riscv, bpf: Add support for far jumps and exits

This commit add support for far (offset > 21b) jumps and exits.
Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Luke Nelson <lukenels@cs.washington.edu>
Link: https://lore.kernel.org/bpf/20191216091343.23260-5-bjorn.topel@gmail.com

arch/riscv/net/bpf_jit_comp.c

@@ -496,16 +496,6 @@ static int is_12b_check(int off, int insn)
 	return 0;
 }
 
-static int is_21b_check(int off, int insn)
-{
-	if (!is_21b_int(off)) {
-		pr_err("bpf-jit: insn=%d 21b < offset=%d not supported yet!\n",
-		       insn, (int)off);
-		return -1;
-	}
-	return 0;
-}
-
 static void emit_imm(u8 rd, s64 val, struct rv_jit_context *ctx)
 {
 	/* Note that the immediate from the add is sign-extended,
@@ -820,6 +810,21 @@ static void emit_sext_32_rd(u8 *rd, struct rv_jit_context *ctx)
 	*rd = RV_REG_T2;
 }
 
+static void emit_jump_and_link(u8 rd, int rvoff, struct rv_jit_context *ctx)
+{
+	s64 upper, lower;
+
+	if (is_21b_int(rvoff)) {
+		emit(rv_jal(rd, rvoff >> 1), ctx);
+		return;
+	}
+
+	upper = (rvoff + (1 << 11)) >> 12;
+	lower = rvoff & 0xfff;
+	emit(rv_auipc(RV_REG_T1, upper), ctx);
+	emit(rv_jalr(rd, RV_REG_T1, lower), ctx);
+}
+
 static bool is_signed_bpf_cond(u8 cond)
 {
 	return cond == BPF_JSGT || cond == BPF_JSLT ||
@@ -1101,13 +1106,7 @@ static int emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 	/* JUMP off */
 	case BPF_JMP | BPF_JA:
 		rvoff = rv_offset(i, off, ctx);
-		if (!is_21b_int(rvoff)) {
-			pr_err("bpf-jit: insn=%d offset=%d not supported yet!\n",
-			       i, rvoff);
-			return -1;
-		}
-
-		emit(rv_jal(RV_REG_ZERO, rvoff >> 1), ctx);
+		emit_jump_and_link(RV_REG_ZERO, rvoff, ctx);
 		break;
 
 	/* IF (dst COND src) JUMP off */
@@ -1245,9 +1244,7 @@ static int emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			break;
 
 		rvoff = epilogue_offset(ctx);
-		if (is_21b_check(rvoff, i))
-			return -1;
-		emit(rv_jal(RV_REG_ZERO, rvoff >> 1), ctx);
+		emit_jump_and_link(RV_REG_ZERO, rvoff, ctx);
 		break;
 
 	/* dst = imm64 */