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

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Date:   Mon, 13 Nov 2017 11:00:39 -0800
From:   Vlad Dumitrescu <vlad@...itrescu.ro>
To:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, brakmo@...com,
        davem@...emloft.net
Cc:     netdev@...r.kernel.org, Craig Gallek <kraigatgoog@...il.com>
Subject: Re: [PATCH net-next] bpf: expose sk_priority through struct bpf_sock_ops

On Sat, Nov 11, 2017 at 2:38 PM, Alexei Starovoitov <ast@...com> wrote:
>
> On 11/12/17 4:46 AM, Daniel Borkmann wrote:
>>
>> On 11/11/2017 05:06 AM, Alexei Starovoitov wrote:
>>>
>>> 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.
>>
>>
>> Hm, on my kernel, struct sock has the 4 bytes sk_priority at offset 440,
>> struct request_sock itself is only 232 byte long in total, and the struct
>> inet_timewait_sock is 208 byte long, so you'd be accessing out of bounds
>> that way, so it cannot be ignored and assumed zero.
>
>
> I thought we always pass fully allocated sock but technically not fullsock yet. My mistake. We do: tcp_timeout_init((struct sock *)req))
> so yeah ctx rewrite approach won't work.
> Let's go back to access via helper.
>

TIL. Thanks!

Is there anything else needed from me to get the helper approach accepted?