[PATCH] Fix ptrace problem

This is indeed a new bug, and it is not architecture-specific.  In my
recent changes to close some race conditions, I overlooked the case of a
process using PTRACE_ATTACH on its own children.  The new PT_ATTACHED flag
does not really mean "PTRACE_ATTACH was used", it means "PTRACE_ATTACH is
changing the ->parent link".

This fixes the problem that Stephane Eranian program demonstrates.
Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/ptrace.c

@@ -132,7 +132,8 @@ int ptrace_attach(struct task_struct *task)
 		goto bad;
 
 	/* Go */
-	task->ptrace |= PT_PTRACED | PT_ATTACHED;
+	task->ptrace |= PT_PTRACED | ((task->real_parent != current)
+				      ? PT_ATTACHED : 0);
 	if (capable(CAP_SYS_PTRACE))
 		task->ptrace |= PT_PTRACE_CAP;
 	task_unlock(task);