netdev - Re: [PATCH net-next] bpf: expose sk_priority through struct bpf_sock

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Date:   Sat, 11 Nov 2017 13:06:22 +0900
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Vlad Dumitrescu <vlad@...itrescu.ro>, <davem@...emloft.net>,
        <brakmo@...com>
CC:     <netdev@...r.kernel.org>, <kraigatgoog@...il.com>
Subject: Re: [PATCH net-next] bpf: expose sk_priority through struct
 bpf_sock_ops

On 11/11/17 6:07 AM, Daniel Borkmann wrote:
> On 11/10/2017 08:17 PM, Vlad Dumitrescu wrote:
>> From: Vlad Dumitrescu <vladum@...gle.com>
>>
>> Allows BPF_PROG_TYPE_SOCK_OPS programs to read sk_priority.
>>
>> Signed-off-by: Vlad Dumitrescu <vladum@...gle.com>
>> ---
>>   include/uapi/linux/bpf.h       |  1 +
>>   net/core/filter.c              | 11 +++++++++++
>>   tools/include/uapi/linux/bpf.h |  1 +
>>   3 files changed, 13 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index e880ae6434ee..9757a2002513 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -947,6 +947,7 @@ struct bpf_sock_ops {
>>       __u32 local_ip6[4];    /* Stored in network byte order */
>>       __u32 remote_port;    /* Stored in network byte order */
>>       __u32 local_port;    /* stored in host byte order */
>> +    __u32 priority;
>>   };
>>     /* List of known BPF sock_ops operators.
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 61c791f9f628..a6329642d047 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -4449,6 +4449,17 @@ static u32 sock_ops_convert_ctx_access(enum
>> bpf_access_type type,
>>           *insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->dst_reg,
>>                         offsetof(struct sock_common, skc_num));
>>           break;
>> +
>> +    case offsetof(struct bpf_sock_ops, priority):
>> +        BUILD_BUG_ON(FIELD_SIZEOF(struct sock, sk_priority) != 4);
>> +
>> +        *insn++ = BPF_LDX_MEM(BPF_FIELD_SIZEOF(
>> +                        struct bpf_sock_ops_kern, sk),
>> +                      si->dst_reg, si->src_reg,
>> +                      offsetof(struct bpf_sock_ops_kern, sk));
>> +        *insn++ = BPF_LDX_MEM(BPF_W, si->dst_reg, si->dst_reg,
>> +                      offsetof(struct sock, sk_priority));
>> +        break;
>
> Hm, I don't think this would work, I actually think your initial patch
> was ok.
> bpf_setsockopt() as well as bpf_getsockopt() check for sk_fullsock(sk)
> right
> before accessing options on either socket or TCP level, and bail out
> with error
> otherwise; in such cases we'd read something else here and assume it's
> sk_priority.

even if it's not fullsock, it will just read zero, no? what's a problem
with that?
In non-fullsock hooks like BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB
the program author will know that it's meaningless to read sk_priority,
so returning zero with minimal checks is fine.
While adding extra runtime if (sk_fullsock(sk)) is unnecessary,
since the safety is not compromised.