Replace gethostbyname(3) by getaddrinfo(3)

gethostbyname(3) was removed in POSIX.1-2008.  It has been obsoleted,
and replaced by getaddrinfo(3), which is superior in several ways:

-  gethostbyname(3) is not reentrant.  There's a GNU extension,
   gethostbyname_r(3) which is reentrant, but it's not likely to be
   standardized for the following reason.  And we don't care too much
   about this point either.

-  gethostbyname(3) only supports IPv4, but getaddrinfo(3) supports both
   IPv4 and IPv6 (and may support other address families in the future).

We don't care about reentrancy, so for keeping the code simple (i.e.,
not touch call site to add code to free(3) an allocated buffer), I added
a static buffer for inet_ntop(3).  We could address that in the future,
but I don't think it's worth it.

BTW, we also replace inet_ntoa(3) by inet_ntop(3), as a consequence of
using getaddrinfo(3).  inet_ntoa(3) is also marked as deprecated, but
that deprecation seems to have been documented only in the manual page,
and POSIX doesn't mark it as deprecated.  The deprecation notice goes
back to when the inet_ntop(3) manual page was added by Sam Varshavchik
to the Linux man-pages in version 1.30 (year 2000).

So, this, apart from updating the code to POSIX.1-2008, is also adding
support for IPv6 :)  Although, probably many other parts of the code are
written for IPv4 only, so I wouldn't yet claim support for it.

A few notes:

-  I didn't check the return value of inet_ntop(3), since it can't fail
   for the given input:

   -  EAFNOSUPPORT:  We only call it with AF_INET and AF_INET6.
   -  ENOSPC:  We calculate the size of the buffer to be wide enough:
               MAX(INET_ADDRSTRLEN, INET6_ADDRSTRLEN) so it always fits.

Cc: Dave Hagewood <admin@arrowweb.com>
Cc: Sam Varshavchik
Cc: Jakub Jelinek <jakub@redhat.com>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

src/login_nopam.c

@@ -52,6 +52,7 @@
 #include <unistd.h>
 #include <stdlib.h>
 #include <sys/socket.h>
+#include <sys/param.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>		/* for inet_ntoa() */
 
@@ -68,6 +69,7 @@
 /* Delimiters for fields and for lists of users, ttys or hosts. */
 static char fs[] = ":";		/* field separator */
 static char sep[] = ", \t";	/* list-element separator */
+static char inet_ntop_buffer[MAX(INET_ADDRSTRLEN, INET6_ADDRSTRLEN)];
 
 static bool list_match (char *list, const char *item, bool (*match_fn) (const char *, const char *));
 static bool user_match (const char *tok, const char *string);
@@ -263,18 +265,41 @@ static bool user_match (const char *tok, const char *string)
 
 static const char *resolve_hostname (const char *string)
 {
-	/*
+	int                  gai;
-	 * Resolve hostname to numeric IP address, as suggested
+	char                 *p;
-	 * by Dave Hagewood <admin@arrowweb.com>.  --marekm
+	struct addrinfo      *addrs;
-	 */
+	struct sockaddr      *sa;
-	struct hostent *hp;
+	struct sockaddr_in   *sin;
+	struct sockaddr_in6  *sin6;
 
-	hp = gethostbyname (string);
+	gai = getaddrinfo(string, NULL, NULL, &addrs);
-	if (NULL != hp) {
+	if (gai != 0)
-		return inet_ntoa (*((struct in_addr *) *(hp->h_addr_list)));
+		goto notfound;
+
+	sa = addrs[0].ai_addr;
+	switch (sa->sa_family) {
+	case AF_INET:
+		sin = (struct sockaddr_in *) sa;
+		inet_ntop(AF_INET, &sin->sin_addr,
+		          inet_ntop_buffer, NITEMS(inet_ntop_buffer));
+		p = inet_ntop_buffer;
+		break;
+	case AF_INET6:
+		sin6 = (struct sockaddr_in6 *) sa;
+		inet_ntop(AF_INET6, &sin6->sin6_addr,
+		          inet_ntop_buffer, NITEMS(inet_ntop_buffer));
+		p = inet_ntop_buffer;
+		break;
+	default:
+		SYSLOG ((LOG_ERR, "Hypothetical future IPv7?"));
+		abort();
 	}
 
-	SYSLOG ((LOG_ERR, "%s - unknown host", string));
+	freeaddrinfo(addrs);
+	return p;
+
+notfound:
+	SYSLOG ((LOG_ERR, "getaddrinfo(%s): %s", string, gai_strerror(gai)));
 	return string;
 }