linux-kernel - Re: [PATCH v2 1/5] bpf: Add new bpf map type to store the pointer to struct perf

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Date:	Wed, 22 Jul 2015 17:48:55 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Kaixu Xia <xiakaixu@...wei.com>, davem@...emloft.net,
	acme@...nel.org, mingo@...hat.com, a.p.zijlstra@...llo.nl,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org
Cc:	wangnan0@...wei.com, linux-kernel@...r.kernel.org,
	pi3orama@....com, hekuang@...wei.com
Subject: Re: [PATCH v2 1/5] bpf: Add new bpf map type to store the pointer to
 struct perf_event

On 7/22/15 1:09 AM, Kaixu Xia wrote:
> Introduce a new bpf map type 'BPF_MAP_TYPE_PERF_EVENT_ARRAY'.
> This map will only store the pointer to struct perf_event.
>
> Signed-off-by: Kaixu Xia <xiakaixu@...wei.com>
> ---
>   include/linux/bpf.h      |  2 ++
>   include/uapi/linux/bpf.h |  1 +
>   kernel/bpf/arraymap.c    | 40 ++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 43 insertions(+)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 4383476..f6a2442 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -11,6 +11,8 @@
>   #include <linux/workqueue.h>
>   #include <linux/file.h>
>
> +#define MAX_PERF_EVENT_ARRAY_ENTRY	(2*NR_CPUS)

why this artificial limit?
Just drop it.

> +static struct bpf_map *perf_event_array_map_alloc(union bpf_attr *attr)
> +{
> +	/* only the pointer to struct perf_event can be stored in
> +	 * perf_event_array map
> +	 */
> +	if (attr->value_size != sizeof(void *))
> +		return ERR_PTR(-EINVAL);

hmm. that's odd. why reinvent things? please do the same as
prog_array does. Namely below:

> +static const struct bpf_map_ops perf_event_array_ops = {
> +	.map_alloc = perf_event_array_map_alloc,
> +	.map_free = array_map_free,
> +	.map_get_next_key = perf_event_array_map_get_next_key,
> +	.map_lookup_elem = array_map_lookup_elem,
> +	.map_delete_elem = array_map_delete_elem,

this is broken. you don't want programs to manipulate
'struct perf_event *' pointers.
lookup/update/delete helpers shouldn't be accessible from the programs
then update/delete can be cleanly implemented and called via syscall.
See how prog_array does it.

Also please collapse patches 1-3 into one. Their logically one piece.
I'll comment on them as well, but it would have been easier for me
and you if their were part of one email thread.

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