Jesper Dangaard Brouer <bro...@redhat.com> writes: > 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?
Well, we already have UABI on the insert side, where the value being inserted has to be an ifindex. And we enforce value_size==4 when creating the map. So IMO I'm just keeping to the already established UAPI here. That being said... > Can we somehow use BTF to help us to make this extensible? ... this would not necessarily be a bad thing, it just needs to be done on both the insert and lookup sides. But I think this is a separate issue, which we need to solve anyway. And I'm still not convinced that the map value is the right place to specify what resources we want ;) -Toke
