test-broken.c

/*
 * Compile with:
 * $ gcc test.c -o /tmp/test -g -O2
 *
 * $ gcc --version
 * gcc (Debian 13.2.0-24) 13.2.0
 *
 * Expected output:
 *
 * $ /tmp/test
 * Before: input.sa_family: 10, output.ss_family: 0
 * After:  input.sa_family: 10, output.ss_family: 10
 *
 * Actual Output:
 *
 * $ /tmp/test
 * Before: input.sa_family: 10, output.ss_family: 0
 * After:  input.sa_family: 10, output.ss_family: 0
 * Error: address family was not copied? 10 vs. 0
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>

static int copy_family_v4(const struct sockaddr_in *input, struct sockaddr_in *output)
{
	in_addr_t pdst = ntohl(input->sin_addr.s_addr);

	/* this breaks it, even for AF_INET6 case? */
	if (((pdst) & 0xffffff00) != 0xe0000000)
		return -1;

	output->sin_family = input->sin_family;
	return 0;
}

static int copy_family_v6(const struct sockaddr_in6 *input, struct sockaddr_in6 *output)
{
	output->sin6_family = input->sin6_family;
	return 0;
}

static int copy_family(struct sockaddr *input, struct sockaddr_storage *output)
{
	switch (input->sa_family) {
	case AF_INET:
		return copy_family_v4((struct sockaddr_in *)input,
				      (struct sockaddr_in *)output);
	case AF_INET6:
		return copy_family_v6((struct sockaddr_in6 *)input,
	                              (struct sockaddr_in6 *)output);
	}

	return -1;
}

static void sap_create_socket(void)
{
	struct sockaddr_storage output = { 0 };
	struct sockaddr input = { 0 };
	int ret;

	input.sa_family = AF_INET6;

	printf("Before:\tinput.sa_family: %i, output.ss_family: %i\n", input.sa_family, output.ss_family);

	ret = copy_family(&input, &output);
	if (ret < 0)
		return;

	printf("After:\tinput.sa_family: %i, output.ss_family: %i\n", input.sa_family, output.ss_family);

	if (input.sa_family != output.ss_family)
		fprintf(stderr, "Error: address family was not copied? %i vs. %i\n", input.sa_family, output.ss_family);

	return;

}

int main(int argc, char *argv[])
{
	sap_create_socket();
	return 0;
}

test-fixed.c

/*
 * Compile with:
 * $ gcc test.c -o /tmp/test -g -O2
 *
 * $ gcc --version
 * gcc (Debian 13.2.0-24) 13.2.0
 *
 * Output:
 *
 * $ /tmp/test
 * Before: input.sa_family: 10, output.ss_family: 0
 * After:  input.sa_family: 10, output.ss_family: 10
 */

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netdb.h>

union sockaddr_union {
	struct sockaddr_in in;
	struct sockaddr_in6 in6;
	struct sockaddr s;
};

static int copy_family_v4(const struct sockaddr_in *input, struct sockaddr_in *output)
{
	/* now this is not undefined behaviour anymore: */
	in_addr_t pdst = ntohl(input->sin_addr.s_addr);
	if (((pdst) & 0xffffff00) != 0xe0000000)
		return -1;

	output->sin_family = input->sin_family;
	return 0;
}

static int copy_family_v6(const struct sockaddr_in6 *input, struct sockaddr_in6 *output)
{
	output->sin6_family = input->sin6_family;
	return 0;
}

static int copy_family(union sockaddr_union *input, union sockaddr_union *output)
{
	switch (input->s.sa_family) {
	case AF_INET:
		return copy_family_v4(&input->in,
				      &output->in);
	case AF_INET6:
		return copy_family_v6(&input->in6,
	                              &output->in6);
	}

	return -1;
}

static void sap_create_socket(void)
{
	union sockaddr_union output = { 0 };
	union sockaddr_union input = { 0 };
	int ret;

	input.s.sa_family = AF_INET6;

	printf("Before:\tinput.sa_family: %i, output.ss_family: %i\n", input.s.sa_family, output.s.sa_family);

	ret = copy_family(&input, &output);
	if (ret < 0)
		return;

	printf("After:\tinput.sa_family: %i, output.ss_family: %i\n", input.s.sa_family, output.s.sa_family);

	if (input.s.sa_family != output.s.sa_family)
		fprintf(stderr, "Error: address family was not copied? %i vs. %i\n", input.s.sa_family, output.s.sa_family);

	return;

}

int main(int argc, char *argv[])
{
	sap_create_socket();
	return 0;
}

test-fixed.diff

--- test-broken.c	2024-05-06 22:21:38.353799962 +0200
+++ test-fixed.c	2024-05-07 15:07:54.020057525 +0200
@@ -9,8 +9,7 @@
  *
  * $ /tmp/test
  * Before: input.sa_family: 10, output.ss_family: 0
- * After:  input.sa_family: 10, output.ss_family: 0
- * Error: address family was not copied? 10 vs. 0
+ * After:  input.sa_family: 10, output.ss_family: 10
  */
 
 #include <sys/types.h>
@@ -21,11 +20,16 @@
 #include <string.h>
 #include <netdb.h>
 
+union sockaddr_union {
+	struct sockaddr_in in;
+	struct sockaddr_in6 in6;
+	struct sockaddr s;
+};
+
 static int copy_family_v4(const struct sockaddr_in *input, struct sockaddr_in *output)
 {
+	/* now this is not undefined behaviour anymore: */
 	in_addr_t pdst = ntohl(input->sin_addr.s_addr);
-
-	/* this breaks it, even for AF_INET6 case? */
 	if (((pdst) & 0xffffff00) != 0xe0000000)
 		return -1;
 
@@ -39,15 +43,15 @@
 	return 0;
 }
 
-static int copy_family(struct sockaddr *input, struct sockaddr_storage *output)
+static int copy_family(union sockaddr_union *input, union sockaddr_union *output)
 {
-	switch (input->sa_family) {
+	switch (input->s.sa_family) {
 	case AF_INET:
-		return copy_family_v4((struct sockaddr_in *)input,
-				      (struct sockaddr_in *)output);
+		return copy_family_v4(&input->in,
+				      &output->in);
 	case AF_INET6:
-		return copy_family_v6((struct sockaddr_in6 *)input,
-	                              (struct sockaddr_in6 *)output);
+		return copy_family_v6(&input->in6,
+	                              &output->in6);
 	}
 
 	return -1;
@@ -55,22 +59,22 @@
 
 static void sap_create_socket(void)
 {
-	struct sockaddr_storage output = { 0 };
-	struct sockaddr input = { 0 };
+	union sockaddr_union output = { 0 };
+	union sockaddr_union input = { 0 };
 	int ret;
 
-	input.sa_family = AF_INET6;
+	input.s.sa_family = AF_INET6;
 
-	printf("Before:\tinput.sa_family: %i, output.ss_family: %i\n", input.sa_family, output.ss_family);
+	printf("Before:\tinput.sa_family: %i, output.ss_family: %i\n", input.s.sa_family, output.s.sa_family);
 
 	ret = copy_family(&input, &output);
 	if (ret < 0)
 		return;
 
-	printf("After:\tinput.sa_family: %i, output.ss_family: %i\n", input.sa_family, output.ss_family);
+	printf("After:\tinput.sa_family: %i, output.ss_family: %i\n", input.s.sa_family, output.s.sa_family);
 
-	if (input.sa_family != output.ss_family)
-		fprintf(stderr, "Error: address family was not copied? %i vs. %i\n", input.sa_family, output.ss_family);
+	if (input.s.sa_family != output.s.sa_family)
+		fprintf(stderr, "Error: address family was not copied? %i vs. %i\n", input.s.sa_family, output.s.sa_family);
 
 	return;