netdev - Re: [PATCH bpf-next v4 21/21] tools/bpf: selftests: add bpf

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Date:   Sat, 9 May 2020 17:34:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>, netdev@...r.kernel.org,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, kernel-team@...com
Subject: Re: [PATCH bpf-next v4 21/21] tools/bpf: selftests: add bpf_iter
 selftests

On Sat, May 09, 2020 at 10:59:23AM -0700, Yonghong Song wrote:
> +static volatile const __u32 ret1;
> +
> +SEC("iter/bpf_map")
> +int dump_bpf_map(struct bpf_iter__bpf_map *ctx)
> +{
> +	struct seq_file *seq = ctx->meta->seq;
> +	struct bpf_map *map = ctx->map;
> +	__u64 seq_num;
> +	int i, ret = 0;
> +
> +	if (map == (void *)0)
> +		return 0;
> +
> +	/* only dump map1_id and map2_id */
> +	if (map->id != map1_id && map->id != map2_id)
> +		return 0;
> +
> +	seq_num = ctx->meta->seq_num;
> +	if (map->id == map1_id) {
> +		map1_seqnum = seq_num;
> +		map1_accessed++;
> +	}
> +
> +	if (map->id == map2_id) {
> +		if (map2_accessed == 0) {
> +			map2_seqnum1 = seq_num;
> +			if (ret1)
> +				ret = 1;
> +		} else {
> +			map2_seqnum2 = seq_num;
> +		}
> +		map2_accessed++;
> +	}
> +
> +	/* fill seq_file buffer */
> +	for (i = 0; i < print_len; i++)
> +		bpf_seq_write(seq, &seq_num, sizeof(seq_num));
> +
> +	return ret;
> +}

I couldn't find where 'return 1' behavior is documented clearly.
I think it's a workaround for overflow.
When bpf prog detects overflow it can request replay of the element?
What if it keeps returning 1 ? read() will never finish?