Generic Data Structure Iterators

[Matt Bobrowski <mattbobrowski@google.com>]

In numerous BPF programs, I've found myself needing to iterate over
some in-kernel generic data structure i.e. list_head, hlist_head,
rbtree, etc. The approach that I generally use to do this consists of
using bpf_loop(), container_of(), and BPF_CORE_READ(). The end result
of this approach is always rather messy as it's mostly implementation
specific and bound to a specific in-kernel type i.e. mount, dentry,
inode, etc.

Recently, I came across the newly added bpf_for_each() open-coded
iterator, which could possibly help out a little with trivially
performing such iterations within BPF programs. However, looking into
the usage of this helper a little more, I realized that this too needs
to be backed by the new kfunc iterator framework
i.e. bpf_iter_##type##_new(), bpf_iter_##type##_destroy(),
bpf_iter_##type##_next(). So, in practice it seems like adopting this
approach to solve this specific iterator problem would lead us into a
situation where we'd be having to define iterator kfuncs for each
in-kernel type and respective field.

Now having said this, I'm wondering whether anyone here has considered
possibly solving this iterator based problem a little more
generically? That is, by exposing a set of kfuncs that allow you to
iterate over a list_head, hlist_head, rbtree, etc, independent of an
underlying in-kernel type and similar to your *list_for_each*() based
helpers that you'd typically find for each of these in-kernel generic
data structures. If so, what were your findings when exploring this
problem space?

/M