Re: [PATCH v3 bpf-next 1/3] bpf: Add bpf_perf_prog_read_branches() helper

[Martin Lau <kafai@fb.com>]

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_prog_read_value_proto = {
> > >           .arg3_type      = ARG_CONST_SIZE,
> > >  };
> > >  
> > > +BPF_CALL_3(bpf_perf_prog_read_branches, struct bpf_perf_event_data_kern *, ctx,
> > > +	   void *, buf, u32, size)
> > > +{
> > > +	struct perf_branch_stack *br_stack = ctx->data->br_stack;
> > > +	u32 to_copy = 0, to_clear = size;
> > > +	int err = -EINVAL;
> > > +
> > > +	if (unlikely(!br_stack))
> > > +		goto clear;
> > > +
> > > +	to_copy = min_t(u32, br_stack->nr * sizeof(struct perf_branch_entry), size);
> > > +	to_clear -= to_copy;
> > > +
> > > +	memcpy(buf, br_stack->entries, to_copy);
> > > +	err = to_copy;
> > > +clear:
> >
> > 
> > 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?

> >
> > 
> > > +	memset(buf + to_copy, 0, to_clear);
> > > +	return err;
> > > +}
> >
> 
> 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.
> 
> Although I think the end result is the same -- now the user has to zero it
> 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.