seq_file, printk to pr_info, disk persistency

README.md

@@ -123,6 +123,19 @@ We have not managed to track this problem down yet, but the following workaround
 
 This started happening when we switched to building QEMU through Buildroot, and has not been observed on later Ubuntu.
 
+## Filesystem persistency
+
+The root filesystem is persistent across:
+
+    ./runqemu
+    ./runqemu
+
+However, when you do:
+
+    ./run
+
+it gets overwritten by a fresh filesystem and you lose all changes.
+
 ## Text mode
 
 Show serial output of QEMU directly on the current terminal, without opening a QEMU window:

kernel_module/README.md

@@ -12,6 +12,8 @@
         1.  [ioctl](ioctl.c)
         1.  [poll](poll.c)
         1.  [anonymous_inode](anonymous_inode.c)
+        1.  [seq_file](seq_file.c)
+            1. [seq_file_single](seq_file_inode.c)
     1.  Asynchronous
         1.  [workqueue](workqueue.c)
         1.  [sleep](sleep.c)

kernel_module/fops.c

@@ -20,7 +20,7 @@ Here we use debugfs.
 #include <asm/uaccess.h> /* copy_from_user, copy_to_user */
 #include <linux/debugfs.h>
 #include <linux/errno.h> /* EFAULT */
-#include <linux/fs.h>
+#include <linux/fs.h> /* file_operations */
 #include <linux/kernel.h> /* min */
 #include <linux/module.h>
 #include <linux/printk.h> /* printk */

kernel_module/hello.c

@@ -15,13 +15,13 @@ MODULE_LICENSE("GPL");
 
 static int myinit(void)
 {
-	printk(KERN_INFO "hello init\n");
+	pr_info("hello init\n");
 	return 0;
 }
 
 static void myexit(void)
 {
-	printk(KERN_INFO "hello exit\n");
+	pr_info("hello exit\n");
 }
 
 module_init(myinit)

kernel_module/hello2.c

@@ -11,13 +11,13 @@ MODULE_LICENSE("GPL");
 
 static int myinit(void)
 {
-	printk(KERN_INFO "hello2 init\n");
+	pr_info("hello2 init\n");
 	return 0;
 }
 
 static void myexit(void)
 {
-	printk(KERN_INFO "hello2 exit\n");
+	pr_info("hello2 exit\n");
 }
 
 module_init(myinit)

kernel_module/seq_file.c

@@ -0,0 +1,90 @@
+/*
+Adapted from: http://allskyee.blogspot.co.uk/2011/12/proc-seqfile-write.html
+
+Writting trivial read fops is repetitive and error prone.
+
+The seq_file API makes the process much easier for those trivial cases.
+
+There is not write version however, as writes are more complex:
+https://stackoverflow.com/questions/30710517/how-to-implement-a-writable-proc-file-by-using-seq-file-in-a-driver-module
+*/
+
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
+#include <linux/debugfs.h>
+#include <linux/errno.h> /* EFAULT */
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/printk.h> /* pr_info */
+#include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */
+#include <uapi/linux/stat.h> /* S_IRUSR */
+
+MODULE_LICENSE("GPL");
+
+static struct dentry *debugfs_file;
+
+static void * next(struct seq_file *s, void *v, loff_t *pos)
+{
+	pr_info("next\n");
+    (*(unsigned long *)v)++;
+    (*pos)++;
+    return *pos < 3 ? v : NULL;
+}
+
+static void * start(struct seq_file *s, loff_t *pos)
+{
+    static unsigned long counter = 0;
+
+	pr_info("start pos = %llx\n", (unsigned long long)*pos);
+    if (*pos == 0) {
+        return &counter;
+	} else {
+        *pos = 0;
+        return NULL;
+    }
+}
+
+static int show(struct seq_file *s, void *v)
+{
+	pr_info("show\n");
+    seq_printf(s, "%lx\n", *(unsigned long *)v);
+    return 0;
+}
+
+static void stop(struct seq_file *s, void *v)
+{
+	pr_info("stop\n");
+}
+
+static struct seq_operations my_seq_ops = {
+    .next  = next,
+    .show  = show,
+    .start = start,
+    .stop  = stop,
+};
+
+static int open(struct inode *inode, struct file *file)
+{
+    return seq_open(file, &my_seq_ops);
+}
+
+static struct file_operations fops = {
+    .owner   = THIS_MODULE,
+    .llseek  = seq_lseek,
+    .open    = open,
+    .read    = seq_read,
+    .release = seq_release
+};
+
+static int myinit(void)
+{
+	debugfs_file = debugfs_create_file("lkmc_seq_file", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
+	return 0;
+}
+
+static void myexit(void)
+{
+	debugfs_remove(debugfs_file);
+}
+
+module_init(myinit)
+module_exit(myexit)

kernel_module/seq_file_single.c

@@ -0,0 +1,49 @@
+/*
+For single reads, single_open is an even more convenient version of seq_file.
+*/
+
+#include <asm/uaccess.h> /* copy_from_user, copy_to_user */
+#include <linux/debugfs.h>
+#include <linux/errno.h> /* EFAULT */
+#include <linux/fs.h>
+#include <linux/module.h>
+#include <linux/printk.h> /* pr_info */
+#include <linux/seq_file.h> /* seq_read, seq_lseek, single_release */
+#include <uapi/linux/stat.h> /* S_IRUSR */
+
+MODULE_LICENSE("GPL");
+
+static struct dentry *debugfs_file;
+
+static int show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "abcd\n");
+	return 0;
+}
+
+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 int myinit(void)
+{
+	debugfs_file = debugfs_create_file("lkmc_seq_file", S_IRUSR | S_IWUSR, NULL, NULL, &fops);
+	return 0;
+}
+
+static void myexit(void)
+{
+	debugfs_remove(debugfs_file);
+}
+
+module_init(myinit)
+module_exit(myexit)

rootfs_overlay/seq_file.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+set -ex
+
+insmod /seq_file.ko
+cd /sys/kernel/debug
+
+cat 'lkmc_seq_file'
+# => 0
+# => 1
+# => 2
+
+cat 'lkmc_seq_file'
+# => 3
+# => 4
+# => 5
+
+# TODO understand, why does this print nothing?
+dd if='lkmc_seq_file' bs=1 count=2 skip=2

rootfs_overlay/seq_file_single.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+set -ex
+insmod /seq_file_single.ko
+cd /sys/kernel/debug
+cat 'lkmc_seq_file_single'
+# => abcd
+dd if='lkmc_seq_file_single' bs=1 skip=2
+# => cd