linux-kernel - Re: [PATCH bpf-next v2 1/3] bpf: Expose bpf_get_socket

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Date:   Fri, 4 Dec 2020 20:03:40 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Florent Revest <revest@...omium.org>, bpf@...r.kernel.org
Cc:     ast@...nel.org, andrii@...nel.org, kpsingh@...omium.org,
        revest@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 1/3] bpf: Expose bpf_get_socket_cookie to
 tracing programs

On 12/4/20 7:56 PM, Daniel Borkmann wrote:
> On 12/3/20 10:33 PM, Florent Revest wrote:
>> This creates a new helper proto because the existing
>> bpf_get_socket_cookie_sock_proto has a ARG_PTR_TO_CTX argument and only
>> works for BPF programs where the context is a sock.
>>
>> This helper could also be useful to other BPF program types such as LSM.
>>
>> Signed-off-by: Florent Revest <revest@...gle.com>
>> ---
>>   include/uapi/linux/bpf.h       | 7 +++++++
>>   kernel/trace/bpf_trace.c       | 4 ++++
>>   net/core/filter.c              | 7 +++++++
>>   tools/include/uapi/linux/bpf.h | 7 +++++++
>>   4 files changed, 25 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index c3458ec1f30a..3e0e33c43998 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -1662,6 +1662,13 @@ union bpf_attr {
>>    *     Return
>>    *         A 8-byte long non-decreasing number.
>>    *
>> + * u64 bpf_get_socket_cookie(void *sk)
>> + *     Description
>> + *         Equivalent to **bpf_get_socket_cookie**\ () helper that accepts
>> + *         *sk*, but gets socket from a BTF **struct sock**.
>> + *     Return
>> + *         A 8-byte long non-decreasing number.
> 
> I would not mention this here since it's not fully correct and we should avoid users
> taking non-decreasing granted in their progs. The only assumption you can make is
> that it can be considered a unique number. See also [0] with reverse counter..
> 
>    [0] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=92acdc58ab11af66fcaef485433fde61b5e32fac

One more thought, in case you plan to use this from sleepable context, you would
need to use sock_gen_cookie() variant in the BPF helper instead.