linux-user: Don't use CMSG_ALIGN(sizeof struct cmsghdr)

The Linux struct cmsghdr is already guaranteed to be sufficiently
aligned that CMSG_ALIGN(sizeof struct cmsghdr) is always equal
to sizeof struct cmsghdr. Stop doing the unnecessary alignment
arithmetic for host and target cmsghdr.

This follows kernel commit 1ff8cebf49ed9e9ca2 and brings our
TARGET_CMSG_* macros back into line with the kernel ones,
as well as making them easier to understand.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Message-Id: <1513345976-22958-3-git-send-email-peter.maydell@linaro.org>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

linux-user/syscall.c

@@ -1692,7 +1692,7 @@ static inline abi_long target_to_host_cmsg(struct msghdr *msgh,
         void *target_data = TARGET_CMSG_DATA(target_cmsg);
 
         int len = tswapal(target_cmsg->cmsg_len)
-                  - TARGET_CMSG_ALIGN(sizeof (struct target_cmsghdr));
+            - sizeof(struct target_cmsghdr);
 
         space += CMSG_SPACE(len);
         if (space > msgh->msg_controllen) {
@@ -1773,7 +1773,7 @@ static inline abi_long host_to_target_cmsg(struct target_msghdr *target_msgh,
         void *data = CMSG_DATA(cmsg);
         void *target_data = TARGET_CMSG_DATA(target_cmsg);
 
-        int len = cmsg->cmsg_len - CMSG_ALIGN(sizeof (struct cmsghdr));
+        int len = cmsg->cmsg_len - sizeof(struct cmsghdr);
         int tgt_len, tgt_space;
 
         /* We never copy a half-header but may copy half-data;

linux-user/syscall_defs.h

@@ -303,9 +303,9 @@ struct target_cmsghdr {
                                __target_cmsg_nxthdr(mhdr, cmsg, cmsg_start)
 #define TARGET_CMSG_ALIGN(len) (((len) + sizeof (abi_long) - 1) \
                                & (size_t) ~(sizeof (abi_long) - 1))
-#define TARGET_CMSG_SPACE(len) (TARGET_CMSG_ALIGN (len) \
-                               + TARGET_CMSG_ALIGN (sizeof (struct target_cmsghdr)))
-#define TARGET_CMSG_LEN(len)   (TARGET_CMSG_ALIGN (sizeof (struct target_cmsghdr)) + (len))
+#define TARGET_CMSG_SPACE(len) (sizeof(struct target_cmsghdr) + \
+                                TARGET_CMSG_ALIGN(len))
+#define TARGET_CMSG_LEN(len) (sizeof(struct target_cmsghdr) + (len))
 
 static __inline__ struct target_cmsghdr *
 __target_cmsg_nxthdr(struct target_msghdr *__mhdr,