bpf: check signal validity in nmi for bpf_send_signal() helper

Commit 8b401f9e ("bpf: implement bpf_send_signal() helper")
introduced bpf_send_signal() helper. If the context is nmi,
the sending signal work needs to be deferred to irq_work.
If the signal is invalid, the error will appear in irq_work
and it won't be propagated to user.

This patch did an early check in the helper itself to notify
user invalid signal, as suggested by Daniel.
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/trace/bpf_trace.c

@@ -600,6 +600,12 @@ BPF_CALL_1(bpf_send_signal, u32, sig)
 		return -EPERM;
 
 	if (in_nmi()) {
+		/* Do an early check on signal validity. Otherwise,
+		 * the error is lost in deferred irq_work.
+		 */
+		if (unlikely(!valid_signal(sig)))
+			return -EINVAL;
+
 		work = this_cpu_ptr(&send_signal_work);
 		if (work->irq_work.flags & IRQ_WORK_BUSY)
 			return -EBUSY;