On 11/13/17, 11:01 AM, "Vlad Dumitrescu" <v...@dumitrescu.ro> wrote:
On Sat, Nov 11, 2017 at 2:38 PM, Alexei Starovoitov <a...@fb.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 <vla...@google.com>
>>>>>
>>>>> Allows BPF_PROG_TYPE_SOCK_OPS programs to read sk_priority.
>>>>>
>>>>> Signed-off-by: Vlad Dumitrescu <vla...@google.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?
I plan to add access to TCP state variables (cwnd, rtt, etc.) and I have been
thinking
about this issue. I think it is possible to access it directly as long as we
use a value
like 0xffffffff to represent an invalid value (e.g. not fullsock). The ctx
rewrite just
needs to add a conditional to determine what to return. I would probably add a
field into the internal kernel struct to indicate if it is fullsock or not (we
should
know when we call tcp_call_bpf whether it is a fullsock or not based on
context).
Let me do a sample patch that I can send for review and get feedback from
Alexi and Daniel.
