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

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?