Fix various edge cases related to system call tracing.

- Always set td_dbg_sc_* when P_TRACED is set on system call entry
  even if the debugger is not tracing system call entries.  This
  ensures the fields are valid when reporting other stops that
  occur at system call boundaries such as for PT_FOLLOW_FORKS or
  when only tracing system call exits.
- Set TDB_SCX when reporting the stop for a new child process in
  fork_return().  This causes the event to be reported as a system
  call exit.
- Report a system call exit event in fork_return() for new threads in
  a traced process.
- Copy td_dbg_sc_* to new threads instead of zeroing.  This ensures
  that td_dbg_sc_code in particular will report the system call that
  created the new thread or process when it reports a system call
  exit event in fork_return().
- Add new ptrace tests to verify that new child processes and threads
  report system call exit events with a valid pl_syscall_code via
  PT_LWPINFO.

Reviewed by:	kib
Differential Revision:	https://reviews.freebsd.org/D3822

1 files changed, 3 insertions, 2 deletions

diff --git a/sys/kern/subr_syscall.c b/sys/kern/subr_syscall.c
index af09613..71f3d59 100644
--- a/sys/kern/subr_syscall.c
+++ b/sys/kern/subr_syscall.c
@@ -83,11 +83,12 @@ syscallenter(struct thread *td, struct syscall_args *sa)
 	if (error == 0) {
 
 		STOPEVENT(p, S_SCE, sa->narg);
-		if (p->p_flag & P_TRACED && p->p_stops & S_PT_SCE) {
+		if (p->p_flag & P_TRACED) {
 			PROC_LOCK(p);
 			td->td_dbg_sc_code = sa->code;
 			td->td_dbg_sc_narg = sa->narg;
-			ptracestop((td), SIGTRAP);
+			if (p->p_stops & S_PT_SCE)
+				ptracestop((td), SIGTRAP);
 			PROC_UNLOCK(p);
 		}
 		if (td->td_dbgflags & TDB_USERWR) {