netdev - Re: [v2 2/3] samples: pktgen: add helper functions for IP(v4/v6) CIDR parsing

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Date:   Fri, 13 Sep 2019 14:43:05 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     "Daniel T. Lee" <danieltimlee@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        brouer@...hat.com
Subject: Re: [v2 2/3] samples: pktgen: add helper functions for IP(v4/v6)
 CIDR parsing

On Thu, 12 Sep 2019 03:48:06 +0900
"Daniel T. Lee" <danieltimlee@...il.com> wrote:

> This commit adds CIDR parsing and IP validate helper function to parse
> single IP or range of IP with CIDR. (e.g. 198.18.0.0/15)
> 
> Helpers will be used in prior to set target address in samples/pktgen.
> 
> Signed-off-by: Daniel T. Lee <danieltimlee@...il.com>
> ---
>  samples/pktgen/functions.sh | 122 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 122 insertions(+)
> 
> diff --git a/samples/pktgen/functions.sh b/samples/pktgen/functions.sh
> index 4af4046d71be..8be5a6b6c097 100644
[...]

> +# Given a single IP(v4/v6) or CIDR, return minimum and maximum IP addr.
> +function parse_addr()
> +{
> +    # check function is called with (funcname)6
> +    [[ ${FUNCNAME[1]: -1} == 6 ]] && local IP6=6
> +    local bitlen=$[ IP6 ? 128 : 32 ]
> +    local octet=$[ IP6 ? 16 : 8 ]
> +
> +    local addr=$1
> +    local net prefix
> +    local min_ip max_ip
> +
> +    IFS='/' read net prefix <<< $addr
> +    [[ $IP6 ]] && net=$(extend_addr6 $net)
> +    validate_addr$IP6 $net
> +
> +    if [[ $prefix -gt $bitlen ]]; then
> +        err 5 "Invalid prefix: $prefix"
> +    elif [[ -z $prefix ]]; then
> +        min_ip=$net
> +        max_ip=$net
> +    else
> +        # defining array for converting Decimal 2 Binary
> +        # 00000000 00000001 00000010 00000011 00000100 ...
> +        local d2b='{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}{0..1}'
> +        [[ $IP6 ]] && d2b+=$d2b
> +        eval local D2B=($d2b)

I must say this is a rather cool shell/bash trick to use an array for
converting decimal numbers into binary.

> +
> +        local shift=$[ bitlen-prefix ]

Using a variable named 'shift' is slightly problematic for shell/bash
code.  It works, but it is just confusing.

> +        local min_mask max_mask
> +        local min max
> +        local ip_bit
> +        local ip sep
> +
> +        # set separator for each IP(v4/v6)
> +        [[ $IP6 ]] && sep=: || sep=.
> +        IFS=$sep read -ra ip <<< $net
> +
> +        min_mask="$(printf '1%.s' $(seq $prefix))$(printf '0%.s' $(seq $shift))"
> +        max_mask="$(printf '0%.s' $(seq $prefix))$(printf '1%.s' $(seq $shift))"

Also a surprising shell trick to get binary numbers out of a prefix number.

> +
> +        # calculate min/max ip with &,| operator
> +        for i in "${!ip[@]}"; do
> +            digit=$[ IP6 ? 16#${ip[$i]} : ${ip[$i]} ]
> +            ip_bit=${D2B[$digit]}
> +
> +            idx=$[ octet*i ]
> +            min[$i]=$[ 2#$ip_bit & 2#${min_mask:$idx:$octet} ]
> +            max[$i]=$[ 2#$ip_bit | 2#${max_mask:$idx:$octet} ]
> +            [[ $IP6 ]] && { min[$i]=$(printf '%X' ${min[$i]});
> +                            max[$i]=$(printf '%X' ${max[$i]}); }
> +        done
> +
> +        min_ip=$(IFS=$sep; echo "${min[*]}")
> +        max_ip=$(IFS=$sep; echo "${max[*]}")
> +    fi
> +
> +    echo $min_ip $max_ip
> +}

If you just fix the variable name 'shift' to something else, then I'm
happy with this patch.

Again, I'm very impressed with your shell/bash skills, I were certainly
challenged when reviewing this :-)

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer