netdev - Re: [PATCH net-next v2 1/3] bpf: implement bpf_get_current_cgroup

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Date:   Mon, 4 Jun 2018 11:08:35 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Yonghong Song <yhs@...com>, ast@...com, netdev@...r.kernel.org
Cc:     kernel-team@...com
Subject: Re: [PATCH net-next v2 1/3] bpf: implement
 bpf_get_current_cgroup_id() helper

On 06/04/2018 12:59 AM, Yonghong Song wrote:
> bpf has been used extensively for tracing. For example, bcc
> contains an almost full set of bpf-based tools to trace kernel
> and user functions/events. Most tracing tools are currently
> either filtered based on pid or system-wide.
> 
> Containers have been used quite extensively in industry and
> cgroup is often used together to provide resource isolation
> and protection. Several processes may run inside the same
> container. It is often desirable to get container-level tracing
> results as well, e.g. syscall count, function count, I/O
> activity, etc.
> 
> This patch implements a new helper, bpf_get_current_cgroup_id(),
> which will return cgroup id based on the cgroup within which
> the current task is running.
> 
> The later patch will provide an example to show that
> userspace can get the same cgroup id so it could
> configure a filter or policy in the bpf program based on
> task cgroup id.
> 
> The helper is currently implemented for tracing. It can
> be added to other program types as well when needed.
> 
> Acked-by: Alexei Starovoitov <ast@...nel.org>
> Signed-off-by: Yonghong Song <yhs@...com>
> ---
>  include/linux/bpf.h      |  1 +
>  include/uapi/linux/bpf.h |  8 +++++++-
>  kernel/bpf/core.c        |  1 +
>  kernel/bpf/helpers.c     | 15 +++++++++++++++
>  kernel/trace/bpf_trace.c |  2 ++
>  5 files changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index bbe2974..995c3b1 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -746,6 +746,7 @@ extern const struct bpf_func_proto bpf_get_stackid_proto;
>  extern const struct bpf_func_proto bpf_get_stack_proto;
>  extern const struct bpf_func_proto bpf_sock_map_update_proto;
>  extern const struct bpf_func_proto bpf_sock_hash_update_proto;
> +extern const struct bpf_func_proto bpf_get_current_cgroup_id_proto;
>  
>  /* Shared helpers among cBPF and eBPF. */
>  void bpf_user_rnd_init_once(void);
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index f0b6608..18712b0 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2070,6 +2070,11 @@ union bpf_attr {
>   * 		**CONFIG_SOCK_CGROUP_DATA** configuration option.
>   * 	Return
>   * 		The id is returned or 0 in case the id could not be retrieved.
> + *
> + * u64 bpf_get_current_cgroup_id(void)
> + * 	Return
> + * 		A 64-bit integer containing the current cgroup id based
> + * 		on the cgroup within which the current task is running.
>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -2151,7 +2156,8 @@ union bpf_attr {
>  	FN(lwt_seg6_action),		\
>  	FN(rc_repeat),			\
>  	FN(rc_keydown),			\
> -	FN(skb_cgroup_id),
> +	FN(skb_cgroup_id),		\
> +	FN(get_current_cgroup_id),
>  
>  /* integer value in 'imm' field of BPF_CALL instruction selects which helper
>   * function eBPF program intends to call
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 527587d..9f14937 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1765,6 +1765,7 @@ const struct bpf_func_proto bpf_get_current_uid_gid_proto __weak;
>  const struct bpf_func_proto bpf_get_current_comm_proto __weak;
>  const struct bpf_func_proto bpf_sock_map_update_proto __weak;
>  const struct bpf_func_proto bpf_sock_hash_update_proto __weak;
> +const struct bpf_func_proto bpf_get_current_cgroup_id_proto __weak;
>  
>  const struct bpf_func_proto * __weak bpf_get_trace_printk_proto(void)
>  {
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 3d24e23..73065e2 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -179,3 +179,18 @@ const struct bpf_func_proto bpf_get_current_comm_proto = {
>  	.arg1_type	= ARG_PTR_TO_UNINIT_MEM,
>  	.arg2_type	= ARG_CONST_SIZE,
>  };
> +
> +#ifdef CONFIG_CGROUPS
> +BPF_CALL_0(bpf_get_current_cgroup_id)
> +{
> +	struct cgroup *cgrp = task_dfl_cgroup(current);
> +
> +	return cgrp->kn->id.id;
> +}
> +
> +const struct bpf_func_proto bpf_get_current_cgroup_id_proto = {
> +	.func		= bpf_get_current_cgroup_id,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +};
> +#endif

Nit: why not moving this function directly to bpf_trace.c?

> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 752992c..e2ab5b7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -564,6 +564,8 @@ tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_get_prandom_u32_proto;
>  	case BPF_FUNC_probe_read_str:
>  		return &bpf_probe_read_str_proto;
> +	case BPF_FUNC_get_current_cgroup_id:
> +		return &bpf_get_current_cgroup_id_proto;

When you have !CONFIG_CGROUPS, then it relies on the weak definition of the
bpf_get_current_cgroup_id_proto, which I would think at latest in fixup_bpf_calls()
bails out with 'kernel subsystem misconfigured func' due to func being NULL.

Can't we just do the #ifdef CONFIG_CGROUPS around BPF_FUNC_get_current_cgroup_id
case instead? Then we bail out normally with 'unknown func' when cgroups are
not configured?

>  	default:
>  		return NULL;
>  	}
>