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

["Daniel Xu" <dxu@dxuuu.xyz>]

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.
>
> 
> > +	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.