Re: [PATCH bpf-next 1/3] bpf: add bpf_progenyof helper

[Javier Honduvilla Coto <javierhonduco@fb.com>]

On Wed, Feb 27, 2019 at 06:26:41AM +0000, Martin Lau wrote:
> On Tue, Feb 26, 2019 at 02:36:49PM -0800, Javier Honduvilla Coto wrote:
> > This patch adds the bpf_progenyof helper which receives a PID and returns
> What is progenof?
>
> > 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@fb.com>
> > ---
> >  include/linux/bpf.h      |  1 +
> >  include/uapi/linux/bpf.h |  3 ++-
> >  kernel/bpf/core.c        |  1 +
> >  kernel/bpf/helpers.c     | 29 +++++++++++++++++++++++++++++
> >  kernel/trace/bpf_trace.c |  2 ++
> >  5 files changed, 35 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;
> It seems only used in bpf_trace.c.  Does it have to be here?
>
> >
> >  /* 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..804e4218eb28 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -2457,7 +2457,8 @@ union bpf_attr {
> >  	FN(spin_lock),			\
> >  	FN(spin_unlock),		\
> >  	FN(sk_fullsock),		\
> > -	FN(tcp_sock),
> > +	FN(tcp_sock),			\
> > +	FN(progenyof),
> Please add doc like other helpers do.

Oops, good catch, thanks! Will send v2 soon!!

>
> >
> >  /* 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))
> hmm.... Could current be NULL?

Wasn't sure about this but added as bpf_get_current_pid_tgid,
bpf_get_current_uid_gid, and bpf_get_current_comm check for this. Texted Alexei
about this and he told me this is probably not necessary anymore, but I
guess it doesn't hurt leaving it?

>
> > +		return -EINVAL;
> > +
> > +	rcu_read_lock();
> > +	while (task != &init_task) {
> I don't know the details of init_task, so qq:
> Could the passed in "pid" be the init_task->pid?
> If possible, what is the expected "result"?
>

Yep! init_task doesn't set a pid for what I could see, so I guess it
will be PID=0. The test in the last patch check bpf_progenyof(0) :)

bpf_progenyof with 0 or 1 will always return 1

> > +		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
> >