Jonathan Lemon <jonathan.le...@gmail.com> writes: > On 4 Jun 2019, at 9:35, Jesper Dangaard Brouer wrote: > >> On Tue, 04 Jun 2019 17:24:10 +0200 >> Toke Høiland-Jørgensen <t...@redhat.com> wrote: >> >>> We don't currently allow lookups into a devmap from eBPF, because the map >>> lookup returns a pointer directly to the dev->ifindex, which shouldn't be >>> modifiable from eBPF. >>> >>> However, being able to do lookups in devmaps is useful to know (e.g.) >>> whether forwarding to a specific interface is enabled. Currently, programs >>> work around this by keeping a shadow map of another type which indicates >>> whether a map index is valid. >>> >>> To allow lookups, simply copy the ifindex into a scratch variable and >>> return a pointer to this. If an eBPF program does modify it, this doesn't >>> matter since it will be overridden on the next lookup anyway. While this >>> does add a write to every lookup, the overhead of this is negligible >>> because the cache line is hot when both the write and the subsequent >>> read happens. >> >> When we choose the return value, here the ifindex, then this basically >> becomes UABI, right? >> >> Can we somehow use BTF to help us to make this extensible? >> >> As Toke mention in the cover letter, we really want to know if the >> chosen egress have actually enabled/allocated resources for XDP >> transmitting, but as we currently don't have in-kernel way to query >> thus (thus, we cannot expose such info). > > Would it be better to add a helper like bpf_map_element_present(), which > just returns a boolean value indicating whether the entry is NULL or not? > > This would solve this problem (and my xskmap problem).
Ah, totally missed that other thread; will go reply there :) -Toke
