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From:   Martin Lau <kafai@fb.com>
To:     Daniel Xu <dxu@dxuuu.xyz>
CC:     John Fastabend <john.fastabend@gmail.com>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        "ast@kernel.org" <ast@kernel.org>,
        "daniel@iogearbox.net" <daniel@iogearbox.net>,
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        Andrii Nakryiko <andriin@fb.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "acme@kernel.org" <acme@kernel.org>
Subject: Re: [PATCH v3 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches()
 helper
Thread-Topic: [PATCH v3 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches()
 helper
Thread-Index: AQHV0jNkMKZbcmePk06Qcg6q4FDVe6f4+78AgAAUpgCAAGq+gA==
Date:   Fri, 24 Jan 2020 08:25:06 +0000
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References: <5e2a3f00a996a_7f9e2ab8c3f9e5c4a6@john-XPS-13-9370.notmuch>
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On Thu, Jan 23, 2020 at 06:02:58PM -0800, Daniel Xu wrote:
> On Thu Jan 23, 2020 at 4:49 PM, John Fastabend wrote:
> [...]
> > >   * function eBPF program intends to call
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index 19e793aa441a..24c51272a1f7 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -1028,6 +1028,35 @@ static const struct bpf_func_proto bpf_perf_pr=
og_read_value_proto =3D {
> > >           .arg3_type      =3D ARG_CONST_SIZE,
> > >  };
> > > =20
> > > +BPF_CALL_3(bpf_perf_prog_read_branches, struct bpf_perf_event_data_k=
ern *, ctx,
> > > +	   void *, buf, u32, size)
> > > +{
> > > +	struct perf_branch_stack *br_stack =3D ctx->data->br_stack;
> > > +	u32 to_copy =3D 0, to_clear =3D size;
> > > +	int err =3D -EINVAL;
> > > +
> > > +	if (unlikely(!br_stack))
> > > +		goto clear;
> > > +
> > > +	to_copy =3D min_t(u32, br_stack->nr * sizeof(struct perf_branch_ent=
ry), size);
> > > +	to_clear -=3D to_copy;
> > > +
> > > +	memcpy(buf, br_stack->entries, to_copy);
> > > +	err =3D to_copy;
> > > +clear:
> >
> >=20
> > There appears to be agreement to clear the extra buffer on error but
> > what about
> > in the non-error case? I expect one usage pattern is to submit a fairly
> > large
> > buffer, large enough to handle worse case nr, in this case we end up
> > zero'ing
> > memory even in the succesful case. Can we skip the clear in this case?
> > Maybe
> > its not too important either way but seems unnecessary.
After some thoughts,  I also think clearing for non-error case
is not ideal.  DanielXu, is it the common use case to always
have a large enough buf size to capture the interested data?

> >
> >=20
> > > +	memset(buf + to_copy, 0, to_clear);
> > > +	return err;
> > > +}
> >
>=20
> Given Yonghong's suggestion of a flag argument, we need to allow users
> to pass in a null ptr while getting buffer size. So I'll change the `buf`
> argument to be ARG_PTR_TO_MEM_OR_NULL, which requires the buffer be
> initialized. We can skip zero'ing out altogether.
>=20
> Although I think the end result is the same -- now the user has to zero i=
t
> out. Unfortunately ARG_PTR_TO_UNINITIALIZED_MEM_OR_NULL is not
> implemented yet.
A "flags" arg can be added but not used to keep our option open in the
future.  Not sure it has to be implemented now though.
I would think whether there is an immediate usecase to learn
br_stack->nr through an extra bpf helper call in every event.

When there is a use case for learning br_stack->nr,
there may have multiple ways to do it also,
this "flags" arg, or another helper,
or br_stack->nr may be read directly with the help of BTF.