static int parse_index(git_index *index, const char *buffer, size_t buffer_size)
{
	int error = 0;
	unsigned int i;
	struct index_header header = { 0 };
	git_oid checksum_calculated, checksum_expected;
	const char *last = NULL;
	const char *empty = "";

#define seek_forward(_increase) { \
	if (_increase >= buffer_size) { \
		error = index_error_invalid("ran out of data while parsing"); \
		goto done; } \
	buffer += _increase; \
	buffer_size -= _increase;\
}

	if (buffer_size < INDEX_HEADER_SIZE + INDEX_FOOTER_SIZE)
		return index_error_invalid("insufficient buffer space");

	/* Precalculate the SHA1 of the files's contents -- we'll match it to
	 * the provided SHA1 in the footer */
	git_hash_buf(&checksum_calculated, buffer, buffer_size - INDEX_FOOTER_SIZE);

	/* Parse header */
	if ((error = read_header(&header, buffer)) < 0)
		return error;

	index->version = header.version;
	if (index->version >= INDEX_VERSION_NUMBER_COMP)
		last = empty;

	seek_forward(INDEX_HEADER_SIZE);

	assert(!index->entries.length);

	if (index->ignore_case)
		git_idxmap_icase_resize((khash_t(idxicase) *) index->entries_map, header.entry_count);
	else
		git_idxmap_resize(index->entries_map, header.entry_count);

	/* Parse all the entries */
	for (i = 0; i < header.entry_count && buffer_size > INDEX_FOOTER_SIZE; ++i) {
		git_index_entry *entry = NULL;
		size_t entry_size;

		if ((error = read_entry(&entry, &entry_size, index, buffer, buffer_size, last)) < 0) {
			error = index_error_invalid("invalid entry");
			goto done;
		}

		if ((error = git_vector_insert(&index->entries, entry)) < 0) {
			index_entry_free(entry);
			goto done;
		}

		INSERT_IN_MAP(index, entry, &error);

		if (error < 0) {
			index_entry_free(entry);
			goto done;
		}
		error = 0;

		if (index->version >= INDEX_VERSION_NUMBER_COMP)
			last = entry->path;

		seek_forward(entry_size);
	}

	if (i != header.entry_count) {
		error = index_error_invalid("header entries changed while parsing");
		goto done;
	}

	/* There's still space for some extensions! */
	while (buffer_size > INDEX_FOOTER_SIZE) {
		size_t extension_size;

		extension_size = read_extension(index, buffer, buffer_size);

		/* see if we have read any bytes from the extension */
		if (extension_size == 0) {
			error = index_error_invalid("extension is truncated");
			goto done;
		}

		seek_forward(extension_size);
	}

	if (buffer_size != INDEX_FOOTER_SIZE) {
		error = index_error_invalid(
			"buffer size does not match index footer size");
		goto done;
	}

	/* 160-bit SHA-1 over the content of the index file before this checksum. */
	git_oid_fromraw(&checksum_expected, (const unsigned char *)buffer);

	if (git_oid__cmp(&checksum_calculated, &checksum_expected) != 0) {
		error = index_error_invalid(
			"calculated checksum does not match expected");
		goto done;
	}

	git_oid_cpy(&index->checksum, &checksum_calculated);

#undef seek_forward

	/* Entries are stored case-sensitively on disk, so re-sort now if
	 * in-memory index is supposed to be case-insensitive
	 */
	git_vector_set_sorted(&index->entries, !index->ignore_case);
	git_vector_sort(&index->entries);

done:
	return error;
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.