static int bpf_core_calc_field_relo()

in contrib/libbpf/src/relo_core.c [678:801]

• 106 lines of code
• 26 McCabe index (conditional complexity)

static int bpf_core_calc_field_relo(const char *prog_name,
				    const struct bpf_core_relo *relo,
				    const struct bpf_core_spec *spec,
				    __u64 *val, __u32 *field_sz, __u32 *type_id,
				    bool *validate)
{
	const struct bpf_core_accessor *acc;
	const struct btf_type *t;
	__u32 byte_off, byte_sz, bit_off, bit_sz, field_type_id;
	const struct btf_member *m;
	const struct btf_type *mt;
	bool bitfield;
	__s64 sz;

	*field_sz = 0;

	if (relo->kind == BPF_CORE_FIELD_EXISTS) {
		*val = spec ? 1 : 0;
		return 0;
	}

	if (!spec)
		return -EUCLEAN; /* request instruction poisoning */

	acc = &spec->spec[spec->len - 1];
	t = btf_type_by_id(spec->btf, acc->type_id);

	/* a[n] accessor needs special handling */
	if (!acc->name) {
		if (relo->kind == BPF_CORE_FIELD_BYTE_OFFSET) {
			*val = spec->bit_offset / 8;
			/* remember field size for load/store mem size */
			sz = btf__resolve_size(spec->btf, acc->type_id);
			if (sz < 0)
				return -EINVAL;
			*field_sz = sz;
			*type_id = acc->type_id;
		} else if (relo->kind == BPF_CORE_FIELD_BYTE_SIZE) {
			sz = btf__resolve_size(spec->btf, acc->type_id);
			if (sz < 0)
				return -EINVAL;
			*val = sz;
		} else {
			pr_warn("prog '%s': relo %d at insn #%d can't be applied to array access\n",
				prog_name, relo->kind, relo->insn_off / 8);
			return -EINVAL;
		}
		if (validate)
			*validate = true;
		return 0;
	}

	m = btf_members(t) + acc->idx;
	mt = skip_mods_and_typedefs(spec->btf, m->type, &field_type_id);
	bit_off = spec->bit_offset;
	bit_sz = btf_member_bitfield_size(t, acc->idx);

	bitfield = bit_sz > 0;
	if (bitfield) {
		byte_sz = mt->size;
		byte_off = bit_off / 8 / byte_sz * byte_sz;
		/* figure out smallest int size necessary for bitfield load */
		while (bit_off + bit_sz - byte_off * 8 > byte_sz * 8) {
			if (byte_sz >= 8) {
				/* bitfield can't be read with 64-bit read */
				pr_warn("prog '%s': relo %d at insn #%d can't be satisfied for bitfield\n",
					prog_name, relo->kind, relo->insn_off / 8);
				return -E2BIG;
			}
			byte_sz *= 2;
			byte_off = bit_off / 8 / byte_sz * byte_sz;
		}
	} else {
		sz = btf__resolve_size(spec->btf, field_type_id);
		if (sz < 0)
			return -EINVAL;
		byte_sz = sz;
		byte_off = spec->bit_offset / 8;
		bit_sz = byte_sz * 8;
	}

	/* for bitfields, all the relocatable aspects are ambiguous and we
	 * might disagree with compiler, so turn off validation of expected
	 * value, except for signedness
	 */
	if (validate)
		*validate = !bitfield;

	switch (relo->kind) {
	case BPF_CORE_FIELD_BYTE_OFFSET:
		*val = byte_off;
		if (!bitfield) {
			*field_sz = byte_sz;
			*type_id = field_type_id;
		}
		break;
	case BPF_CORE_FIELD_BYTE_SIZE:
		*val = byte_sz;
		break;
	case BPF_CORE_FIELD_SIGNED:
		*val = (btf_is_any_enum(mt) && BTF_INFO_KFLAG(mt->info)) ||
		       (btf_int_encoding(mt) & BTF_INT_SIGNED);
		if (validate)
			*validate = true; /* signedness is never ambiguous */
		break;
	case BPF_CORE_FIELD_LSHIFT_U64:
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
		*val = 64 - (bit_off + bit_sz - byte_off  * 8);
#else
		*val = (8 - byte_sz) * 8 + (bit_off - byte_off * 8);
#endif
		break;
	case BPF_CORE_FIELD_RSHIFT_U64:
		*val = 64 - bit_sz;
		if (validate)
			*validate = true; /* right shift is never ambiguous */
		break;
	case BPF_CORE_FIELD_EXISTS:
	default:
		return -EOPNOTSUPP;
	}

	return 0;
}