Re: [PATCH bpf-next v5 09/12] libbpf: Update gen_loader to emit BTF_KIND_FUNC relocations

+static void emit_relo_kfunc_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo, int insn)
+{
+	struct kfunc_desc *kdesc;
+	int btf_fd_idx;
+
+	kdesc = get_kfunc_desc(gen, relo->name);
+	if (!kdesc)
+		return;
+
+	btf_fd_idx = kdesc->ref > 1 ? kdesc->off : add_kfunc_btf_fd(gen);
+	if (btf_fd_idx > INT16_MAX) {
+		pr_warn("BTF fd off %d for kfunc %s exceeds INT16_MAX, cannot process relocation\n",
+			btf_fd_idx, relo->name);
+		gen->error = -E2BIG;
+		return;
+	}
+	/* load slot pointer */
+	emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_8, BPF_PSEUDO_MAP_IDX_VALUE,
+					 0, 0, 0, blob_fd_array_off(gen, btf_fd_idx)));
+	/* Try to map one insn->off to one insn->imm */
+	if (kdesc->ref > 1) {
+		emit(gen, BPF_MOV64_REG(BPF_REG_9, BPF_REG_7));
+		goto skip_btf_fd;
+	} else {
+		/* cannot use index == 0 */
+		if (!btf_fd_idx) {
+			btf_fd_idx = add_kfunc_btf_fd(gen);
+			/* shift to next slot */
+			emit(gen, BPF_ALU64_IMM(BPF_ADD, BPF_REG_8, sizeof(int)));
+		}
+		kdesc->off = btf_fd_idx;
+	}
+
+	/* set a default value */
+	emit(gen, BPF_ST_MEM(BPF_W, BPF_REG_8, 0, 0));
+	emit(gen, BPF_MOV64_REG(BPF_REG_9, BPF_REG_7));
+	/* store BTF fd if ret < 0 */
+	emit(gen, BPF_JMP_IMM(BPF_JSLT, BPF_REG_7, 0, 3));
+	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_7, 32));
+	/* store BTF fd in slot */
+	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_7, 0));
+	emit(gen, BPF_MOV64_REG(BPF_REG_7, BPF_REG_9));
+skip_btf_fd:
+	/* remember BTF fd to skip insn->off store for vmlinux case */
+	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_9, 32));
+	/* set a default value */
+	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_0, offsetof(struct bpf_insn, off), 0));
+	/* skip insn->off store if ret < 0 */
+	emit(gen, BPF_JMP_IMM(BPF_JSLT, BPF_REG_7, 0, 2));
+	/* skip if vmlinux BTF */
+	emit(gen, BPF_JMP_IMM(BPF_JEQ, BPF_REG_9, 0, 1));
+	/* store index into insn[insn_idx].off */
+	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_0, offsetof(struct bpf_insn, off), btf_fd_idx));
+	/* close fd that we skipped storing in fd_array */
+	if (kdesc->ref > 1) {
+		emit(gen, BPF_MOV64_REG(BPF_REG_1, BPF_REG_9));
+		__emit_sys_close(gen);
+	}