um: ubd: use 64-bit time_t where possible

The ubd code suffers from a possible y2038 overflow on 32-bit
architectures, both for the cow header and the os_file_modtime()
function.

Replace time_t with time64_t to extend the ubd_kern side as much
as possible.

Whether this makes a difference for the user side depends on
the host libc implementation that may use either 32-bit or 64-bit
time_t.

For the cow file format, the header contains an unsigned 32-bit
timestamp, which is good until y2106, passing this through a
'long long' gives us a consistent interpretation between 32-bit
and 64-bit um kernels.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>

arch/um/drivers/ubd_kern.c

@@ -561,7 +561,7 @@ static inline int ubd_file_size(struct ubd *ubd_dev, __u64 *size_out)
 	__u32 version;
 	__u32 align;
 	char *backing_file;
-	time_t mtime;
+	time64_t mtime;
 	unsigned long long size;
 	int sector_size;
 	int bitmap_offset;
@@ -600,9 +600,9 @@ static int read_cow_bitmap(int fd, void *buf, int offset, int len)
 	return 0;
 }
 
-static int backing_file_mismatch(char *file, __u64 size, time_t mtime)
+static int backing_file_mismatch(char *file, __u64 size, time64_t mtime)
 {
-	unsigned long modtime;
+	time64_t modtime;
 	unsigned long long actual;
 	int err;
 
@@ -628,7 +628,7 @@ static int backing_file_mismatch(char *file, __u64 size, time_t mtime)
 		return -EINVAL;
 	}
 	if (modtime != mtime) {
-		printk(KERN_ERR "mtime mismatch (%ld vs %ld) of COW header vs "
+		printk(KERN_ERR "mtime mismatch (%lld vs %lld) of COW header vs "
 		       "backing file\n", mtime, modtime);
 		return -EINVAL;
 	}
@@ -671,7 +671,7 @@ static int open_ubd_file(char *file, struct openflags *openflags, int shared,
 		  unsigned long *bitmap_len_out, int *data_offset_out,
 		  int *create_cow_out)
 {
-	time_t mtime;
+	time64_t mtime;
 	unsigned long long size;
 	__u32 version, align;
 	char *backing_file;