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Date:   Sat, 2 Mar 2019 01:10:59 +0000
From:   Javier Honduvilla Coto <javierhonduco@...com>
To:     Martin Lau <kafai@...com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Yonghong Song <yhs@...com>, Kernel Team <Kernel-team@...com>,
        Javier Honduvilla Coto <javierhonduco@...il.com>
Subject: Re: [PATCH v2 bpf-next 1/3] bpf: add bpf_progenyof helper

On Sat, Mar 02, 2019 at 12:12:57AM +0000, Martin Lau wrote:
> On Fri, Mar 01, 2019 at 10:06:12AM -0800, Javier Honduvilla Coto wrote:
> > From: Javier Honduvilla Coto <javierhonduco@...il.com>
> >
> > 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@...il.com>
> > ---
> >  include/linux/bpf.h      |  1 +
> >  include/uapi/linux/bpf.h | 11 ++++++++++-
> >  kernel/bpf/core.c        |  1 +
> >  kernel/bpf/helpers.c     | 29 +++++++++++++++++++++++++++++
> >  kernel/trace/bpf_trace.c |  2 ++
> >  5 files changed, 43 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index de18227b3d95..447395ba202b 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -921,6 +921,7 @@ 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 bcdd2474eee7..354ec295864c 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2359,6 +2359,14 @@ union bpf_attr {
> >   *	Return
> >   *		A **struct bpf_tcp_sock** pointer on success, or NULL in
> >   *		case of failure.
> > + *
> > + * 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.
> >   */
> >  #define __BPF_FUNC_MAPPER(FN)		\
> >  	FN(unspec),			\
> > @@ -2457,7 +2465,8 @@ union bpf_attr {
> >  	FN(spin_lock),			\
> >  	FN(spin_unlock),		\
> >  	FN(sk_fullsock),		\
> > -	FN(tcp_sock),
> > +	FN(tcp_sock),			\
> > +	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 ef88b167959d..69e209fbd128 100644
> > --- a/kernel/bpf/core.c
> > +++ b/kernel/bpf/core.c
> > @@ -2015,6 +2015,7 @@ 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..3899787e8dbf 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 +365,31 @@ const struct bpf_func_proto bpf_get_local_storage_proto = {
> >  };
> >  #endif
> >  #endif
> > +
> > +BPF_CALL_1(bpf_progenyof, int, pid)
> > +{
> > +	int result = 0;
> > +	struct task_struct *task = current;
> > +
> > +	if (unlikely(!task))
> > +		return -EINVAL;
> > +
> > +	rcu_read_lock();
> I don't think it is needed.  It should already be under rcu_read_lock().
> You may add 'WARN_ON_ONCE(!rcu_read_lock_held());' to be safe,
> like other "bpf_map_xxx_elem()" helpers in the same file.
>

Spot on! Just checked that we are rcu locking in include/linux/bpf.h
being called kernel/trace/bpf_trace.c. Thanks!!

> > +	while (task != &init_task) {
> > +		if (task->pid == pid) {
> > +			result = 1;
> > +			break;
> > +		}
> > +		task = rcu_dereference(task->real_parent);
> > +	}
> > +	rcu_read_unlock();
> > +
> > +	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 f1a86a0d881d..8602ae83c799 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -600,6 +600,8 @@ 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;
> > --
> > 2.17.1
> >

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