riscv, bpf: Fix missing exception handling and redundant zext for LDX_B/H/W

For LDX_B/H/W, when zext has been inserted by verifier, it'll return 1,
and no exception handling will continue. Also, when the offset is 12-bit
value, the redundant zext inserted by the verifier is not removed. Fix
both scenarios by moving down the removal of redundant zext.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20230824095001.3408573-2-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Pu Lehui

arch/riscv/net/bpf_jit_comp64.c

@@ -1512,8 +1512,6 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			insns_start = ctx->ninsns;
 			emit(rv_lbu(rd, 0, RV_REG_T1), ctx);
 			insn_len = ctx->ninsns - insns_start;
-			if (insn_is_zext(&insn[1]))
-				return 1;
 			break;
 		case BPF_H:
 			if (is_12b_int(off)) {
@@ -1528,8 +1526,6 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			insns_start = ctx->ninsns;
 			emit(rv_lhu(rd, 0, RV_REG_T1), ctx);
 			insn_len = ctx->ninsns - insns_start;
-			if (insn_is_zext(&insn[1]))
-				return 1;
 			break;
 		case BPF_W:
 			if (is_12b_int(off)) {
@@ -1544,8 +1540,6 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			insns_start = ctx->ninsns;
 			emit(rv_lwu(rd, 0, RV_REG_T1), ctx);
 			insn_len = ctx->ninsns - insns_start;
-			if (insn_is_zext(&insn[1]))
-				return 1;
 			break;
 		case BPF_DW:
 			if (is_12b_int(off)) {
@@ -1566,6 +1560,9 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		ret = add_exception_handler(insn, ctx, rd, insn_len);
 		if (ret)
 			return ret;
+
+		if (BPF_SIZE(code) != BPF_DW && insn_is_zext(&insn[1]))
+			return 1;
 		break;
 	}
 	/* speculation barrier */