Implement the PT_LWPINFO request. This request can be used by the

tracing process to obtain information about the LWP that caused the
traced process to stop. Debuggers can use this information to select
the thread currently running on the LWP as the current thread.

The request has been made compatible with NetBSD for as much as
possible. This implementation differs from NetBSD in the following
ways:
1.  The data argument is allowed to be smaller than the size of the
    ptrace_lwpinfo structure known to the kernel, but not 0. This
    is opposite to what NetBSD allows. The reason for this is that
    we can extend the structure without affecting older binaries.
2.  On NetBSD the tracing process is to set the pl_lwpid field to
    the Id of the LWP it wants information of. We don't do that.
    Our ptrace interface allows passing the LWP Id instead of the
    PID. The tracing process is to set the PID to the LWP Id it
    wants information of.
3.  When the PID is actually the PID of the tracing process, this
    request returns the information about the LWP that caused the
    process to stop. This was the whole purpose of the request in
    the first place.

When the traced process has exited, this request will return the
LWP Id 0, indicating that the process state is not the result of
an event specific to a LWP.

sys/kern/kern_exit.c

@@ -429,6 +429,7 @@ exit1(struct thread *td, int rv)
 	mtx_lock(&Giant);	
 	PROC_LOCK(p);
 	p->p_xstat = rv;
+	p->p_xlwpid = td->td_tid;
 	*p->p_ru = p->p_stats->p_ru;
 	mtx_lock_spin(&sched_lock);
 	calcru(p, &p->p_ru->ru_utime, &p->p_ru->ru_stime, NULL);

sys/kern/kern_sig.c

@@ -1882,6 +1882,7 @@ do_tdsignal(struct thread *td, int sig, sigtarget_t target)
 				goto out;
 			p->p_flag |= P_STOPPED_SIG;
 			p->p_xstat = sig;
+			p->p_xlwpid = td->td_tid;
 			mtx_lock_spin(&sched_lock);
 			FOREACH_THREAD_IN_PROC(p, td0) {
 				if (TD_IS_SLEEPING(td0) &&
@@ -2011,6 +2012,7 @@ ptracestop(struct thread *td, int sig)
 	    &p->p_mtx.mtx_object, "Stopping for traced signal");
 
 	p->p_xstat = sig;
+	p->p_xlwpid = td->td_tid;
 	PROC_LOCK(p->p_pptr);
 	psignal(p->p_pptr, SIGCHLD);
 	PROC_UNLOCK(p->p_pptr);
@@ -2154,6 +2156,7 @@ issignal(td)
 				    &p->p_mtx.mtx_object, "Catching SIGSTOP");
 				p->p_flag |= P_STOPPED_SIG;
 				p->p_xstat = sig;
+				p->p_xlwpid = td->td_tid;
 				mtx_lock_spin(&sched_lock);
 				FOREACH_THREAD_IN_PROC(p, td0) {
 					if (TD_IS_SLEEPING(td0) &&

sys/kern/sys_process.c

@@ -305,6 +305,7 @@ ptrace(struct thread *td, struct ptrace_args *uap)
 	 */
 	union {
 		struct ptrace_io_desc piod;
+		struct ptrace_lwpinfo pl;
 		struct dbreg dbreg;
 		struct fpreg fpreg;
 		struct reg reg;
@@ -317,6 +318,7 @@ ptrace(struct thread *td, struct ptrace_args *uap)
 	case PT_GETREGS:
 	case PT_GETFPREGS:
 	case PT_GETDBREGS:
+	case PT_LWPINFO:
 		break;
 	case PT_SETREGS:
 		error = copyin(uap->addr, &r.reg, sizeof r.reg);
@@ -354,6 +356,9 @@ ptrace(struct thread *td, struct ptrace_args *uap)
 	case PT_GETDBREGS:
 		error = copyout(&r.dbreg, uap->addr, sizeof r.dbreg);
 		break;
+	case PT_LWPINFO:
+		error = copyout(&r.pl, uap->addr, uap->data);
+		break;
 	}
 
 	return (error);
@@ -367,6 +372,7 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 	struct proc *curp, *p, *pp;
 	struct thread *td2 = NULL;
 	struct ptrace_io_desc *piod;
+	struct ptrace_lwpinfo *pl;
 	int error, write, tmp;
 	int proctree_locked = 0;
 	lwpid_t tid = 0;
@@ -606,6 +612,7 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 		/* deliver or queue signal */
 		if (P_SHOULDSTOP(p)) {
 			p->p_xstat = data;
+			p->p_xlwpid = 0;
 			p->p_flag &= ~(P_STOPPED_TRACE|P_STOPPED_SIG);
 			mtx_lock_spin(&sched_lock);
 			thread_unsuspend(p);
@@ -727,6 +734,17 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 		PROC_UNLOCK(p);
 		return (error);
 
+	case PT_LWPINFO:
+		if (data == 0 || data > sizeof(*pl))
+			return (EINVAL);
+		pl = addr;
+		_PHOLD(p);
+		pl->pl_lwpid = p->p_xlwpid;
+		_PRELE(p);
+		PROC_UNLOCK(p);
+		pl->pl_event = PL_EVENT_SIGNAL;
+		return (0);
+
 	default:
 #ifdef __HAVE_PTRACE_MACHDEP
 		if (req >= PT_FIRSTMACH) {
@@ -764,6 +782,7 @@ stopevent(struct proc *p, unsigned int event, unsigned int val)
 	p->p_step = 1;
 	do {
 		p->p_xstat = val;
+		p->p_xlwpid = 0;
 		p->p_stype = event;	/* Which event caused the stop? */
 		wakeup(&p->p_stype);	/* Wake up any PIOCWAIT'ing procs */
 		msleep(&p->p_step, &p->p_mtx, PWAIT, "stopevent", 0);

sys/sys/proc.h

@@ -602,6 +602,7 @@ struct proc {
 #define	p_endcopy	p_xstat
 
 	u_short		p_xstat;	/* (c) Exit status; also stop sig. */
+	lwpid_t		p_xlwpid;	/* (c) Thread corresponding p_xstat. */
 	int		p_numthreads;	/* (j) Number of threads. */
 	int		p_numksegrps;	/* (?) number of ksegrps */
 	struct mdproc	p_md;		/* Any machine-dependent fields. */

sys/sys/ptrace.h

@@ -47,6 +47,7 @@
 #define	PT_ATTACH	10	/* trace some running process */
 #define	PT_DETACH	11	/* stop tracing a process */
 #define PT_IO		12	/* do I/O to/from stopped process. */
+#define	PT_LWPINFO	13	/* Info about the LWP that stopped. */
 
 #define	PT_TO_SCE	20
 #define	PT_TO_SCX	21
@@ -77,6 +78,14 @@ struct ptrace_io_desc {
 #define PIOD_READ_I	3	/* Read from I space */
 #define PIOD_WRITE_I	4	/* Write to I space */
 
+/* Argument structure for PT_LWPINFO. */
+struct ptrace_lwpinfo {
+	lwpid_t	pl_lwpid;	/* LWP described. */
+	int	pl_event;	/* Event that stopped the LWP. */
+#define	PL_EVENT_NONE	0
+#define	PL_EVENT_SIGNAL	1
+};
+
 #ifdef _KERNEL
 
 #define	PTRACESTOP_SC(p, td, flag)				\