zsock_getaddrinfo() returns garbage values if IPv4 address is passed and hints->ai_family == AF_INET6

[kFYatek]

Describe the bug
When zsock_getaddrinfo() is passed a node value that is already a valid stringified IPv4 address, but hints->ai_family is configured to AF_INET6, the returned address is unusable.

To Reproduce

1. Build and run the following application:

CMakeLists.txt:

cmake_minimum_required(VERSION 3.13.1)
include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
project(pollout_bug_demo)
target_sources(app PRIVATE main.c)

prj.conf:

CONFIG_NEWLIB_LIBC=y
CONFIG_MAIN_STACK_SIZE=16384
CONFIG_NETWORKING=y
CONFIG_NET_IPV4=y
CONFIG_NET_IPV6=y
CONFIG_NET_SOCKETS=y
CONFIG_DNS_RESOLVER=y
CONFIG_DNS_SERVER_IP_ADDRESSES=y
CONFIG_DNS_SERVER1="8.8.8.8"
CONFIG_NET_CONFIG_NEED_IPV4=y
CONFIG_NET_CONFIG_SETTINGS=y
CONFIG_NET_CONFIG_MY_IPV4_ADDR="192.0.2.1"

main.c:

#include <assert.h>
#include <stdio.h>
#include <net/socket.h>

void main(void) {
    struct zsock_addrinfo *result = NULL;
    int ret = zsock_getaddrinfo("192.168.1.1", NULL,
                                &(struct zsock_addrinfo) {
                                    .ai_family = AF_INET6
                                }, &result);
    assert(!ret);
    assert(result);
    assert(result->ai_addr->sa_family == AF_INET6);
    
    char buf[256];
    puts(zsock_inet_ntop(
            AF_INET6,
            &((const struct sockaddr_in6 *) result->ai_addr)->sin6_addr,
            buf, sizeof(buf)));
}

2. Observe a garbage address being printed out, for example c0a8:101:196:8:5830:8:0:41. Note that the first two segments, c0a8:101, is 192.168.1.1 printed in IPv6 notation. The rest are random bytes from uninitialized memory.

Expected behavior
zsock_getaddrinfo() in this usage shall fail.

If AI_V4MAPPED flag is passed through hints->ai_flags, and if Zephyr supported IPv4-mapped IPv6 addresses, then returning a proper IPv4-mapped IPv6 address would also be acceptable, but AFAIK, Zephyr does not support IPv4-mapped IPv6 addresses.

Impact
If both IPv4 and IPv6 are configured, zsock_getaddrinfo() prefers IPv4 addresses (i.e., returns them first). If an application wants to support IPv4/v6 dual stack, wants to support both domain names and numeric addresses, and wants to prefer IPv6 addresses, one possible approach is to call getaddrinfo() with AF_INET6 hint first, and then fall back to AF_INET if that does not yield any results. This approach does not work on Zephyr because of this bug.

A different approach is necessary as a workaround, e.g. attempting to use inet_pton() first, or using getaddrinfo() without family hint and reordering the results. However, this can be an obstacle when attempting to use existing code that would otherwise be trivially portable to Zephyr.

Environment (please complete the following information):

• OS: Ubuntu 18.04
• Toolchain: Zephyr SDK 0.10.3
• Commit SHA or Version used: aa9b762d12