On 06/28/2019 09:17 AM, Toke Høiland-Jørgensen wrote:
> Daniel Borkmann <dan...@iogearbox.net> writes:
>
>> On 06/23/2019 04:17 AM, Toke Høiland-Jørgensen wrote:
>>> From: Toke Høiland-Jørgensen <t...@redhat.com>
>>>
>>> The bpf_redirect_map() helper used by XDP programs doesn't return any
>>> indication of whether it can successfully redirect to the map index it was
>>> given. Instead, BPF programs have to track this themselves, leading to
>>> programs using duplicate maps to track which entries are populated in the
>>> devmap.
>>>
>>> This patch fixes this by moving the map lookup into the bpf_redirect_map()
>>> helper, which makes it possible to return failure to the eBPF program. The
>>> lower bits of the flags argument is used as the return code, which means
>>> that existing users who pass a '0' flag argument will get XDP_ABORTED.
>>>
>>> With this, a BPF program can check the return code from the helper call and
>>> react by, for instance, substituting a different redirect. This works for
>>> any type of map used for redirect.
>>>
>>> Signed-off-by: Toke Høiland-Jørgensen <t...@redhat.com>
>>
>> Overall series looks good to me. Just very small things inline here & in the
>> other two patches:
>>
>> [...]
>>> @@ -3750,9 +3742,16 @@ BPF_CALL_3(bpf_xdp_redirect_map, struct bpf_map *,
>>> map, u32, ifindex,
>>> {
>>> struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>>>
>>> - if (unlikely(flags))
>>> + /* Lower bits of the flags are used as return code on lookup failure */
>>> + if (unlikely(flags > XDP_TX))
>>> return XDP_ABORTED;
>>>
>>> + ri->item = __xdp_map_lookup_elem(map, ifindex);
>>> + if (unlikely(!ri->item)) {
>>> + WRITE_ONCE(ri->map, NULL);
>>
>> This WRITE_ONCE() is not needed. We never set it before at this point.
>
> You mean the WRITE_ONCE() wrapper is not needed, or the set-to-NULL is
> not needed? The reason I added it is in case an eBPF program calls the
> helper twice before returning, where the first lookup succeeds but the
> second fails; in that case we want to clear the ->map pointer, no?
Yeah I meant the set-to-NULL. So if first call succeeds, and the second one
fails, then the expected semantics wrt the first call are as if the program
would have called bpf_xdp_redirect() only?
Looking at the code again, if we set ri->item to NULL, then we /must/ also
set ri->map to NULL. I guess there are two options: i) leave as is, ii) keep
the __xdp_map_lookup_elem() result in a temp var, if it's NULL return flags,
otherwise only /then/ update ri->item, so that semantics are similar to the
invalid flags check earlier. I guess fine either way, in case of i) there
should probably be a comment since it's less obvious.
Thanks,
Daniel
