Merge pull request #15623 from poettering/cmsg-cleanup

various CMSG_xyz clean-ups, split out of #15571

src/basic/socket-util.c

@@ -818,10 +818,7 @@ ssize_t send_one_fd_iov_sa(
                 const struct sockaddr *sa, socklen_t len,
                 int flags) {
 
-        union {
-                struct cmsghdr cmsghdr;
-                uint8_t buf[CMSG_SPACE(sizeof(int))];
-        } control = {};
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(int))) control = {};
         struct msghdr mh = {
                 .msg_name = (struct sockaddr*) sa,
                 .msg_namelen = len,
@@ -850,8 +847,6 @@ ssize_t send_one_fd_iov_sa(
                 cmsg->cmsg_type = SCM_RIGHTS;
                 cmsg->cmsg_len = CMSG_LEN(sizeof(int));
                 memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
-
-                mh.msg_controllen = CMSG_SPACE(sizeof(int));
         }
         k = sendmsg(transport_fd, &mh, MSG_NOSIGNAL | flags);
         if (k < 0)
@@ -877,10 +872,7 @@ ssize_t receive_one_fd_iov(
                 int flags,
                 int *ret_fd) {
 
-        union {
-                struct cmsghdr cmsghdr;
-                uint8_t buf[CMSG_SPACE(sizeof(int))];
-        } control = {};
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(int))) control;
         struct msghdr mh = {
                 .msg_control = &control,
                 .msg_controllen = sizeof(control),

src/basic/socket-util.h

@@ -166,6 +166,17 @@ struct cmsghdr* cmsg_find(struct msghdr *mh, int level, int type, socklen_t leng
                 (ctype*) (_found ? CMSG_DATA(_found) : NULL);         \
         })
 
+/* Resolves to a type that can carry cmsghdr structures. Make sure things are properly aligned, i.e. the type
+ * itself is placed properly in memory and the size is also aligned to what's appropriate for "cmsghdr"
+ * structures. */
+#define CMSG_BUFFER_TYPE(size)                                          \
+        union {                                                         \
+                struct cmsghdr cmsghdr;                                 \
+                uint8_t buf[size];                                      \
+                uint8_t align_check[(size) >= CMSG_SPACE(0) &&          \
+                                    (size) == CMSG_ALIGN(size) ? 1 : -1]; \
+        }
+
 /*
  * Certain hardware address types (e.g Infiniband) do not fit into sll_addr
  * (8 bytes) and run over the structure. This macro returns the correct size that