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). -- Jonathan
