If the current thread has the trap bit set (i.e. a debugger had

single stepped the process to the system call), we need to clear
the trap flag from the new frame. Otherwise, the new thread will
receive a (likely unexpected) SIGTRAP when it executes the first
instruction after returning to userland.

1 files changed, 8 insertions, 0 deletions

diff --git a/sys/amd64/amd64/vm_machdep.c b/sys/amd64/amd64/vm_machdep.c
index adddc1c..0cc9b03 100644
--- a/sys/amd64/amd64/vm_machdep.c
+++ b/sys/amd64/amd64/vm_machdep.c
@@ -280,6 +280,14 @@ cpu_set_upcall(struct thread *td, struct thread *td0)
 	 */
 	bcopy(td0->td_frame, td->td_frame, sizeof(struct trapframe));
 
+	/* If the current thread has the trap bit set (i.e. a debugger had
+	 * single stepped the process to the system call), we need to clear
+	 * the trap flag from the new frame. Otherwise, the new thread will
+	 * receive a (likely unexpected) SIGTRAP when it executes the first
+	 * instruction after returning to userland.
+	 */
+	td->td_frame->tf_rflags &= ~PSL_T;
+
 	/*
 	 * Set registers for trampoline to user mode.  Leave space for the
 	 * return address on stack.  These are the kernel mode register values.