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

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Message-ID: <20171026122442.7e2ac2d0@shemminger-XPS-13-9360>
Date:   Thu, 26 Oct 2017 12:24:42 +0200
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.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 Wed, 25 Oct 2017 20:59:01 -0600
David Ahern <dsahern@...il.com> wrote:

> 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.

Actually this has little impact because old code was doing implicit zero
of whole buffer, new code does not.