Hangbin Liu <liuhangbin@xxxxxxxxx> writes:
> On Wed, Apr 14, 2021 at 05:23:50PM -0700, Martin KaFai Lau wrote:
>> On Wed, Apr 14, 2021 at 08:26:08PM +0800, Hangbin Liu wrote:
>> [ ... ]
>>
>> > +static __always_inline int __bpf_xdp_redirect_map(struct bpf_map *map, u32 ifindex,
>> > + u64 flags, u64 flag_mask,
>> > void *lookup_elem(struct bpf_map *map, u32 key))
>> > {
>> > struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>> >
>> > /* Lower bits of the flags are used as return code on lookup failure */
>> > - if (unlikely(flags > XDP_TX))
>> > + if (unlikely(flags & ~(BPF_F_ACTION_MASK | flag_mask)))
>> > return XDP_ABORTED;
>> >
>> > ri->tgt_value = lookup_elem(map, ifindex);
>> > - if (unlikely(!ri->tgt_value)) {
>> > + if (unlikely(!ri->tgt_value) && !(flags & BPF_F_BROADCAST)) {
>> > /* If the lookup fails we want to clear out the state in the
>> > * redirect_info struct completely, so that if an eBPF program
>> > * performs multiple lookups, the last one always takes
>> > @@ -1482,13 +1484,21 @@ static __always_inline int __bpf_xdp_redirect_map(struct bpf_map *map, u32 ifind
>> > */
>> > ri->map_id = INT_MAX; /* Valid map id idr range: [1,INT_MAX[ */
>> > ri->map_type = BPF_MAP_TYPE_UNSPEC;
>> > - return flags;
>> > + return flags & BPF_F_ACTION_MASK;
>> > }
>> >
>> > ri->tgt_index = ifindex;
>> > ri->map_id = map->id;
>> > ri->map_type = map->map_type;
>> >
>> > + if (flags & BPF_F_BROADCAST) {
>> > + WRITE_ONCE(ri->map, map);
>> Why only WRITE_ONCE on ri->map? Is it needed?
>
> I think this is make sure the map pointer assigned to ri->map safely.
> which starts from commit f6069b9aa993 ("bpf: fix redirect to map under tail
> calls")
The reason WRITE_ONCE() is only on the map field is because that's the
one that could be changed by a remote CPU (in bpf_clear_redirect_map())
- everything else is only accessed on the local CPU.
As for whether it's strictly needed from a memory model PoV, I'm not
actually sure (and should we be using smp_{store_release,load_acquire}()
instead?); I view it mostly as an annotation to make it clear that the
map field is 'special' in this respect...
-Toke
