fix kernel modules build after updating linux to v5.9.2

- `dep.c` and `dep2.c`: `debugfs_create_u32` does not return anymore, not
  sure why:
  https://unix.stackexchange.com/questions/593983/creating-a-debugfs-file-that-is-used-to-read-write-u32-value/621282#621282
  So just doing `debugfs_lookup` for now
- vermagic.c:
  cirosantilli/linux@51161bf
  prevents its usage in v5.8. Just migrating to `init_utsname` for now
- procfs.c: `struct file_operations` moved to a new `struct proc_ops` at:
  cirosantilli/linux@b567e07
- myprintk.c: `pr_warning` dropped for `pr_warn`:
  cirosantilli/linux@61ff72f
- `poll.c`: `kthread_func` is not defined in kernel, prefix with lkmc to
  avoid conflict

Fix #136

Fix #137

Author: cirosantilli

README.adoc

@@ -6765,7 +6765,22 @@ Bibliography:
 
 ==== vermagic
 
-Vermagic is a magic string present in the kernel and on <<module-info>> of kernel modules. It is used to verify that the kernel module was compiled against a compatible kernel version and relevant configuration:
+link:kernel_modules/vermagic.c[]
+
+As of kernel v5.8, you can't use `VERMAGIC_STRING` string from modules anymore as per: https://github.com/cirosantilli/linux/commit/51161bfc66a68d21f13d15a689b3ea7980457790[]. So instead we just showcase `init_utsname`.
+
+Sample insmod output as of LKMC fa8c2ee521ea83a74a2300e7a3be9f9ab86e2cb6 + 1 aarch64:
+
+....
+<6>[   25.180697] sysname    = Linux
+<6>[   25.180697] nodename   = buildroot
+<6>[   25.180697] release    = 5.9.2
+<6>[   25.180697] version    = #1 SMP Thu Jan 1 00:00:00 UTC 1970
+<6>[   25.180697] machine    = aarch64
+<6>[   25.180697] domainname = (none)
+....
+
+Vermagic is a magic string present in the kernel and previously visible in <<module-info>> on kernel modules. It is used to verify that the kernel module was compiled against a compatible kernel version and relevant configuration:
 
 ....
 insmod vermagic.ko
@@ -6777,8 +6792,6 @@ Possible dmesg output:
 VERMAGIC_STRING = 4.17.0 SMP mod_unload modversions
 ....
 
-Source: link:kernel_modules/vermagic.c[]
-
 If we artificially create a mismatch with `MODULE_INFO(vermagic`, the insmod fails with:
 
 ....

kernel_modules/debugfs.c

@@ -33,17 +33,12 @@ static const struct file_operations fops = {
 
 static int myinit(void)
 {
-	struct dentry *file;
 	dir = debugfs_create_dir("lkmc_debugfs", 0);
 	if (!dir) {
 		pr_alert("debugfs_create_dir failed");
 		return -1;
 	}
-	file = debugfs_create_u32("myfile", S_IRUSR | S_IWUSR, dir, &value);
-	if (!file) {
-		pr_alert("debugfs_create_u32 failed");
-		return -1;
-	}
+	debugfs_create_u32("myfile", S_IRUSR | S_IWUSR, dir, &value);
 
 	/* Created on the toplevel of the debugfs mount,
 	 * and with explicit fops instead of a fixed integer value.

kernel_modules/dep.c

@@ -6,17 +6,18 @@
 
 u32 lkmc_dep = 0;
 EXPORT_SYMBOL(lkmc_dep);
-static struct dentry *debugfs_file;
+static char *debugfs_filename = "lkmc_dep";
 
 static int myinit(void)
 {
-	debugfs_file = debugfs_create_u32("lkmc_dep", S_IRUSR | S_IWUSR, NULL, &lkmc_dep);
+	debugfs_create_u32(debugfs_filename, S_IRUSR | S_IWUSR, NULL, &lkmc_dep);
 	return 0;
 }
 
 static void myexit(void)
 {
-	debugfs_remove(debugfs_file);
+
+	debugfs_remove(debugfs_lookup(debugfs_filename, NULL));
 }
 
 module_init(myinit)

kernel_modules/dep2.c

@@ -5,17 +5,17 @@
 #include <linux/module.h>
 
 extern u32 lkmc_dep;
-static struct dentry *debugfs_file;
+static char *debugfs_filename = "lkmc_dep2";
 
 static int myinit(void)
 {
-	debugfs_file = debugfs_create_u32("lkmc_dep2", S_IRUSR | S_IWUSR, NULL, &lkmc_dep);
+	debugfs_create_u32(debugfs_filename, S_IRUSR | S_IWUSR, NULL, &lkmc_dep);
 	return 0;
 }
 
 static void myexit(void)
 {
-	debugfs_remove(debugfs_file);
+	debugfs_remove(debugfs_lookup(debugfs_filename, NULL));
 }
 
 module_init(myinit)

kernel_modules/mmap.c

@@ -1,5 +1,6 @@
 /* https://cirosantilli.com/linux-kernel-module-cheat#mmap */
 
+#include <asm-generic/io.h> /* virt_to_phys */
 #include <linux/fs.h>
 #include <linux/init.h>
 #include <linux/kernel.h> /* min */
@@ -120,17 +121,17 @@ static int release(struct inode *inode, struct file *filp)
 	return 0;
 }
 
-static const struct file_operations fops = {
-	.mmap = mmap,
-	.open = open,
-	.release = release,
-	.read = read,
-	.write = write,
+static const struct proc_ops pops = {
+	.proc_mmap = mmap,
+	.proc_open = open,
+	.proc_release = release,
+	.proc_read = read,
+	.proc_write = write,
 };
 
 static int myinit(void)
 {
-	proc_create(filename, 0, NULL, &fops);
+	proc_create(filename, 0, NULL, &pops);
 	return 0;
 }
 

kernel_modules/myprintk.c

@@ -8,7 +8,7 @@ static int myinit(void)
 	pr_alert("pr_alert\n");
 	pr_crit("pr_crit\n");
 	pr_err("pr_err");
-	pr_warning("pr_warning\n");
+	pr_warn("pr_warning\n");
 	pr_notice("pr_notice\n");
 	pr_info("pr_info\n");
 	pr_debug("pr_debug\n");

kernel_modules/poll.c

@@ -54,7 +54,7 @@ unsigned int poll(struct file *filp, struct poll_table_struct *wait)
 	}
 }
 
-static int kthread_func(void *data)
+static int lkmc_kthread_func(void *data)
 {
 	while (!kthread_should_stop()) {
 		readbuflen = snprintf(
@@ -81,7 +81,7 @@ static int myinit(void)
 	debugfs_file = debugfs_create_file(
 		"lkmc_poll", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
 	init_waitqueue_head(&waitqueue);
-	kthread = kthread_create(kthread_func, NULL, "mykthread");
+	kthread = kthread_create(lkmc_kthread_func, NULL, "mykthread");
 	wake_up_process(kthread);
 	return 0;
 }

kernel_modules/procfs.c

@@ -1,6 +1,5 @@
 /* https://cirosantilli.com/linux-kernel-module-cheat#procfs */
 
-#include <linux/debugfs.h>
 #include <linux/module.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h> /* seq_read, seq_lseek, single_open, single_release */
@@ -19,17 +18,16 @@ static int open(struct inode *inode, struct  file *file)
 	return single_open(file, show, NULL);
 }
 
-static const struct file_operations fops = {
-	.llseek = seq_lseek,
-	.open = open,
-	.owner = THIS_MODULE,
-	.read = seq_read,
-	.release = single_release,
+static const struct proc_ops pops = {
+	.proc_lseek = seq_lseek,
+	.proc_open = open,
+	.proc_read = seq_read,
+	.proc_release = single_release,
 };
 
 static int myinit(void)
 {
-	proc_create(filename, 0, NULL, &fops);
+	proc_create(filename, 0, NULL, &pops);
 	return 0;
 }
 

kernel_modules/vermagic.c

@@ -2,11 +2,34 @@
 
 #include <linux/module.h>
 #include <linux/kernel.h>
-#include <linux/vermagic.h>
+#include <linux/utsname.h>
+#if 0
+#include <linux/vermagic.h> /* VERMAGIC_STRING */
+#endif
 
 static int myinit(void)
 {
+    struct new_utsname *uts = init_utsname();
+	pr_info(
+        "sysname    = %s\n"
+        "nodename   = %s\n"
+        "release    = %s\n"
+        "version    = %s\n"
+        "machine    = %s\n"
+        "domainname = %s\n",
+        uts->sysname,
+        uts->nodename,
+        uts->release,
+        uts->version,
+        uts->machine,
+        uts->domainname
+    );
+
+#if 0
+    /* Possible before v5.8, but was buggy apparently, not sure why:
+     * https://github.com/cirosantilli/linux/commit/51161bfc66a68d21f13d15a689b3ea7980457790 */
 	pr_info("VERMAGIC_STRING = " VERMAGIC_STRING "\n");
+#endif
 	/* Nice try, but it is not a member. */
 	/*pr_info("THIS_MODULE->vermagic = %s\n", THIS_MODULE->vermagic);*/
 	return 0;