Avoid using variable-length array

Using variable-length array is not a good call
(https://lkml.org/lkml/2011/10/23/25). So remove it.

Signed-off-by: Eric Lin <ericlinsechs@gmail.com>

Author: ericlinsechs

client.c

@@ -29,6 +29,10 @@ int main()
     for (int i = 0; i <= offset; i++) {
         lseek(fd, i, SEEK_SET);
         sz = read(fd, buf, 1);
+        if (sz < 0) {
+            perror("Failed to read character device");
+            exit(1);
+        }
         printf("Reading from " FIB_DEV
                " at offset %d, returned the sequence "
                "%lld.\n",
@@ -38,6 +42,10 @@ int main()
     for (int i = offset; i >= 0; i--) {
         lseek(fd, i, SEEK_SET);
         sz = read(fd, buf, 1);
+        if (sz < 0) {
+            perror("Failed to read character device");
+            exit(1);
+        }
         printf("Reading from " FIB_DEV
                " at offset %d, returned the sequence "
                "%lld.\n",

fibdrv.c

@@ -6,6 +6,7 @@
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/mutex.h>
+#include <linux/slab.h>
 #include <linux/version.h>
 
 MODULE_LICENSE("Dual MIT/GPL");
@@ -27,8 +28,9 @@ static int major = 0, minor = 0;
 
 static long long fib_sequence(long long k)
 {
-    /* FIXME: C99 variable-length array (VLA) is not allowed in Linux kernel. */
-    long long f[k + 2];
+    long long *f = kmalloc(sizeof(*f) * (k + 2), GFP_KERNEL);
+    if (!f)
+        return -1;
 
     f[0] = 0;
     f[1] = 1;
@@ -37,7 +39,11 @@ static long long fib_sequence(long long k)
         f[i] = f[i - 1] + f[i - 2];
     }
 
-    return f[k];
+    long long ret = f[k];
+
+    kfree(f);
+
+    return ret;
 }
 
 static int fib_open(struct inode *inode, struct file *file)