netdev - Re: [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support

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Message-ID: <20200522011552.ak4dkxhqwg6j2koy@ast-mbp.dhcp.thefacebook.com>
Date:   Thu, 21 May 2020 18:15:52 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, andrii.nakryiko@...il.com,
        kernel-team@...com, "Paul E . McKenney" <paulmck@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support

On Sun, May 17, 2020 at 12:57:24PM -0700, Andrii Nakryiko wrote:
> +
> +static inline int roundup_len(__u32 len)
> +{
> +	/* clear out top 2 bits */
> +	len <<= 2;
> +	len >>= 2;

what this is for?
Overflow prevention?
but kernel checked the size already?

> +	/* add length prefix */
> +	len += BPF_RINGBUF_HDR_SZ;
> +	/* round up to 8 byte alignment */
> +	return (len + 7) / 8 * 8;
> +}
> +