Re: [PATCH net-next 1/4] netlink: truncate overlength attribute list in nla_nest_end()

[David Ahern <dsahern@gmail.com>]

On 1/22/21 9:53 PM, Edwin Peer wrote:
> If a nested list of attributes is too long, then the length will
> exceed the 16-bit nla_len of the parent nlattr. In such cases,
> determine how many whole attributes can fit and truncate the
> message to this length. This properly maintains the nesting
> hierarchy, keeping the entire message valid, while fitting more
> subelements inside the nest range than may result if the length
> is wrapped modulo 64KB.
> 
> Marking truncated attributes, such that user space can determine
> the precise attribute truncated, by means of an additional bit in
> the nla_type was considered and rejected. The NLA_F_NESTED and
> NLA_F_NET_BYTEORDER flags are supposed to be mutually exclusive.
> So, in theory, the latter bit could have been redefined for nested
> attributes in order to indicate truncation, but user space tools
> (most notably iproute2) cannot be relied on to honor NLA_TYPE_MASK,
> resulting in alteration of the perceived nla_type and subsequent
> catastrophic failure.
> 

Did you look at using NETLINK_CB / netlink_skb_parms to keep a running
length of nested attributes to avoid the need to trim?