Re: [PATCH bpf-next v3 2/4] bpf: Expose bpf_get_socket_cookie to tracing programs

[KP Singh]

On Tue, Dec 8, 2020 at 9:20 PM Florent Revest <revest@xxxxxxxxxxxx> wrote:
>
> This needs two new helpers, one that works in a sleepable context (using
> sock_gen_cookie which disables/enables preemption) and one that does not
> (for performance reasons). Both take a struct sock pointer and need to
> check it for NULLness.
>
> This helper could also be useful to other BPF program types such as LSM.
>
> Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> ---
>  include/linux/bpf.h            |  2 ++
>  include/uapi/linux/bpf.h       |  7 +++++++
>  kernel/trace/bpf_trace.c       |  4 ++++
>  net/core/filter.c              | 24 ++++++++++++++++++++++++
>  tools/include/uapi/linux/bpf.h |  7 +++++++
>  5 files changed, 44 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index d05e75ed8c1b..2ecda549b773 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1859,6 +1859,8 @@ extern const struct bpf_func_proto bpf_snprintf_btf_proto;
>  extern const struct bpf_func_proto bpf_per_cpu_ptr_proto;
>  extern const struct bpf_func_proto bpf_this_cpu_ptr_proto;
>  extern const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto;
> +extern const struct bpf_func_proto bpf_get_socket_ptr_cookie_sleepable_proto;
> +extern const struct bpf_func_proto bpf_get_socket_ptr_cookie_proto;
>
>  const struct bpf_func_proto *bpf_tracing_func_proto(
>         enum bpf_func_id func_id, const struct bpf_prog *prog);
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index ba59309f4d18..9ac66cf25959 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1667,6 +1667,13 @@ union bpf_attr {
>   *     Return
>   *             A 8-byte long unique 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 unique number.
> + *
>   * u32 bpf_get_socket_uid(struct sk_buff *skb)
>   *     Return
>   *             The owner UID of the socket associated to *skb*. If the socket
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 0cf0a6331482..99accc2146bc 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1778,6 +1778,10 @@ tracing_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>                 return &bpf_sk_storage_get_tracing_proto;
>         case BPF_FUNC_sk_storage_delete:
>                 return &bpf_sk_storage_delete_tracing_proto;
> +       case BPF_FUNC_get_socket_cookie:
> +               return prog->aux->sleepable ?
> +                      &bpf_get_socket_ptr_cookie_sleepable_proto :
> +                      &bpf_get_socket_ptr_cookie_proto;
>  #endif
>         case BPF_FUNC_seq_printf:
>                 return prog->expected_attach_type == BPF_TRACE_ITER ?
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 77001a35768f..34877796ab5b 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4631,6 +4631,30 @@ static const struct bpf_func_proto bpf_get_socket_cookie_sock_proto = {
>         .arg1_type      = ARG_PTR_TO_CTX,
>  };
>
> +BPF_CALL_1(bpf_get_socket_ptr_cookie_sleepable, struct sock *, sk)
> +{
> +       return sk ? sock_gen_cookie(sk) : 0;

My understanding is you can simply always call sock_gen_cookie and not
have two protos.

This will disable preemption in sleepable programs and not have any effect
in non-sleepable programs since preemption will already be disabled.