Re: [PATCH v2 bpf-next 1/5] devmap: Formalize map value as a named struct

[David Ahern <dsahern@gmail.com>]

On 5/28/20 1:01 AM, Andrii Nakryiko wrote:
> On Wed, May 27, 2020 at 5:15 PM David Ahern <dsahern@kernel.org> wrote:
>>
>> From: David Ahern <dsahern@gmail.com>
>>
>> Add 'struct devmap_val' to the bpf uapi to formalize the
>> expected values that can be passed in for a DEVMAP.
>> Update devmap code to use the struct.
>>
>> Signed-off-by: David Ahern <dsahern@gmail.com>
>> ---
>>  include/uapi/linux/bpf.h       |  5 ++++
>>  kernel/bpf/devmap.c            | 43 ++++++++++++++++++++--------------
>>  tools/include/uapi/linux/bpf.h |  5 ++++
>>  3 files changed, 35 insertions(+), 18 deletions(-)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 54b93f8b49b8..d27302ecaa9c 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -3625,6 +3625,11 @@ struct xdp_md {
>>         __u32 rx_queue_index;  /* rxq->queue_index  */
>>  };
>>
>> +/* DEVMAP values */
>> +struct devmap_val {
>> +       __u32 ifindex;   /* device index */
>> +};
>> +
> 
> can DEVMAP be used outside of BPF ecosystem? If not, shouldn't this be
> `struct bpf_devmap_val`, to be consistent with the rest of the type
> names?

sure, added 'bpf_' to the name.

> 
>>  enum sk_action {
>>         SK_DROP = 0,
>>         SK_PASS,
>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>> index a51d9fb7a359..069a50113e26 100644
>> --- a/kernel/bpf/devmap.c
>> +++ b/kernel/bpf/devmap.c
>> @@ -66,6 +66,7 @@ struct bpf_dtab_netdev {
>>         struct bpf_dtab *dtab;
>>         struct rcu_head rcu;
>>         unsigned int idx;
>> +       struct devmap_val val;
>>  };
>>
>>  struct bpf_dtab {
>> @@ -110,7 +111,8 @@ static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
>>
>>         /* check sanity of attributes */
>>         if (attr->max_entries == 0 || attr->key_size != 4 ||
>> -           attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
>> +           attr->value_size > sizeof(struct devmap_val) ||
> 
> So is 0, 1, 2, 3, and after next patch 5, 6, and 7 all allowed as
> well? Isn't that a bit too permissive?

sure, I should check that it is at least 4-bytes - the existing size of
the values. After that the struct can vary as user and kernel differ.
The key is that newer userspace can not send down a higher value size
than the kernel supports  and older userspace can send fewer bytes
(e.g., 4-byte ifindex only vs 8-byte ifindex + fd). I'll revert this to
v1 where I check for specific known value sizes.

> 
>> +           attr->map_flags & ~DEV_CREATE_FLAG_MASK)
>>                 return -EINVAL;
>>
> 
> [...]
>