x86: msr: correct return value on partial operations

Return the correct return value when the MSR driver partially
completes a request (we should return the number of bytes actually
read or written, instead of the error code.)
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

arch/x86/kernel/msr.c

@@ -72,7 +72,8 @@ static ssize_t msr_read(struct file *file, char __user *buf,
 	u32 data[2];
 	u32 reg = *ppos;
 	int cpu = iminor(file->f_path.dentry->d_inode);
-	int err;
+	int err = 0;
+	ssize_t bytes = 0;
 
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
@@ -82,14 +83,17 @@ static ssize_t msr_read(struct file *file, char __user *buf,
 		if (err) {
 			if (err == -EFAULT) /* Fix idiotic error code */
 				err = -EIO;
-			return err;
+			break;
+		}
+		if (copy_to_user(tmp, &data, 8)) {
+			err = -EFAULT;
+			break;
 		}
-		if (copy_to_user(tmp, &data, 8))
-			return -EFAULT;
 		tmp += 2;
+		bytes += 8;
 	}
 
-	return ((char __user *)tmp) - buf;
+	return bytes ? bytes : err;
 }
 
 static ssize_t msr_write(struct file *file, const char __user *buf,
@@ -99,24 +103,28 @@ static ssize_t msr_write(struct file *file, const char __user *buf,
 	u32 data[2];
 	u32 reg = *ppos;
 	int cpu = iminor(file->f_path.dentry->d_inode);
-	int err;
+	int err = 0;
+	ssize_t bytes = 0;
 
 	if (count % 8)
 		return -EINVAL;	/* Invalid chunk size */
 
 	for (; count; count -= 8) {
-		if (copy_from_user(&data, tmp, 8))
-			return -EFAULT;
+		if (copy_from_user(&data, tmp, 8)) {
+			err = -EFAULT;
+			break;
+		}
 		err = wrmsr_safe_on_cpu(cpu, reg, data[0], data[1]);
 		if (err) {
 			if (err == -EFAULT) /* Fix idiotic error code */
 				err = -EIO;
-			return err;
+			break;
 		}
 		tmp += 2;
+		bytes += 8;
 	}
 
-	return ((char __user *)tmp) - buf;
+	return bytes ? bytes : err;
 }
 
 static int msr_open(struct inode *inode, struct file *file)