netdev - Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if size is not enough

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Date:   Wed, 25 Oct 2017 20:59:01 -0600
From:   David Ahern <dsahern@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Michal Kubecek <mkubecek@...e.cz>, Phil Sutter <phil@....cc>
Subject: Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if
 size is not enough

On 10/25/17 7:41 PM, Hangbin Liu wrote:
> +static int rtnl_recvmsg(int fd, struct msghdr *msg, char **answer)
> +{
> +	struct iovec *iov = msg->msg_iov;
> +	char *buf;
> +	int len;
> +
> +	iov->iov_base = NULL;
> +	iov->iov_len = 0;
> +
> +	len = __rtnl_recvmsg(fd, msg, MSG_PEEK | MSG_TRUNC);
> +	if (len < 0)
> +		return len;
> +
> +	buf = malloc(len);
> +	if (!buf) {
> +		fprintf(stderr, "malloc error: not enough buffer\n");
> +		return -ENOMEM;
> +	}
> +
> +	iov->iov_base = buf;
> +	iov->iov_len = len;
> +
> +	len = __rtnl_recvmsg(fd, msg, 0);
> +	if (len < 0) {
> +		free(buf);
> +		return len;
> +	}

The kernel needs a flag that says "give me the message of the buffer is
large enough; if not just PEEK and tell me the length." That would avoid
the double call in most cases.