listen does not allow specifying a negative backlog

[asomers]

The listen function, in C, takes a "backlog" argument as an int. It normally must be positive. However, it may be ngative:

• On FreeBSD, since version 2.2, to use the maximum value
• On Linux, as an undocumented feature, to use a default value, at least according to @devnexen .

But when listen was added to Nix in change f85e9bf , the backlog argument's type was specified as usize, which precludes negative values. It also will rollover at i32::MAX_VALUE with unpredictable consequences.

[asomers]

See rust-lang/rust#97963 for a discussion of the behavior on Linux.

[SteveLauC]

So we know that on FreeBSD, a negative backlog or a value bigger than the max one means the default value. And from that PR, Linux will see -1 as the default value, what about other negative values, will they also be seen the default value?

Maybe we should use a wrapper type, just like the one introduced in #1876

[SteveLauC]

Ok, from the Linux source code, they directly cast the backlog argument from int to unsigned int, for a somaxconn of value 4096, all the negative values will be bigger than it and then set to it:

This behavior depends on the value of somaxconn, so it is different from the FreeBSD one, we should probably not use other negative value

int __sys_listen(int fd, int backlog)
{
	struct socket *sock;
	int err, fput_needed;
	int somaxconn;

	sock = sockfd_lookup_light(fd, &err, &fput_needed);
	if (sock) {
		somaxconn = READ_ONCE(sock_net(sock->sk)->core.sysctl_somaxconn);
		if ((unsigned int)backlog > somaxconn)
			backlog = somaxconn;

		err = security_socket_listen(sock, backlog);
		if (!err)
			err = READ_ONCE(sock->ops)->listen(sock, backlog);

		fput_light(sock->file, fput_needed);
	}
	return err;
}