netdev - Re: [PATCH bpf-next]: add sock_ops R/W access to ipv4 tos

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Date:   Wed, 28 Mar 2018 17:41:46 +0000
From:   Nikita Shirokov <tehnerd@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Lawrence Brakmo <brakmo@...com>,
        "ast@...nel.org" <ast@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next]: add sock_ops R/W access to ipv4 tos


>>On 03/26/2018 05:36 PM, Nikita V. Shirokov wrote:
>>     bpf: Add sock_ops R/W access to ipv4 tos
>>
>>     Sample usage for tos:
>>
>>       bpf_getsockopt(skops, SOL_IP, IP_TOS, &v, sizeof(v))
>>
>>     where skops is a pointer to the ctx (struct bpf_sock_ops).
>>
>> Signed-off-by: Nikita V. Shirokov <tehnerd@...com>
>> ---
>>  net/core/filter.c | 35 +++++++++++++++++++++++++++++++++++
>>  1 file changed, 35 insertions(+)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 00c711c..afd8255 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3462,6 +3462,27 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
>>                        ret = -EINVAL;
>>                }
>>  #ifdef CONFIG_INET
>> +     } else if (level == SOL_IP) {
>> +             if (optlen != sizeof(int) || sk->sk_family != AF_INET)
>> +                     return -EINVAL;
>> +
>> +             val = *((int *)optval);
>> +             /* Only some options are supported */
>> +             switch (optname) {
>> +             case IP_TOS:
>> +                     if (val < -1 || val > 0xff) {
>> +                             ret = -EINVAL;
>> +                     } else {
>> +                             struct inet_sock *inet = inet_sk(sk);
>> +
>> +                             if (val == -1)
>> +                                     val = 0;
>> +                             inet->tos = val;
>
>Should this not have the exact same semantics given the helper resembles
>the normal setsockopt? do_ip_setsockopt() does the following when setting
>IP_TOS:
>
>        case IP_TOS:    /* This sets both TOS and Precedence */
>                if (sk->sk_type == SOCK_STREAM) {
>                        val &= ~INET_ECN_MASK;
>                        val |= inet->tos & INET_ECN_MASK;
>                }
>                if (inet->tos != val) {
>                        inet->tos = val;
>                        sk->sk_priority = rt_tos2priority(val);
>                        sk_dst_reset(sk);
>                }
>                break;
>
>E.g. why we don't need to set sk->sk_priority as well or reset the dst
>entry here?
>

it feels like initially (w/ commit for IP_TOS in ip_sockglue.c) there were some usecase in mind
where reflection of tos to prio was needed + some policy based routing (thats why dst_reset).
but e.g. for ipv6 (IPV6_TCLASS, same as TOS but in ipv6 world) we do just this - set new tclass value
and call it the day. in my opinion this aproach is more flexible (e.g. we have separate
bpf_setsockopt for SOL_PRIORITY) as it did only what we want (i can imagine few usecases
where we want just to change TOS w/o changing priority)

however if you feel strong and want to reproduce the same behavior as in regular setsockopt, i can
totally make v2. please let me know

>> +                     }
>> +                     break;
>> +             default:
>> +                     ret = -EINVAL;
>> +             }
>>  #if IS_ENABLED(CONFIG_IPV6)
>>        } else if (level == SOL_IPV6) {
>>                if (optlen != sizeof(int) || sk->sk_family != AF_INET6)
>> @@ -3561,6 +3582,20 @@ BPF_CALL_5(bpf_getsockopt, struct bpf_sock_ops_kern *, bpf_sock,
>>                } else {
>>                        goto err_clear;
>>                }
>> +     } else if (level == SOL_IP) {
>> +             struct inet_sock *inet = inet_sk(sk);
>> +
>> +             if (optlen != sizeof(int) || sk->sk_family != AF_INET)
>> +                     goto err_clear;
>> +
>> +             /* Only some options are supported */
>> +             switch (optname) {
>> +             case IP_TOS:
>> +                     *((int *)optval) = (int)inet->tos;
>
>This part is fine though, same as in do_ip_getsockopt().
>
>> +                     break;
>> +             default:
>> +                     goto err_clear;
>> +             }
>>  #if IS_ENABLED(CONFIG_IPV6)
>>        } else if (level == SOL_IPV6) {
>>                struct ipv6_pinfo *np = inet6_sk(sk);
>>