af_unix: Use offsetof() instead of sizeof().

The length of the AF_UNIX socket address contains an offset to the member
sun_path of struct sockaddr_un.

Currently, the preceding member is just sun_family, and its type is
sa_family_t and resolved to short.  Therefore, the offset is represented by
sizeof(short).  However, it is not clear and fragile to changes in struct
sockaddr_storage or sockaddr_un.

This commit makes it clear and robust by rewriting sizeof() with
offsetof().
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/unix/af_unix.c

@@ -231,7 +231,8 @@ static int unix_mkname(struct sockaddr_un *sunaddr, int len, unsigned int *hashp
 {
 	*hashp = 0;
 
-	if (len <= sizeof(short) || len > sizeof(*sunaddr))
+	if (len <= offsetof(struct sockaddr_un, sun_path) ||
+	    len > sizeof(*sunaddr))
 		return -EINVAL;
 	if (!sunaddr || sunaddr->sun_family != AF_UNIX)
 		return -EINVAL;
@@ -244,7 +245,8 @@ static int unix_mkname(struct sockaddr_un *sunaddr, int len, unsigned int *hashp
 		 * kernel address buffer.
 		 */
 		((char *)sunaddr)[len] = 0;
-		len = strlen(sunaddr->sun_path)+1+sizeof(short);
+		len = strlen(sunaddr->sun_path) +
+			offsetof(struct sockaddr_un, sun_path) + 1;
 		return len;
 	}
 
@@ -966,7 +968,8 @@ static int unix_autobind(struct socket *sock)
 		goto out;
 
 	err = -ENOMEM;
-	addr = kzalloc(sizeof(*addr) + sizeof(short) + 16, GFP_KERNEL);
+	addr = kzalloc(sizeof(*addr) +
+		       offsetof(struct sockaddr_un, sun_path) + 16, GFP_KERNEL);
 	if (!addr)
 		goto out;
 
@@ -974,7 +977,8 @@ static int unix_autobind(struct socket *sock)
 	refcount_set(&addr->refcnt, 1);
 
 retry:
-	addr->len = sprintf(addr->name->sun_path+1, "%05x", ordernum) + 1 + sizeof(short);
+	addr->len = sprintf(addr->name->sun_path + 1, "%05x", ordernum) +
+		offsetof(struct sockaddr_un, sun_path) + 1;
 	addr->hash = unix_hash_fold(csum_partial(addr->name, addr->len, 0));
 	addr->hash ^= sk->sk_type;
 
@@ -1156,7 +1160,7 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	    sunaddr->sun_family != AF_UNIX)
 		return -EINVAL;
 
-	if (addr_len == sizeof(short))
+	if (addr_len == offsetof(struct sockaddr_un, sun_path))
 		return unix_autobind(sock);
 
 	err = unix_mkname(sunaddr, addr_len, &hash);
@@ -1600,7 +1604,7 @@ static int unix_getname(struct socket *sock, struct sockaddr *uaddr, int peer)
 	if (!addr) {
 		sunaddr->sun_family = AF_UNIX;
 		sunaddr->sun_path[0] = 0;
-		err = sizeof(short);
+		err = offsetof(struct sockaddr_un, sun_path);
 	} else {
 		err = addr->len;
 		memcpy(sunaddr, addr->name, addr->len);
@@ -3228,7 +3232,8 @@ static int unix_seq_show(struct seq_file *seq, void *v)
 			seq_putc(seq, ' ');
 
 			i = 0;
-			len = u->addr->len - sizeof(short);
+			len = u->addr->len -
+				offsetof(struct sockaddr_un, sun_path);
 			if (!UNIX_ABSTRACT(s))
 				len--;
 			else {

net/unix/diag.c

@@ -19,7 +19,8 @@ static int sk_diag_dump_name(struct sock *sk, struct sk_buff *nlskb)
 	if (!addr)
 		return 0;
 
-	return nla_put(nlskb, UNIX_DIAG_NAME, addr->len - sizeof(short),
+	return nla_put(nlskb, UNIX_DIAG_NAME,
+		       addr->len - offsetof(struct sockaddr_un, sun_path),
 		       addr->name->sun_path);
 }