net: add a CMSG_USER_DATA macro

Add a variant of CMSG_DATA that operates on user pointer to avoid
sparse warnings about casting to/from user pointers.  Also fix up
CMSG_DATA to rely on the gcc extension that allows void pointer
arithmetics to cut down on the amount of casts.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Christoph Hellwig

include/linux/socket.h

@@ -94,7 +94,10 @@ struct cmsghdr {
 
 #define CMSG_ALIGN(len) ( ((len)+sizeof(long)-1) & ~(sizeof(long)-1) )
 
-#define CMSG_DATA(cmsg)	((void *)((char *)(cmsg) + sizeof(struct cmsghdr)))
+#define CMSG_DATA(cmsg) \
+	((void *)(cmsg) + sizeof(struct cmsghdr))
+#define CMSG_USER_DATA(cmsg) \
+	((void __user *)(cmsg) + sizeof(struct cmsghdr))
 #define CMSG_SPACE(len) (sizeof(struct cmsghdr) + CMSG_ALIGN(len))
 #define CMSG_LEN(len) (sizeof(struct cmsghdr) + (len))
 

net/core/scm.c

@@ -236,7 +236,7 @@ int put_cmsg(struct msghdr * msg, int level, int type, int len, void *data)
 	err = -EFAULT;
 	if (copy_to_user(cm, &cmhdr, sizeof cmhdr))
 		goto out;
-	if (copy_to_user(CMSG_DATA(cm), data, cmlen - sizeof(struct cmsghdr)))
+	if (copy_to_user(CMSG_USER_DATA(cm), data, cmlen - sizeof(*cm)))
 		goto out;
 	cmlen = CMSG_SPACE(len);
 	if (msg->msg_controllen < cmlen)
@@ -300,7 +300,7 @@ void scm_detach_fds(struct msghdr *msg, struct scm_cookie *scm)
 	if (fdnum < fdmax)
 		fdmax = fdnum;
 
-	for (i=0, cmfptr=(__force int __user *)CMSG_DATA(cm); i<fdmax;
+	for (i=0, cmfptr =(int __user *)CMSG_USER_DATA(cm); i<fdmax;
 	     i++, cmfptr++)
 	{
 		struct socket *sock;