Re: [PATCH bpf] xdp: Handle device unregister for devmap_hash map type

[Alexei Starovoitov <ast@fb.com>]

On 10/17/19 3:28 AM, Toke Høiland-Jørgensen wrote:
> Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> writes:
> 
>> On 2019/10/16 22:28, Toke Høiland-Jørgensen wrote:
>>> It seems I forgot to add handling of devmap_hash type maps to the device
>>> unregister hook for devmaps. This omission causes devices to not be
>>> properly released, which causes hangs.
>>>
>>> Fix this by adding the missing handler.
>>>
>>> Fixes: 6f9d451ab1a3 ("xdp: Add devmap_hash map type for looking up devices by hashed index")
>>> Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>
>> Well, regarding 6f9d451ab1a3, I think that we want explicit "(u64)" cast
>>
>> @@ -97,6 +123,14 @@ static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
>>          cost = (u64) dtab->map.max_entries * sizeof(struct bpf_dtab_netdev *);
>>          cost += sizeof(struct list_head) * num_possible_cpus();
>>
>> +       if (attr->map_type == BPF_MAP_TYPE_DEVMAP_HASH) {
>> +               dtab->n_buckets = roundup_pow_of_two(dtab->map.max_entries);
>> +
>> +               if (!dtab->n_buckets) /* Overflow check */
>> +                       return -EINVAL;
>> +               cost += sizeof(struct hlist_head) * dtab->n_buckets;
>>
>>                                                      ^here
>>
>> +       }
>> +
>>          /* if map size is larger than memlock limit, reject it */
>>          err = bpf_map_charge_init(&dtab->map.memory, cost);
>>          if (err)
>>
>> like "(u64) dtab->map.max_entries * sizeof(struct bpf_dtab_netdev *)" does.
>> Otherwise, on 32bits build, "sizeof(struct hlist_head) * dtab->n_buckets" can become 0.
> 
> Oh, right. I kinda assumed the compiler would be smart enough to figure
> that out based on the type of the LHS; will send a separate fix for this.

compiler smart enough?! you must be kidding.
It's a C standard. Compiler has to do 32 bit multiply because n_buckets
is u32 and sizeof is 32 bit in 32bit arches as Tetsuo explained.