Re: [PATCH bpf-next] bpf, lpm: fix check prefixlen before walking trie

[Florian Lehner <dev@der-flo.net>]

On Sun, Nov 05, 2023 at 08:08:43PM +0100, David Rheinsberg wrote:
> Hi
> 
> On Sun, Nov 5, 2023, at 9:58 AM, Florian Lehner wrote:
> > When looking up an element in LPM trie, the condition 'matchlen ==
> > trie->max_prefixlen' will never return true, if key->prefixlen is larger
> > than trie->max_prefixlen. Consequently all elements in the LPM trie will
> > be visited and no element is returned in the end.
> >
> 
> Am I understanding you right that this is an optimization to avoid walking the entire trie? Because the way I read your commit-message I assume the output has always been NULL? Or am I missing something.
> 
> Do you have a specific use-case where such lookups are common? Can you explain why it is important to optimize this case? Because you now add a condition for every lookup just to optimize for the lookup-miss of a special case. I don't think I understand your reasoning here, but I might be missing some context.
> 
> Thanks!
> David

Hi David,

Your understanding is correct. The return value currently and with this patch is
in both cases the same for the case where key->prefixlen > trie->max_prefixlen.

The optimization is to avoid the locking mechanism, walking the trie and
checking its elements. It might not be the most common use case, so I see your
point.

> 
> > Fixes: b95a5c4db09b ("bpf: add a longest prefix match trie map implementation")
> > Signed-off-by: Florian Lehner <dev@der-flo.net>
> > ---
> >  kernel/bpf/lpm_trie.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> > index 17c7e7782a1f..b32be680da6c 100644
> > --- a/kernel/bpf/lpm_trie.c
> > +++ b/kernel/bpf/lpm_trie.c
> > @@ -231,6 +231,9 @@ static void *trie_lookup_elem(struct bpf_map *map, 
> > void *_key)
> >  	struct lpm_trie_node *node, *found = NULL;
> >  	struct bpf_lpm_trie_key *key = _key;
> > 
> > +	if (key->prefixlen > trie->max_prefixlen)
> > +		return NULL;
> > +
> >  	/* Start walking the trie from the root node ... */
> > 
> >  	for (node = rcu_dereference_check(trie->root, rcu_read_lock_bh_held());
> > -- 
> > 2.39.2