ugdb, pid, tid);
	mutex_unlock(&ugdb->u_mutex);

	if (*pthread)
		return "OK";

err:
	return "E01";
}

static const char *handle_ck_alive(struct ugdb *ugdb, char *cmd)
{
	struct ugdb_thread *thread;
	int pid = 0, tid;

	if (!parse_pid_tid(cmd, &pid, &tid, false))
		goto err;

	mutex_lock(&ugdb->u_mutex);
	thread = ugdb_find_thread(ugdb, pid, tid);
	mutex_unlock(&ugdb->u_mutex);

	if (thread)
		return "OK";

err:
	return "E01";
}

static int parse_pid(char *str)
{
	int pid;

	if (!parse_xid(str, &pid, false))
		return 0;

	return pid;
}

static const char *handle_vattach(struct ugdb *ugdb, char *cmd)
{
	int pid = parse_pid(cmd);

	if (pid && !ugdb_attach(ugdb, pid))
		return "OK";

	return "E01";
}

static const char *handle_detach(struct ugdb *ugdb, char *cmd)
{
	int pid;

	if (*cmd++ != ';')
		goto err;

	pid = parse_pid(cmd);
	if (pid && !ugdb_detach(ugdb, pid))
		return "OK";

err:
	return "E01";
}

typedef int (*each_func_t)(struct ugdb_thread *, void *);

static int ugdb_do_each_thread(struct ugdb *ugdb, int pid, int tid,
				each_func_t func, void *arg)
{
	struct ugdb_process *process;
	struct ugdb_thread *thread;
	int ret = -ESRCH;

	list_for_each_entry(process, &ugdb->u_processes, p_processes) {
		if (unlikely(!process_alive(process)))
			continue;
		if (pid > 0 && process->p_pid != pid)
			continue;

		list_for_each_entry(thread, &process->p_threads, t_threads) {
			if (WARN_ON(!thread_alive(thread)))
				continue;
			if (tid > 0 && thread->t_tid != tid)
				continue;

			ret = func(thread, arg);
			if (ret)
				goto out;

			if (tid >= 0)
				break;
		}

		if (pid >= 0)
			break;
	}

out:
	return ret;
}

static int do_stop_thread(struct ugdb_thread *thread, void *arg)
{
	ugdb_stop_thread(thread, arg != NULL);
	return 0;
}

static int do_cont_thread(struct ugdb_thread *thread, void *arg)
{
	ugdb_cont_thread(thread, arg != NULL);
	return 0;
}

static const char *handle_vcont(struct ugdb *ugdb, char *cmd)
{
	int pid, tid;
	void *arg;
	int ret;

	switch (*cmd ++) {
	default:
		return "E01";
	case '?':
		return "vCont;t";
	case ';':
		break;
	}

	// XXX: Discuss the generic case! currently trivial.

	if (*cmd++ != 't')
		return "E01";

	pid = tid = -1;
	if (*cmd++ == ':') {
		if (!parse_pid_tid(cmd, &pid, &tid, true))
			return "E01";
	}
	arg = (tid >= 0) ? NULL : (void*)1;

	mutex_lock(&ugdb->u_mutex);
	// XXX: currently we only report -ESRCH
	ret = ugdb_do_each_thread(ugdb, pid, tid, do_stop_thread, arg);
	mutex_unlock(&ugdb->u_mutex);

	return ret < 0 ? "E01" : "OK";
}

static const char *handle_c(struct ugdb *ugdb, char *cmd)
{
	const char *rc = "E01";

	mutex_lock(&ugdb->u_mutex);
	if (ugdb->u_cur_hc)
		if (ugdb_cont_thread(ugdb->u_cur_hc, false) > 0)
			rc = "OK";
	mutex_unlock(&ugdb->u_mutex);

	return rc;
}

static const char *handle_qpass_signals(struct ugdb *ugdb, char *cmd)
{
	sigset_t *set = &ugdb->u_sig_ign;

	sigemptyset(set);
	while (*cmd) {
		char *end;
		int sig = simple_strtoul(cmd, &end, 16);

		if (cmd == end || *end != ';')
			return "E01";
		cmd = end + 1;

		sig = sig_from_gdb(sig);
		if (!sig)
			return "E01";

		sigaddset(set, sig);
	}

	return "OK";
}

// -----------------------------------------------------------------------------
static struct task_struct *
ugdb_prepare_examine(struct ugdb *ugdb, struct utrace_examiner *exam)
{
	struct ugdb_thread *thread;
	struct task_struct *task;
	int err;

	mutex_lock(&ugdb->u_mutex);
	thread = ugdb->u_cur_hg;
	if (!thread || !(thread->t_stop_state & T_STOP_STOPPED))
		goto err;

	// XXX: u_cur_hg can't exit, we hold the mutex
	task = thread_to_task(thread);
	if (!task)
		goto err;

	for (;;) {
		if (fatal_signal_pending(current))
			goto err;

		err = utrace_prepare_examine(task, thread->t_engine, exam);
		if (!err)
			break;

		if (err == -ESRCH)
			goto err;

		schedule_timeout_interruptible(1);
	}

	return task;
err:
	mutex_unlock(&ugdb->u_mutex);
	return NULL;
}

// XXX: we hold the mutex in between, but only because we can't
// use get_task_struct/put_task_struct.

static int
ugdb_finish_examine(struct ugdb *ugdb, struct utrace_examiner *exam)
{
	// XXX: u_cur_hg can't exit, we hold the mutex
	struct ugdb_thread *thread = ugdb->u_cur_hg;
	struct task_struct *task = thread_to_task(thread);
	int ret = -ESRCH;

	if (task)
		ret = utrace_finish_examine(task, thread->t_engine, exam);

	mutex_unlock(&ugdb->u_mutex);
	return ret;
}

#define REGSET_GENERAL	0
// stolen from gdb-7.1/gdb/gdbserver/linux-x86-low.c
static int x86_64_regmap[] = {
	80, 40, 88, 96, 104, 112, 32, 152, 72, 64, 56, 48, 24, 16,
	8, 0, 128, 144, 136, 160, 184, 192, 200, 208, -1, -1, -1, -1,
	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 120,
};

static char *handle_getregs(struct ugdb *ugdb)
{
	struct utrace_examiner exam;
	struct task_struct *task;
	const struct user_regset_view *view;
	const struct user_regset *rset;
	struct user_regs_struct *pregs;
	int rn;

	static int pkt_size;
	if (!pkt_size) {
		int sz = 0;
		for (rn = 0; rn < ARRAY_SIZE(x86_64_regmap); ++rn) {
			int offs = x86_64_regmap[rn];
			if (offs < 0)
				continue;
			if (offs > (sizeof(*pregs) - sizeof(long))) {
				printk(KERN_INFO "XXX: x86_64_regmap is wrong!\n");
				ugdb->u_err = -EINVAL;
				goto err;
			}
			sz += sizeof(long) * 2;
		}
		pkt_size = sz;
	}

	if (pb_room(&ugdb->u_pbuf) < 4 + pkt_size + sizeof(*pregs)) {
		printk(KERN_INFO "XXX: getregs ENOMEM %d %ld\n",
					pkt_size, sizeof(*pregs));
		goto err;
	}

	pregs = pb_alloc_tmp(&ugdb->u_pbuf, sizeof(*pregs));
	BUG_ON(pregs + 1 != (void*)ugdb->u_pbuf.cur + BUFFER_SIZE);

	task = ugdb_prepare_examine(ugdb, &exam);
	if (!task)
		goto err;

	view = task_user_regset_view(task);
	rset = view->regsets + REGSET_GENERAL;

	rset->get(task, rset, 0, sizeof(*pregs), pregs, NULL);

	if (ugdb_finish_examine(ugdb, &exam))
		goto err;

	pb_start(&ugdb->u_pbuf);
	for (rn = 0; rn < ARRAY_SIZE(x86_64_regmap); ++rn) {
		int offs = x86_64_regmap[rn];
		if (offs >= 0)
			pb_putbs(&ugdb->u_pbuf, (void*)pregs + offs,
					sizeof(long));
	}

	WARN_ON(pb_room(&ugdb->u_pbuf) < sizeof(*pregs));
	pb_end(&ugdb->u_pbuf);
	return NULL;
err:
	return "E01";
}

static typeof(access_process_vm) *u_access_process_vm;

static const char *apvm(struct ugdb *ugdb, struct task_struct *task,
			unsigned long addr, int size)
{
	unsigned char *mbuf;

	mbuf = pb_alloc_bs(&ugdb->u_pbuf, size);
	if (!mbuf) {
		printk(KERN_INFO "XXX: apvm(%d) ENOMEM\n", size);
		goto err;
	}

	size = u_access_process_vm(task, addr, mbuf, size, 0);
	if (size <= 0)
		goto err;

	pb_start(&ugdb->u_pbuf);
	pb_putbs(&ugdb->u_pbuf, mbuf, size);
	pb_end(&ugdb->u_pbuf);
	return NULL;
err:
	return "E01";
}

static const char *handle_readmem(struct ugdb *ugdb, char *cmd)
{
	struct utrace_examiner exam;
	struct task_struct *task;
	unsigned long addr, size;
	const char *ret = "E01";

	if (sscanf(cmd, "m%lx,%lx", &addr, &size) != 2)
		goto out;

	task = ugdb_prepare_examine(ugdb, &exam);
	if (!task)
		goto out;

	ret = apvm(ugdb, task, addr, size);

	/* Too late to report the error*/
	if (ugdb_finish_examine(ugdb, &exam))
		;
out:
	return ret;
}

// -----------------------------------------------------------------------------
#define EQ(cmd, str)					\
	(strncmp((cmd), (str), sizeof(str)-1) ?	false :	\
		((cmd) += sizeof(str)-1, true))

static void handle_command(struct ugdb *ugdb, char *cmd, int len)
{
	struct pbuf *pb = &ugdb->u_pbuf;
	const char *rc = "";

	switch (cmd[0]) {
	case '!':
	case '?':
		rc = "OK";
		break;

	case 'H':
		rc = handle_set_cur(ugdb, cmd + 1);
		break;

	case 'T':
		rc = handle_ck_alive(ugdb, cmd + 1);
		break;

	case 'D':
		rc = handle_detach(ugdb, cmd + 1);
		break;

	case 'g':
		rc = handle_getregs(ugdb);
		break;

	case 'm':
		rc = handle_readmem(ugdb, cmd);
		break;

	case 'c':
		rc = handle_c(ugdb, cmd);
		break;

	case 'q':
		if (EQ(cmd, "qSupported")) {
			if (!strstr(cmd, "multiprocess+")) {
				printk(KERN_INFO "ugdb: can't work without multiprocess\n");
				ugdb->u_err = -EPROTONOSUPPORT;
			}

			pb_packf(&ugdb->u_pbuf, "PacketSize=%x;%s",
				PACKET_SIZE,
				"QStartNoAckMode+;QNonStop+;multiprocess+;"
				"QPassSignals+");
			rc = NULL;
		}
		else if (EQ(cmd, "qfThreadInfo")) {
			rc = handle_thread_info(ugdb, true);
		}
		else if (EQ(cmd, "qsThreadInfo")) {
			rc = handle_thread_info(ugdb, false);
		}
		else if (EQ(cmd, "qTStatus")) {
			rc = "T0";
		}

		break;

	case 'Q':
		if (EQ(cmd, "QStartNoAckMode")) {
			ugdb->u_no_ack = true;
			rc = "OK";
		}
		else if (EQ(cmd, "QNonStop:")) {
			if (*cmd != '1') {
				printk(KERN_INFO "ugdb: all-stop is not implemented.\n");
				ugdb->u_err = -EPROTONOSUPPORT;
			}

			rc = "OK";
		}
		else if (EQ(cmd, "QPassSignals:")) {
			rc = handle_qpass_signals(ugdb, cmd);
		}

		break;

	case 'v':
		if (EQ(cmd, "vAttach;")) {
			rc = handle_vattach(ugdb, cmd);
		}
		else if (EQ(cmd, "vStopped")) {
			rc = handle_vstopped(ugdb);
		}
		else if (EQ(cmd, "vCont")) {
			rc = handle_vcont(ugdb, cmd);
		}

		break;

	default:
		;
	}

	if (rc)
		pb_packs(pb, rc);
}

static void process_commands(struct ugdb *ugdb)
{
	char *cmds = ugdb->u_cbuf;
	int todo = ugdb->u_clen;

	if (o_remote_debug)
		printk(KERN_INFO "=> %.*s\n", ugdb->u_clen, ugdb->u_cbuf);

	while (todo) {
		char first;
		char *c_cmd, *c_end;
		int c_len;

		first = *cmds++;
		todo--;

		switch (first) {
		default:
			printk(KERN_INFO "XXX: unknown chr %02x\n", first);
			pb_putc(&ugdb->u_pbuf, '-');
			break;

		case '-':
			printk(KERN_INFO "XXX: got NACK!\n");
			ugdb->u_err = -EPROTO;
		case '+':
			break;

		case 0x3:
			printk(KERN_INFO "XXX: unexpected CTRL-C\n");
			break;

		case '$':
			c_cmd = cmds;
			c_end = strnchr(c_cmd, todo, '#');
			c_len = c_end ? c_end - cmds : -1;

			if (c_len < 0 || todo < c_len + 3) {
				printk(KERN_INFO "XXX: can't find '#cs'\n");
				++todo;
				--cmds;
				goto out;
			}

			// XXX: verify checksum ?
			todo -= c_len + 3;
			cmds += c_len + 3;
			*c_end = 0;

			if (!ugdb->u_no_ack)
				pb_putc(&ugdb->u_pbuf, '+');

			handle_command(ugdb, c_cmd, c_len);
		}
	}
out:
	ugdb->u_clen = todo;
	if (todo && cmds > ugdb->u_cbuf)
		memmove(ugdb->u_cbuf, cmds, todo);
}

// -----------------------------------------------------------------------------
static int xxx_tinfo(struct ugdb *ugdb)
{
	struct ugdb_thread *thread;
	int tid = 0;

	mutex_lock(&ugdb->u_mutex);
	thread = ugdb_advance_tinfo(ugdb);
	if (thread)
		tid = thread->t_tid;
	mutex_unlock(&ugdb->u_mutex);

	return tid;
}

static int xxx_sc_threads(struct ugdb *ugdb, int tid, bool sc)
{
	void *arg = NULL;
	int pid = 0;
	int ret;

	if (tid < 0) {
		pid = -tid;
		tid = -1;
		arg = (void*)1;
	}

	mutex_lock(&ugdb->u_mutex);
	ret = ugdb_do_each_thread(ugdb, pid, tid,
				sc ? do_stop_thread : do_cont_thread,
				arg);
	mutex_unlock(&ugdb->u_mutex);

	return ret;
}

static int xxx_stop(struct ugdb *ugdb, int tid)
{
	return xxx_sc_threads(ugdb, tid, true);
}

static int xxx_cont(struct ugdb *ugdb, int tid)
{
	return xxx_sc_threads(ugdb, tid, false);
}

static int xxx_get_stopped(struct ugdb *ugdb)
{
	struct ugdb_thread *thread;
	int tid = 1;

	if (ugdb->u_stop_state == U_STOP_IDLE)
		return -1;

	if (ugdb->u_stop_state == U_STOP_PENDING)
		tid = 1000;

	thread = ugdb_next_stopped(ugdb);
	if (!thread)
		return 0;
	return tid * thread->t_tid;
}

static int xxx_show_all(struct ugdb *ugdb)
{
	struct ugdb_process *process;
	struct ugdb_thread *thread;

	printk(KERN_INFO "SHOW start ----------------------------------------\n");

	mutex_lock(&ugdb->u_mutex);
	list_for_each_entry(process, &ugdb->u_processes, p_processes) {
		printk(KERN_INFO "PROC: %x\n", process->p_pid);

		list_for_each_entry(thread, &process->p_threads, t_threads) {
			printk(KERN_INFO "    T: %x %p; %p %p\n",
					thread->t_tid, thread,
					thread->t_spid, pid_task(thread->t_spid, PIDTYPE_PID));
		}

	}
	mutex_unlock(&ugdb->u_mutex);

	printk(KERN_INFO "SHOW end ----------------------------------------\n");
	return 0;
}

static long ugdb_f_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
	struct ugdb *ugdb = file->private_data;
	// XXX: otherwise gdb->get_tty_state(TCGETS, TCSETS, TCFLSH) complains
	int ret = 0;

	// XXX: temporary debugging hooks, ignore.
	switch (cmd) {
		case 0x666 + 0:
			ret = ugdb_attach(ugdb, arg);
			break;

		case 0x666 + 1:
			ret = ugdb_detach(ugdb, arg);
			break;

		case 0x666 + 2:
			ret = xxx_tinfo(ugdb);
			break;

		case 0x666 + 3:
			ret = xxx_stop(ugdb, arg);
			break;

		case 0x666 + 4:
			ret = xxx_cont(ugdb, arg);
			break;

		case 0x666 + 5:
			ret = xxx_get_stopped(ugdb);
			break;

		case 0x666 + 6:
			ret = xxx_show_all(ugdb);
			break;
	}

	return ret;
}

static unsigned int ugdb_f_poll(struct file *file, poll_table *wait)
{
	struct ugdb *ugdb = file->private_data;
	unsigned int mask;

	poll_wait(file, &ugdb->u_wait, wait);

	mask = (POLLOUT | POLLWRNORM);

	if (pb_size(&ugdb->u_pbuf) || ugdb->u_stop_state == U_STOP_PENDING)
		mask |= (POLLIN | POLLRDNORM);

	if (ugdb->u_err)
		mask |= POLLERR;

	return mask;
}

static ssize_t ugdb_f_read(struct file *file, char __user *ubuf,
				size_t count, loff_t *ppos)
{
	struct ugdb *ugdb = file->private_data;
	struct pbuf *pb = &ugdb->u_pbuf;

	if (ugdb->u_err)
		return ugdb->u_err;

	if (ugdb->u_stop_state == U_STOP_PENDING)
		ugdb_report_stopped(ugdb, true);

	if (pb_size(pb) > count) {
		printk(KERN_INFO "XXX: short read %d %ld\n",
					pb_size(pb), count);
	}

	return pb_copy_to_user(pb, ubuf, count);
}

static ssize_t ugdb_f_write(struct file *file, const char __user *ubuf,
				size_t count, loff_t *ppos)
{
	struct ugdb *ugdb = file->private_data;

	if (ugdb->u_err)
		return ugdb->u_err;

	if (count > PACKET_SIZE - ugdb->u_clen) {
		count = PACKET_SIZE - ugdb->u_clen;
		printk("XXX: write(%ld,%d) enospc\n", count, ugdb->u_clen);
	}

	if (copy_from_user(ugdb->u_cbuf + ugdb->u_clen, ubuf, count))
		return -EFAULT;

	ugdb->u_clen += count;
	process_commands(ugdb);

	return count;
}

static int ugdb_f_open(struct inode *inode, struct file *file)
{
	nonseekable_open(inode, file);

	file->private_data = ugdb_create();

	return	IS_ERR(file->private_data) ?
		PTR_ERR(file->private_data) : 0;
}

static int ugdb_f_release(struct inode *inode, struct file *file)
{
	ugdb_destroy(file->private_data);

	return 0;
}

static const struct file_operations ugdb_f_ops = {
	.open			= ugdb_f_open,
	.unlocked_ioctl		= ugdb_f_ioctl,
	.poll			= ugdb_f_poll,
	.read			= ugdb_f_read,
	.write			= ugdb_f_write,
	.release		= ugdb_f_release,
};

#include <linux/kallsyms.h>

struct kallsyms_sym {
	const char	*name;
	unsigned long	addr;
};

static int kallsyms_on_each_symbol_cb(void *data, const char *name,
				struct module *mod, unsigned long addr)
{
	struct kallsyms_sym *sym = data;

	if (strcmp(name, sym->name))
		return 0;

	sym->addr = addr;
	return 1;
}

// XXX: kallsyms_lookup_name() is not exported in 2.6.32
static bool lookup_unexported(void)
{
	struct kallsyms_sym sym;

	sym.name = "access_process_vm";
	if (!kallsyms_on_each_symbol(kallsyms_on_each_symbol_cb, &sym))
		goto err;
	u_access_process_vm = (void*)sym.addr;

	return true;
err:
	printk(KERN_ERR "ugdb: can't lookup %s\n", sym.name);
	return false;
}

#define PROC_NAME	"ugdb"
struct proc_dir_entry *ugdb_pde;

static int __init ugdb_init(void)
{
	if (!lookup_unexported())
		return -ESRCH;

	ugdb_pde = proc_create(PROC_NAME, S_IFREG|S_IRUGO|S_IWUGO,
				NULL, &ugdb_f_ops);
	if (!ugdb_pde)
		return -EBADF;

	return 0;
}

static void __exit ugdb_exit(void)
{
	remove_proc_entry(PROC_NAME, NULL);
}

MODULE_LICENSE("GPL");
module_init(ugdb_init);
module_exit(ugdb_exit);

Re: gdbstub initial code, v7

[Jan Kratochvil]

On Mon, 30 Aug 2010 20:58:50 +0200, Oleg Nesterov wrote:
> 	- report signals. A bit more code changes than I expected.

BTW not sure if it is already the right time for it but to keep ugdb on-par
with my linux-nat's re-post today (still not accepted in FSF GDB)
	[0/9]#2 Fix lost siginfo_t
	http://sourceware.org/ml/gdb-patches/2010-08/msg00480.html

ugdb should support qXfer:siginfo, currently accessible only via $_siginfo
print/set, though.


Thanks,
Jan

Re: gdbstub initial code, v7

[Jan Kratochvil]

On Mon, 30 Aug 2010 21:20:40 +0200, Jan Kratochvil wrote:
> On Mon, 30 Aug 2010 20:58:50 +0200, Oleg Nesterov wrote:
> > 	- report signals. A bit more code changes than I expected.
> 
> BTW not sure if it is already the right time for it but to keep ugdb on-par
> with my linux-nat's re-post today (still not accepted in FSF GDB)

That's not true, this functionality needs no gdb/remote.c changes and its
correctnes relies just on ugdb (and it is probably not a problem for ugdb).


> ugdb should support qXfer:siginfo, currently accessible only via $_siginfo
> print/set, though.

Still sure this feature should be also implemented one day.


Thanks,
Jan

Re: gdbstub initial code, v7

[Oleg Nesterov]

Sorry for the delay, I was distracted. Trying to switch back to ugdb.

On 08/31, Jan Kratochvil wrote:
>
> > ugdb should support qXfer:siginfo, currently accessible only via $_siginfo
> > print/set, though.
>
> Still sure this feature should be also implemented one day.

Yes sure. This should be simple, although I didn't expect qXfer
needs remote_escape_output() and x86_siginfo_fixup(). I assume
that qXfer:siginfo:read always mean Hg thread. It is not clear
to me what should ugdb report if there is no a valid siginfo.
linux_xfer_siginfo() return E01, but gdbserver uses SIGSTOP to
stop the tracee, so it always has something to report. But ugdb
stop the tracee somewhere else, not in get_signal_to_deliver()
path.

Likewise, it is not clear what should ugdb do if gdb sends
$CSIG in this case. But this all is minor, I think.

I was going to send v8 which implements qXfer:siginfo:read and
continue with signal, but (oh, as always) hit the unexpected
problems. To the point, I had to add printk's to utrace.c to
understand what is wrong. Hopefully tomorrow.

Oleg.

Re: gdbstub initial code, v7

[Jan Kratochvil]

On Thu, 02 Sep 2010 22:06:32 +0200, Oleg Nesterov wrote:
> I assume that qXfer:siginfo:read always mean Hg thread.

It seems so.

> It is not clear to me what should ugdb report if there is no a valid
> siginfo.  linux_xfer_siginfo() return E01, but gdbserver uses SIGSTOP to
> stop the tracee,

I find error more appropriate in such case.

> Likewise, it is not clear what should ugdb do if gdb sends
> $CSIG in this case.

Currently GDB does not do anything special, that is if there is siginfo for
signal SIGUSR1 but one does $C0B (SIGSEGV) does ptrace reset the siginfo or is
left the SIGUSR1 siginfo for SIGSEGV?

> But this all is minor, I think.

As this is being discussed for GDB I would find enough to just make $_siginfo
accessible without these details.


Thanks,
Jan

Re: gdbstub initial code, v7

[Frank Ch. Eigler]

Oleg Nesterov <oleg@redhat.com> writes:

> [...]  To the point, I had to add printk's to utrace.c to understand
> what is wrong. Hopefully tomorrow.

You might wish to try out systemtap for such purposes.  It's easy to
insert printk-like tracing at almost any point; monitor variables for
changes, pretty-print structs, etc.  No recompilation/reboot.

- FChE

Re: gdbstub initial code, v7

[Roland McGrath]

> Currently GDB does not do anything special, that is if there is siginfo for
> signal SIGUSR1 but one does $C0B (SIGSEGV) does ptrace reset the siginfo or is
> left the SIGUSR1 siginfo for SIGSEGV?

The kernel considers this sloppy behavior on the debugger's part.  If
you inject a different signal, we expect you should PTRACE_SETSIGINFO
to something appropriate, or else that you really didn't care about
the bits being accurate.  If the resumption signal does not match the
siginfo_t.si_signo, then the kernel resets the siginfo as if the
debugger had just used kill with the new signal (i.e. si_pid, si_uid
point to the ptracer).


Thanks,
Roland

Re: gdbstub initial code, v7

[Jan Kratochvil]

On Fri, 03 Sep 2010 21:59:06 +0200, Roland McGrath wrote:
> > Currently GDB does not do anything special, that is if there is siginfo for
> > signal SIGUSR1 but one does $C0B (SIGSEGV) does ptrace reset the siginfo or is
> > left the SIGUSR1 siginfo for SIGSEGV?
> 
> The kernel considers this sloppy behavior on the debugger's part.  If
> you inject a different signal, we expect you should PTRACE_SETSIGINFO
> to something appropriate, or else that you really didn't care about
> the bits being accurate.  If the resumption signal does not match the
> siginfo_t.si_signo, then the kernel resets the siginfo as if the
> debugger had just used kill with the new signal (i.e. si_pid, si_uid
> point to the ptracer).

OK, that seems to me as the best choice.  Sorry I did not test/read it.


Thanks,
Jan

Re: gdbstub initial code, v7

[Roland McGrath]

> OK, that seems to me as the best choice.  Sorry I did not test/read it.

No need for you to do so.  That's why you have kernel hackers to consult.

Thanks,
Roland

Re: gdbstub initial code, v7

[Oleg Nesterov]

On 09/03, Frank Ch. Eigler wrote:
>
> Oleg Nesterov <oleg@redhat.com> writes:
>
> > [...]  To the point, I had to add printk's to utrace.c to understand
> > what is wrong. Hopefully tomorrow.
>
> You might wish to try out systemtap for such purposes.  It's easy to
> insert printk-like tracing at almost any point; monitor variables for
> changes, pretty-print structs, etc.

Probably I should try the new versions...

Last time I tried systemtap more than year ago. It didn't work for me
without the "properly installed" kernel/etc which I don't have. And,
I doubt I can insert the probe at the needed line without DEBUG_INFO.

> No recompilation/reboot.

CONFIG_KPROBE ;)

But anyway I agree, systemtap is the great tool.

Oleg.

Re: gdbstub initial code, v7

[Oleg Nesterov]

On 09/03, Jan Kratochvil wrote:
>
> On Thu, 02 Sep 2010 22:06:32 +0200, Oleg Nesterov wrote:
>
> > Likewise, it is not clear what should ugdb do if gdb sends
> > $CSIG in this case.
>
> Currently GDB does not do anything special, that is if there is siginfo for
> signal SIGUSR1 but one does $C0B (SIGSEGV) does ptrace reset the siginfo or is
> left the SIGUSR1 siginfo for SIGSEGV?

The kernel changes siginfo->si_signo (and updates other fields). So
does ugdb.

But I meant another case, when the stopped tracee doesn't have siginfo.
Currently ugdb just sends this signal to tracee, and then it will be
reported to gdb. Not sure if this is right or not, I can change this.
(or perhap this doesn't matter, I dunno).

Oleg.

Re: gdbstub initial code, v7

[Roland McGrath]

> But I meant another case, when the stopped tracee doesn't have siginfo.
> Currently ugdb just sends this signal to tracee, and then it will be
> reported to gdb. Not sure if this is right or not, I can change this.
> (or perhap this doesn't matter, I dunno).

What do you mean by "doesn't have siginfo"?  You mean non-signal stops?
What non-signal stops does ugdb report?


Thanks,
Roland

Re: gdbstub initial code, v7

[Oleg Nesterov]

On 09/10, Roland McGrath wrote:
>
> > But I meant another case, when the stopped tracee doesn't have siginfo.
> > Currently ugdb just sends this signal to tracee, and then it will be
> > reported to gdb. Not sure if this is right or not, I can change this.
> > (or perhap this doesn't matter, I dunno).
>
> What do you mean by "doesn't have siginfo"?  You mean non-signal stops?

Yes.

> What non-signal stops does ugdb report?

	(gdb) interrupt

ugdb sets "please stop" flag and does utrace_control(INTERRUPT). However,
in unlikely case the tracee can stop before ->report_signal() reporting
loop (especially in multitracing case). Or it can be already stopped
(note: this needs a separate discussion, currently ugdb intentionally
doesn't handle this case).

And. With the current implementation, even if the tracee stops after
ugdb_report_signal() was called, it doesn't setup ->t_siginfo.

IOW. If the tracee actually recieves a signal, then

	- qXfer:siginfo:read works

	- "signal SIG" works as expected (delivered to tracee)

Otherwise

	- qXfer:siginfo:read reports E01

	- "signal XX" means TXX report.

Once again, this can be changed (fixed?), but I am not sure this
should be changed.

Oleg.

Re: gdbstub initial code, v7

[Roland McGrath]

> ugdb sets "please stop" flag and does utrace_control(INTERRUPT). However,
> in unlikely case the tracee can stop before ->report_signal() reporting

I don't think this is the right thing to do.  When the intent is explicitly
to interrupt, there is no reason to stop before the interruption is
complete, i.e. report_signal.  If you only stop there, then you can always
process a signal injection with complete flexibility.  The gdb model and
the remote protocol doesn't currently have any concept of requesting a stop
that is not an interruption.


Thanks,
Roland

Re: gdbstub initial code, v7

[Oleg Nesterov]

On 09/10, Roland McGrath wrote:
>
> > ugdb sets "please stop" flag and does utrace_control(INTERRUPT). However,
> > in unlikely case the tracee can stop before ->report_signal() reporting
>
> I don't think this is the right thing to do.  When the intent is explicitly
> to interrupt, there is no reason to stop before the interruption is
> complete, i.e. report_signal.

This means that ugdb_report_quiesce() should never return UTRACE_STOP,
and that is all.

But what about multitracing? Suppose that "(gdb) interrupt" happens
just before, say, do_report_syscall_entry() and another engine wants
to stop. If ugdb_report_quiesce() doesn't return UTRACE_STOP, then
gdb will wait until another debugger resumes the tracee.

What do you think?

> If you only stop there, then you can always
> process a signal injection with complete flexibility.

Yes, sure (again, currently ugdb does not injection a signal even
if the tracee was stopped in report_signal, but of course we can
change this).

Oleg.

Re: gdbstub initial code, v7

[Roland McGrath]

> On 09/10, Roland McGrath wrote:
> >
> > > ugdb sets "please stop" flag and does utrace_control(INTERRUPT). However,
> > > in unlikely case the tracee can stop before ->report_signal() reporting
> >
> > I don't think this is the right thing to do.  When the intent is explicitly
> > to interrupt, there is no reason to stop before the interruption is
> > complete, i.e. report_signal.
> 
> This means that ugdb_report_quiesce() should never return UTRACE_STOP,
> and that is all.

I'm not sure about this.

> But what about multitracing? Suppose that "(gdb) interrupt" happens
> just before, say, do_report_syscall_entry() and another engine wants
> to stop. If ugdb_report_quiesce() doesn't return UTRACE_STOP, then
> gdb will wait until another debugger resumes the tracee.

Yes, I do think that's a problem.  We want gdb to report back promptly.
One possibility is to have report_quiesce notice its argument is
UTRACE_EVENT(SYSCALL_ENTRY) and roll back to before the syscall.
That is, it enables SYSCALL_ENTRY and SYSCALL_EXIT reporting, then
its report_syscall_entry uses UTRACE_SIGNAL_ABORT, report_syscall_exit
does syscall_set_return_value(-ERESTARTNOHAND, 0) and then returns 
UTRACE_INTERRUPT.  Now, we'll reenter a UTRACE_SIGNAL_REPORT callback
"before" the system call, and we can stop there without being in any
sticky situation.


Thanks,
Roland

Re: gdbstub initial code, v7

[Oleg Nesterov]

On 10/13, Roland McGrath wrote:
>
> > On 09/10, Roland McGrath wrote:
> > >
> > > > ugdb sets "please stop" flag and does utrace_control(INTERRUPT). However,
> > > > in unlikely case the tracee can stop before ->report_signal() reporting
> > >
> > > I don't think this is the right thing to do.  When the intent is explicitly
> > > to interrupt, there is no reason to stop before the interruption is
> > > complete, i.e. report_signal.
> >
> > This means that ugdb_report_quiesce() should never return UTRACE_STOP,
> > and that is all.
>
> I'm not sure about this.

Ignoring the problems below, why?

> > But what about multitracing? Suppose that "(gdb) interrupt" happens
> > just before, say, do_report_syscall_entry() and another engine wants
> > to stop. If ugdb_report_quiesce() doesn't return UTRACE_STOP, then
> > gdb will wait until another debugger resumes the tracee.
>
> Yes, I do think that's a problem.  We want gdb to report back promptly.
> One possibility is to have report_quiesce notice its argument is
> UTRACE_EVENT(SYSCALL_ENTRY) and roll back to before the syscall.
> That is, it enables SYSCALL_ENTRY and SYSCALL_EXIT reporting, then
> its report_syscall_entry uses UTRACE_SIGNAL_ABORT, report_syscall_exit
> does syscall_set_return_value(-ERESTARTNOHAND, 0) and then returns
> UTRACE_INTERRUPT.  Now, we'll reenter a UTRACE_SIGNAL_REPORT callback
> "before" the system call, and we can stop there without being in any
> sticky situation.

Well, but this doesn't look friendly to other engines...

And at first glance this looks a bit too hairy. And, this doesn't
cover another case: gdb asks to stop the tracee when it is already
stopped by another engine and sleeps in utrace_resume() path.

So, I think ugdb should be changed so that "signal SIG" always works
(without reporing this signal) even when the stopped tracee doesn't
have the signal context.

As for $_siginfo (qXfer:siginfo:read::), I do not know what ugdb
should do. Probably it can just report the all-zeroes siginfo or
report si_signo = SIGSTOP.

Oleg.