tools, bpf_asm: Warn when jumps are out of range

When compiling larger programs with bpf_asm, it's possible to
accidentally exceed jt/jf range, in which case it won't complain, but
rather silently emit a truncated offset, leading to a "happy debugging"
situation.

Add a warning to help detecting such issues. It could be made an error
instead, but this might break compilation of existing code (which might
be working by accident).

Signed-off-by: Ilya Leoshkevich <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/bpf/bpf_exp.y

···
 	}
 }
 
+static uint8_t bpf_encode_jt_jf_offset(int off, int i)
+{
+	int delta = off - i - 1;
+
+	if (delta < 0 || delta > 255)
+		fprintf(stderr, "warning: insn #%d jumps to insn #%d, "
+				"which is out of range\n", i, off);
+	return (uint8_t) delta;
+}
+
 static void bpf_reduce_jt_jumps(void)
 {
 	int i;
···
 	for (i = 0; i < curr_instr; i++) {
 		if (labels_jt[i]) {
 			int off = bpf_find_insns_offset(labels_jt[i]);
-			out[i].jt = (uint8_t) (off - i -1);
+			out[i].jt = bpf_encode_jt_jf_offset(off, i);
 		}
 	}
 }
···
 	for (i = 0; i < curr_instr; i++) {
 		if (labels_jf[i]) {
 			int off = bpf_find_insns_offset(labels_jf[i]);
-			out[i].jf = (uint8_t) (off - i - 1);
+			out[i].jf = bpf_encode_jt_jf_offset(off, i);
 		}
 	}
 }