ptrace: don't send SIGTRAP on exec if SEIZED

ptrace_event(PTRACE_EVENT_EXEC) sends SIGTRAP if PT_TRACE_EXEC is not
set.  This is because this SIGTRAP predates PTRACE_O_TRACEEXEC option,
we do not need/want this with PT_SEIZED which can set the options during
attach.

Suggested-by: Pedro Alves <palves@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Chris Evans <scarybeasts@gmail.com>
Cc: Indan Zupancic <indan@nul.nu>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Pedro Alves <palves@redhat.com>
Cc: Jan Kratochvil <jan.kratochvil@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/ptrace.h

@@ -199,9 +199,10 @@ static inline void ptrace_event(int event, unsigned long message)
 	if (unlikely(ptrace_event_enabled(current, event))) {
 		current->ptrace_message = message;
 		ptrace_notify((event << 8) | SIGTRAP);
-	} else if (event == PTRACE_EVENT_EXEC && unlikely(current->ptrace)) {
+	} else if (event == PTRACE_EVENT_EXEC) {
 		/* legacy EXEC report via SIGTRAP */
-		send_sig(SIGTRAP, current, 0);
+		if ((current->ptrace & (PT_PTRACED|PT_SEIZED)) == PT_PTRACED)
+			send_sig(SIGTRAP, current, 0);
 	}
 }