bpf: implement insn_is_cast_user() helper for JITs

Implement a helper function to check if an instruction is
addr_space_cast from as(0) to as(1). Use this helper in the x86 JIT.

Other JITs can use this helper when they add support for this instruction.

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Link: https://lore.kernel.org/r/20240324183226.29674-1-puranjay12@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

arch/x86/net/bpf_jit_comp.c

@@ -1351,8 +1351,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image, u8 *rw_image
 			break;
 
 		case BPF_ALU64 | BPF_MOV | BPF_X:
-			if (insn->off == BPF_ADDR_SPACE_CAST &&
-			    insn->imm == 1U << 16) {
+			if (insn_is_cast_user(insn)) {
 				if (dst_reg != src_reg)
 					/* 32-bit mov */
 					emit_mov_reg(&prog, false, dst_reg, src_reg);

include/linux/filter.h

@@ -228,6 +228,16 @@ static inline bool insn_is_zext(const struct bpf_insn *insn)
 	return insn->code == (BPF_ALU | BPF_MOV | BPF_X) && insn->imm == 1;
 }
 
+/* addr_space_cast from as(0) to as(1) is for converting bpf arena pointers
+ * to pointers in user vma.
+ */
+static inline bool insn_is_cast_user(const struct bpf_insn *insn)
+{
+	return insn->code == (BPF_ALU64 | BPF_MOV | BPF_X) &&
+			      insn->off == BPF_ADDR_SPACE_CAST &&
+			      insn->imm == 1U << 16;
+}
+
 /* BPF_LD_IMM64 macro encodes single 'load 64-bit immediate' insn */
 #define BPF_LD_IMM64(DST, IMM)					\
 	BPF_LD_IMM64_RAW(DST, 0, IMM)