s390/bpf: Tolerate not converging code shrinking

"BPF_MAXINSNS: Maximum possible literals" unnecessarily falls back to
the interpreter because of failing sanity check in bpf_set_addr. The
problem is that there are a lot of branches that can be shrunk, and
doing so opens up the possibility to shrink even more. This process
does not converge after 3 passes, causing code offsets to change during
the codegen pass, which must never happen.

Fix by inserting nops during codegen pass in order to preserve code
offets.

Fixes: 4e9b4a68 ("s390/bpf: Use relative long branches")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200717165326.6786-5-iii@linux.ibm.com

arch/s390/net/bpf_jit_comp.c

@@ -489,6 +489,24 @@ static void save_restore_regs(struct bpf_jit *jit, int op, u32 stack_depth)
 	} while (re <= last);
 }
 
+static void bpf_skip(struct bpf_jit *jit, int size)
+{
+	if (size >= 6 && !is_valid_rel(size)) {
+		/* brcl 0xf,size */
+		EMIT6_PCREL_RIL(0xc0f4000000, size);
+		size -= 6;
+	} else if (size >= 4 && is_valid_rel(size)) {
+		/* brc 0xf,size */
+		EMIT4_PCREL(0xa7f40000, size);
+		size -= 4;
+	}
+	while (size >= 2) {
+		/* bcr 0,%0 */
+		_EMIT2(0x0700);
+		size -= 2;
+	}
+}
+
 /*
  * Emit function prologue
  *
@@ -1503,7 +1521,14 @@ static bool bpf_is_new_addr_sane(struct bpf_jit *jit, int i)
  */
 static int bpf_set_addr(struct bpf_jit *jit, int i)
 {
-	if (!bpf_is_new_addr_sane(jit, i))
+	int delta;
+
+	if (is_codegen_pass(jit)) {
+		delta = jit->prg - jit->addrs[i];
+		if (delta < 0)
+			bpf_skip(jit, -delta);
+	}
+	if (WARN_ON_ONCE(!bpf_is_new_addr_sane(jit, i)))
 		return -1;
 	jit->addrs[i] = jit->prg;
 	return 0;