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Date:   Mon, 25 Mar 2019 15:17:03 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Javier Honduvilla Coto <javierhonduco@...com>,
        netdev@...r.kernel.org
Cc:     yhs@...com, kernel-team@...com
Subject: Re: [PATCH v4 bpf-next 1/3] bpf: add bpf_progenyof helper

On 03/22/2019 11:38 PM, Javier Honduvilla Coto wrote:
> This patch adds the bpf_progenyof helper which receives a PID and returns
> 1 if the process currently being executed is in the process hierarchy
> including itself or 0 if not.
> 
> This is very useful in tracing programs when we want to filter by a
> given PID and all the children it might spawn. The current workarounds
> most people implement for this purpose have issues:
> 
> - Attaching to process spawning syscalls and dynamically add those PIDs
>   to some bpf map that would be used to filter is cumbersome and
> potentially racy.
> - Unrolling some loop to perform what this helper is doing consumes lots
>   of instructions. That and the impossibility to jump backwards makes it
> really hard to be correct in really large process chains.
> 
> Signed-off-by: Javier Honduvilla Coto <javierhonduco@...com>
> ---
>  include/linux/bpf.h      |  1 +
>  include/uapi/linux/bpf.h | 10 +++++++++-
>  kernel/bpf/core.c        |  1 +
>  kernel/bpf/helpers.c     | 32 ++++++++++++++++++++++++++++++++
>  kernel/trace/bpf_trace.c |  2 ++
>  5 files changed, 45 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f62897198844..bd0d2b38e7d5 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -930,6 +930,7 @@ extern const struct bpf_func_proto bpf_sk_redirect_map_proto;
>  extern const struct bpf_func_proto bpf_spin_lock_proto;
>  extern const struct bpf_func_proto bpf_spin_unlock_proto;
>  extern const struct bpf_func_proto bpf_get_local_storage_proto;
> +extern const struct bpf_func_proto bpf_progenyof_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 3c04410137d9..cf54cc739bf4 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2463,6 +2463,13 @@ union bpf_attr {
>   * 	Return
>   * 		0 if iph and th are a valid SYN cookie ACK, or a negative error
>   * 		otherwise.
> + * int bpf_progenyof(int pid)
> + *	Description
> + *		This helper is useful in programs that want to filter events
> + *		happening to a pid of any of its descendants.
> + *	Return
> + *		1 if the currently executing process' pid is in the process
> + *		hierarchy of the passed pid. 0 Otherwise.

What about the -EINVAL?

>   */
>  #define __BPF_FUNC_MAPPER(FN)		\
>  	FN(unspec),			\
> @@ -2565,7 +2572,8 @@ union bpf_attr {
>  	FN(skb_ecn_set_ce),		\
>  	FN(get_listener_sock),		\
>  	FN(skc_lookup_tcp),		\
> -	FN(tcp_check_syncookie),
> +	FN(tcp_check_syncookie),	\
> +	FN(progenyof),
>  
>  /* 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 ff09d32a8a1b..437986497468 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2044,6 +2044,7 @@ 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_get_current_cgroup_id_proto __weak;
>  const struct bpf_func_proto bpf_get_local_storage_proto __weak;
> +const struct bpf_func_proto bpf_progenyof_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 a411fc17d265..f093b35d1ba8 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -18,6 +18,7 @@
>  #include <linux/sched.h>
>  #include <linux/uidgid.h>
>  #include <linux/filter.h>
> +#include <linux/init_task.h>
>  
>  /* If kernel subsystem is allowing eBPF programs to call this function,
>   * inside its own verifier_ops->get_func_proto() callback it should return
> @@ -364,3 +365,34 @@ const struct bpf_func_proto bpf_get_local_storage_proto = {
>  };
>  #endif
>  #endif
> +
> +BPF_CALL_1(bpf_progenyof, int, pid)

Nit: could we add a more descriptive helper name? What's progenyof? Also s/int/pid_t/?

> +{
> +	int result = 0;
> +	struct task_struct *task = current;
> +
> +	WARN_ON(!rcu_read_lock_held());

This is not needed since it's guaranteed in BPF.

> +	if (unlikely(!task))
> +		return -EINVAL;
> +
> +	if (pid == 0)
> +		return 1;
> +
> +	while (task != &init_task) {
> +		if (task->pid == pid) {
> +			result = 1;
> +			break;
> +		}
> +		task = rcu_dereference(task->real_parent);
> +	}
> +
> +	return result;
> +}
> +
> +const struct bpf_func_proto bpf_progenyof_proto = {
> +	.func		= bpf_progenyof,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_ANYTHING,
> +};
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index d64c00afceb5..e69283d423ff 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -599,6 +599,8 @@ 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_progenyof:
> +		return &bpf_progenyof_proto;
>  #ifdef CONFIG_CGROUPS
>  	case BPF_FUNC_get_current_cgroup_id:
>  		return &bpf_get_current_cgroup_id_proto;
> 

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