If you have pressed CTRL+Z and a process is suspended, then you use gdb

to attach to the process, it is surprising that the process is resumed
without inputting any gdb commands, however ptrace manual said:
  The tracing process will see the newly-traced process stop and may
  then control it as if it had been traced all along.
But the current code does not work in this way, unless traced process
received a signal later, it will continue to run as a background task.
To fix this problem, just send signal SIGSTOP to the traced process after
we resumed it, this works like that you are attaching to a running process,
it is not perfect but better than nothing.

1 files changed, 4 insertions, 4 deletions

diff --git a/sys/kern/sys_process.c b/sys/kern/sys_process.c
index 9c6c204..10209ec 100644
--- a/sys/kern/sys_process.c
+++ b/sys/kern/sys_process.c
@@ -635,7 +635,7 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 	struct iovec iov;
 	struct uio uio;
 	struct proc *curp, *p, *pp;
-	struct thread *td2 = NULL;
+	struct thread *td2 = NULL, *td3;
 	struct ptrace_io_desc *piod = NULL;
 	struct ptrace_lwpinfo *pl;
 	int error, write, tmp, num;
@@ -953,10 +953,8 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 			td2->td_xsig = data;
 
 			if (req == PT_DETACH) {
-				struct thread *td3;
-				FOREACH_THREAD_IN_PROC(p, td3) {
+				FOREACH_THREAD_IN_PROC(p, td3)
 					td3->td_dbgflags &= ~TDB_SUSPEND; 
-				}
 			}
 			/*
 			 * unsuspend all threads, to not let a thread run,
@@ -967,6 +965,8 @@ kern_ptrace(struct thread *td, int req, pid_t pid, void *addr, int data)
 			p->p_flag &= ~(P_STOPPED_TRACE|P_STOPPED_SIG|P_WAITED);
 			thread_unsuspend(p);
 			PROC_SUNLOCK(p);
+			if (req == PT_ATTACH)
+				kern_psignal(p, data);
 		} else {
 			if (data)
 				kern_psignal(p, data);