Avoid kernel panic when attaching a process which may not be stopped

by debugger, e.g process is dumping core. Only access p_xthread if
P_STOPPED_TRACE is set, this means thread is ready to exchange signal
with debugger, print a warning if P_STOPPED_TRACE is not set due to
some bugs in other code, if there is.

The patch has been tested by Anish Mistry mistry.7 at osu dot edu, and
is slightly adjusted.

2 files changed, 30 insertions, 28 deletions

diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index 4c7f6a9..cd2402e 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -2272,7 +2272,6 @@ do_tdsignal(struct proc *p, struct thread *td, int sig, ksiginfo_t *ksi)
 				goto out;
 			p->p_flag |= P_STOPPED_SIG;
 			p->p_xstat = sig;
-			p->p_xthread = td;
 			mtx_lock_spin(&sched_lock);
 			FOREACH_THREAD_IN_PROC(p, td0) {
 				if (TD_IS_SLEEPING(td0) &&
@@ -2599,7 +2598,6 @@ issignal(td)
 				    &p->p_mtx.mtx_object, "Catching SIGSTOP");
 				p->p_flag |= P_STOPPED_SIG;
 				p->p_xstat = sig;
-				p->p_xthread = td;
 				mtx_lock_spin(&sched_lock);
 				FOREACH_THREAD_IN_PROC(p, td0) {
 					if (TD_IS_SLEEPING(td0) &&
diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
index 0176525..0b19ab1 100644
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -491,7 +491,6 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 	int error, write, tmp, num;
 	int proctree_locked = 0;
 	lwpid_t tid = 0, *buf;
-	pid_t saved_pid = pid;
 #ifdef COMPAT_IA32
 	int wrap32 = 0, safe = 0;
 	struct ptrace_io_desc32 *piod32 = NULL;
@@ -567,7 +566,12 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 	}
 
 	if (tid == 0) {
-		td2 = FIRST_THREAD_IN_PROC(p);
+		if ((p->p_flag & P_STOPPED_TRACE) != 0) {
+			KASSERT(p->p_xthread != NULL, ("NULL p_xthread"));
+			td2 = p->p_xthread;
+		} else {
+			td2 = FIRST_THREAD_IN_PROC(p);
+		}
 		tid = td2->td_tid;
 	}
 
@@ -637,12 +641,19 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 		}
 
 		/* not currently stopped */
-		if (!P_SHOULDSTOP(p) || p->p_suspcount != p->p_numthreads ||
+		if ((p->p_flag & (P_STOPPED_SIG | P_STOPPED_TRACE)) == 0 ||
+		    p->p_suspcount != p->p_numthreads  ||
 		    (p->p_flag & P_WAITED) == 0) {
 			error = EBUSY;
 			goto fail;
 		}
 
+		if ((p->p_flag & P_STOPPED_TRACE) == 0) {
+			static int count = 0;
+			if (count++ == 0)
+				printf("P_STOPPED_TRACE not set.\n");
+		}
+
 		/* OK */
 		break;
 	}
@@ -795,17 +806,14 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 		if (proctree_locked)
 			sx_xunlock(&proctree_lock);
 		/* deliver or queue signal */
-		if (P_SHOULDSTOP(p)) {
-			p->p_xstat = data;
+		mtx_lock_spin(&sched_lock);
+		td2->td_flags &= ~TDF_XSIG;
+		mtx_unlock_spin(&sched_lock);
+		td2->td_xsig = data;
+		p->p_xstat = data;
+		p->p_xthread = NULL;
+		if ((p->p_flag & (P_STOPPED_SIG | P_STOPPED_TRACE)) != 0) {
 			mtx_lock_spin(&sched_lock);
-			if (saved_pid <= PID_MAX) {
-				p->p_xthread->td_flags &= ~TDF_XSIG;
-				p->p_xthread->td_xsig = data;
-			} else {
-				td2->td_flags &= ~TDF_XSIG;
-				td2->td_xsig = data;
-			}
-			p->p_xthread = NULL;
 			if (req == PT_DETACH) {
 				struct thread *td3;
 				FOREACH_THREAD_IN_PROC(p, td3)
@@ -818,15 +826,16 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 			 */
 			mtx_unlock_spin(&sched_lock);
 			thread_continued(p);
-			p->p_flag &= ~(P_STOPPED_TRACE|P_STOPPED_SIG);
+			p->p_flag &= ~(P_STOPPED_TRACE|P_STOPPED_SIG|P_WAITED);
 			mtx_lock_spin(&sched_lock);
 			thread_unsuspend(p);
 			mtx_unlock_spin(&sched_lock);
-		} else if (data) {
-			psignal(p, data);
 		}
-		PROC_UNLOCK(p);
 
+		if (data)
+			psignal(p, data);
+
+		PROC_UNLOCK(p);
 		return (0);
 
 	case PT_WRITE_I:
@@ -964,16 +973,11 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 			return (EINVAL);
 		pl = addr;
 		_PHOLD(p);
-		if (saved_pid <= PID_MAX) {
-			pl->pl_lwpid = p->p_xthread->td_tid;
+		pl->pl_lwpid = td2->td_tid;
+		if (td2->td_flags & TDF_XSIG)
 			pl->pl_event = PL_EVENT_SIGNAL;
-		} else {
-			pl->pl_lwpid = td2->td_tid;
-			if (td2->td_flags & TDF_XSIG)
-				pl->pl_event = PL_EVENT_SIGNAL;
-			else
-				pl->pl_event = 0;
-		}
+		else
+			pl->pl_event = 0;
 		if (td2->td_pflags & TDP_SA) {
 			pl->pl_flags = PL_FLAG_SA;
 			if (td2->td_upcall && !TD_CAN_UNBIND(td2))