MFC 292894,292896: Add ptrace(2) reporting for LWP events.

292894:
Add ptrace(2) reporting for LWP events.

Add two new LWPINFO flags: PL_FLAG_BORN and PL_FLAG_EXITED for reporting
thread creation and destruction. Newly created threads will stop to report
PL_FLAG_BORN before returning to userland and exiting threads will stop to
report PL_FLAG_EXIT before exiting completely. Both of these events are
only enabled and reported if PT_LWP_EVENTS is enabled on a process.

292896:
Document the recently added support for ptrace(2) LWP events.

4 files changed, 70 insertions, 22 deletions

diff --git a/sys/kern/kern_fork.c b/sys/kern/kern_fork.c
index a84b619..3d7bf09 100644
--- a/sys/kern/kern_fork.c
+++ b/sys/kern/kern_fork.c
@@ -1083,7 +1083,7 @@ fork_return(struct thread *td, struct trapframe *frame)
 			cv_broadcast(&p->p_dbgwait);
 		}
 		PROC_UNLOCK(p);
-	} else if (p->p_flag & P_TRACED) {
+	} else if (p->p_flag & P_TRACED || td->td_dbgflags & TDB_BORN) {
  		/*
 		 * This is the start of a new thread in a traced
 		 * process.  Report a system call exit event.
@@ -1091,9 +1091,10 @@ fork_return(struct thread *td, struct trapframe *frame)
 		PROC_LOCK(p);
 		td->td_dbgflags |= TDB_SCX;
 		_STOPEVENT(p, S_SCX, td->td_dbg_sc_code);
-		if ((p->p_stops & S_PT_SCX) != 0)
+		if ((p->p_stops & S_PT_SCX) != 0 ||
+		    (td->td_dbgflags & TDB_BORN) != 0)
 			ptracestop(td, SIGTRAP);
-		td->td_dbgflags &= ~TDB_SCX;
+		td->td_dbgflags &= ~(TDB_SCX | TDB_BORN);
 		PROC_UNLOCK(p);
 	}
 
diff --git a/sys/kern/kern_sig.c b/sys/kern/kern_sig.c
index 29783f8..65675dd 100644
--- a/sys/kern/kern_sig.c
+++ b/sys/kern/kern_sig.c
@@ -2488,7 +2488,12 @@ ptracestop(struct thread *td, int sig)
 	    td->td_tid, p->p_pid, td->td_dbgflags, sig);
 	PROC_SLOCK(p);
 	while ((p->p_flag & P_TRACED) && (td->td_dbgflags & TDB_XSIG)) {
-		if (p->p_flag & P_SINGLE_EXIT) {
+		if (p->p_flag & P_SINGLE_EXIT &&
+		    !(td->td_dbgflags & TDB_EXIT)) {
+			/*
+			 * Ignore ptrace stops except for thread exit
+			 * events when the process exits.
+			 */
 			td->td_dbgflags &= ~TDB_XSIG;
 			PROC_SUNLOCK(p);
 			return (sig);
diff --git a/sys/kern/kern_thr.c b/sys/kern/kern_thr.c
index 98965b1..8b19967 100644
--- a/sys/kern/kern_thr.c
+++ b/sys/kern/kern_thr.c
@@ -252,6 +252,8 @@ thread_create(struct thread *td, struct rtprio *rtp,
 	thread_unlock(td);
 	if (P_SHOULDSTOP(p))
 		newtd->td_flags |= TDF_ASTPENDING | TDF_NEEDSUSPCHK;
+	if (p->p_flag2 & P2_LWP_EVENTS)
+		newtd->td_dbgflags |= TDB_BORN;
 	PROC_UNLOCK(p);
 
 	tidhash_add(newtd);
@@ -314,29 +316,54 @@ kern_thr_exit(struct thread *td)
 
 	p = td->td_proc;
 
-	rw_wlock(&tidhash_lock);
+	/*
+	 * If all of the threads in a process call this routine to
+	 * exit (e.g. all threads call pthread_exit()), exactly one
+	 * thread should return to the caller to terminate the process
+	 * instead of the thread.
+	 *
+	 * Checking p_numthreads alone is not sufficient since threads
+	 * might be committed to terminating while the PROC_LOCK is
+	 * dropped in either ptracestop() or while removing this thread
+	 * from the tidhash.  Instead, the p_pendingexits field holds
+	 * the count of threads in either of those states and a thread
+	 * is considered the "last" thread if all of the other threads
+	 * in a process are already terminating.
+	 */
 	PROC_LOCK(p);
-
-	if (p->p_numthreads != 1) {
-		racct_sub(p, RACCT_NTHR, 1);
-		LIST_REMOVE(td, td_hash);
-		rw_wunlock(&tidhash_lock);
-		tdsigcleanup(td);
-		umtx_thread_exit(td);
-		PROC_SLOCK(p);
-		thread_stopped(p);
-		thread_exit();
-		/* NOTREACHED */
+	if (p->p_numthreads == p->p_pendingexits + 1) {
+		/*
+		 * Ignore attempts to shut down last thread in the
+		 * proc.  This will actually call _exit(2) in the
+		 * usermode trampoline when it returns.
+		 */
+		PROC_UNLOCK(p);
+		return (0);
 	}
 
+	p->p_pendingexits++;
+	td->td_dbgflags |= TDB_EXIT;
+	if (p->p_flag & P_TRACED && p->p_flag2 & P2_LWP_EVENTS)
+		ptracestop(td, SIGTRAP);
+	PROC_UNLOCK(p);
+	tidhash_remove(td);
+	PROC_LOCK(p);
+	p->p_pendingexits--;
+
 	/*
-	 * Ignore attempts to shut down last thread in the proc.  This
-	 * will actually call _exit(2) in the usermode trampoline when
-	 * it returns.
+	 * The check above should prevent all other threads from this
+	 * process from exiting while the PROC_LOCK is dropped, so
+	 * there must be at least one other thread other than the
+	 * current thread.
 	 */
-	PROC_UNLOCK(p);
-	rw_wunlock(&tidhash_lock);
-	return (0);
+	KASSERT(p->p_numthreads > 1, ("too few threads"));
+	racct_sub(p, RACCT_NTHR, 1);
+	tdsigcleanup(td);
+	umtx_thread_exit(td);
+	PROC_SLOCK(p);
+	thread_stopped(p);
+	thread_exit();
+	/* NOTREACHED */
 }
 
 int
diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
index 96e0181..66e9097 100644
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -666,6 +666,7 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 	case PT_TO_SCX:
 	case PT_SYSCALL:
 	case PT_FOLLOW_FORK:
+	case PT_LWP_EVENTS:
 	case PT_DETACH:
 		sx_xlock(&proctree_lock);
 		proctree_locked = 1;
@@ -905,6 +906,16 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 			p->p_flag &= ~P_FOLLOWFORK;
 		break;
 
+	case PT_LWP_EVENTS:
+		CTR3(KTR_PTRACE, "PT_LWP_EVENTS: pid %d %s -> %s", p->p_pid,
+		    p->p_flag2 & P2_LWP_EVENTS ? "enabled" : "disabled",
+		    data ? "enabled" : "disabled");
+		if (data)
+			p->p_flag2 |= P2_LWP_EVENTS;
+		else
+			p->p_flag2 &= ~P2_LWP_EVENTS;
+		break;
+
 	case PT_STEP:
 	case PT_CONTINUE:
 	case PT_TO_SCE:
@@ -1227,6 +1238,10 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 		}
 		if (td2->td_dbgflags & TDB_CHILD)
 			pl->pl_flags |= PL_FLAG_CHILD;
+		if (td2->td_dbgflags & TDB_BORN)
+			pl->pl_flags |= PL_FLAG_BORN;
+		if (td2->td_dbgflags & TDB_EXIT)
+			pl->pl_flags |= PL_FLAG_EXITED;
 		pl->pl_sigmask = td2->td_sigmask;
 		pl->pl_siglist = td2->td_siglist;
 		strcpy(pl->pl_tdname, td2->td_name);