uprobes: Send SIGILL if handle_trampoline() fails

1. It doesn't make sense to continue if handle_trampoline()
   fails, change handle_swbp() to always return after this call.

2. Turn pr_warn() into uprobe_warn(), and change
   handle_trampoline() to send SIGILL on failure. It is pointless to
   return to user mode with the corrupted instruction_pointer() which
   we can't restore.

Tested-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Anton Arapov <arapov@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150721134008.GA4745@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/uprobes.c

@@ -1770,19 +1770,19 @@ handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
 	up_read(&uprobe->register_rwsem);
 }
 
-static bool handle_trampoline(struct pt_regs *regs)
+static void handle_trampoline(struct pt_regs *regs)
 {
 	struct uprobe_task *utask;
 	struct return_instance *ri;
 	bool chained;
 
 	utask = current->utask;
 	if (!utask)
-		return false;
+		goto sigill;
 
 	ri = utask->return_instances;
 	if (!ri)
-		return false;
+		goto sigill;
 
 	/*
 	 * TODO: we should throw out return_instance's invalidated by
@@ -1804,8 +1804,12 @@ static bool handle_trampoline(struct pt_regs *regs)
 	}
 
 	utask->return_instances = ri;
+	return;
+
+ sigill:
+	uprobe_warn(current, "handle uretprobe, sending SIGILL.");
+	force_sig_info(SIGILL, SEND_SIG_FORCED, current);
 
-	return true;
 }
 
 bool __weak arch_uprobe_ignore(struct arch_uprobe *aup, struct pt_regs *regs)
@@ -1824,13 +1828,8 @@ static void handle_swbp(struct pt_regs *regs)
 	int uninitialized_var(is_swbp);
 
 	bp_vaddr = uprobe_get_swbp_addr(regs);
-	if (bp_vaddr == get_trampoline_vaddr()) {
-		if (handle_trampoline(regs))
-			return;
-
-		pr_warn("uprobe: unable to handle uretprobe pid/tgid=%d/%d\n",
-						current->pid, current->tgid);
-	}
+	if (bp_vaddr == get_trampoline_vaddr())
+		return handle_trampoline(regs);
 
 	uprobe = find_active_uprobe(bp_vaddr, &is_swbp);
 	if (!uprobe) {