From 75afeba635bfe22a678a2312398ba0dfc5febec8 Mon Sep 17 00:00:00 2001
From: Ciro Santilli <ciro.santilli@gmail.com>
Date: Thu, 27 Jul 2017 15:10:00 +0100
Subject: [PATCH] sysfs

---
 kernel_module/README.md |  1 +
 kernel_module/sysfs.c   | 84 +++++++++++++++++++++++++++++++++++++++++
 rootfs_overlay/sysfs.sh | 11 ++++++
 3 files changed, 96 insertions(+)
 create mode 100644 kernel_module/sysfs.c
 create mode 100755 rootfs_overlay/sysfs.sh

diff --git a/kernel_module/README.md b/kernel_module/README.md
index a2f9964..892ea3e 100644
--- a/kernel_module/README.md
+++ b/kernel_module/README.md
@@ -10,6 +10,7 @@
     1.  Pseudo filesystems
         1.  [debugfs](debugfs.c)
         1.  [procfs](procfs.c)
+        1.  [sysfs](sysfs.c)
     1.  [fops](fops.c)
         1.  [ioctl](ioctl.c)
         1.  [poll](poll.c)
diff --git a/kernel_module/sysfs.c b/kernel_module/sysfs.c
new file mode 100644
index 0000000..79bd56e
--- /dev/null
+++ b/kernel_module/sysfs.c
@@ -0,0 +1,84 @@
+/*
+Adapted from: https://github.com/t3rm1n4l/kern-dev-tutorial/blob/1f036ef40fc4378f5c8d2842e55bcea7c6f8894a/05-sysfs/sysfs.c
+
+Vs procfs:
+
+- https://unix.stackexchange.com/questions/4884/what-is-the-difference-between-procfs-and-sysfs
+- https://stackoverflow.com/questions/37237835/how-to-attach-file-operations-to-sysfs-attribute-in-platform-driver
+
+This example shows how sysfs is more restricted, as it does not take a file_operations.
+
+So you basically can only do open, close, read, write, and lseek on sysfs files.
+
+It is kind of similar to a seq_file file_operations, except that write is also implemented.
+
+TODO: what are those kobject structs? Make a more complex example that shows what they can do.
+
+- https://www.kernel.org/doc/Documentation/kobject.txt
+- https://www.quora.com/What-are-kernel-objects-Kobj
+- http://www.makelinux.net/ldd3/chp-14-sect-1
+- https://www.win.tue.nl/~aeb/linux/lk/lk-13.html
+*/
+
+#include <linux/init.h>
+#include <linux/kobject.h>
+#include <linux/module.h>
+#include <linux/stat.h>
+#include <linux/string.h>
+#include <linux/sysfs.h>
+#include <uapi/linux/stat.h> /* S_IRUSR, S_IWUSR  */
+
+enum { FOO_SIZE_MAX = 4 };
+static int foo_size;
+static char foo_tmp[FOO_SIZE_MAX];
+
+static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
+		char *buff)
+{
+	strncpy(buff, foo_tmp, foo_size);
+	return foo_size;
+}
+
+static ssize_t foo_store(struct  kobject *kobj, struct kobj_attribute *attr,
+		const char *buff, size_t count)
+{
+	foo_size = min(count, (size_t)FOO_SIZE_MAX);
+	strncpy(foo_tmp, buff, foo_size);
+	return count;
+}
+
+static struct kobj_attribute foo_attribute =
+	__ATTR(foo, S_IRUGO | S_IWUSR, foo_show, foo_store);
+
+static struct attribute *attrs[] = {
+	&foo_attribute.attr,
+	NULL,
+};
+
+static struct attribute_group attr_group = {
+	.attrs = attrs,
+};
+
+static struct kobject *kobj;
+
+static int myinit(void)
+{
+	int ret;
+
+	kobj = kobject_create_and_add("lkmc_sysfs", kernel_kobj);
+	if (!kobj)
+		return -ENOMEM;
+	ret = sysfs_create_group(kobj, &attr_group);
+	if (ret)
+		kobject_put(kobj);
+	return ret;
+}
+
+static void myexit(void)
+{
+	kobject_put(kobj);
+}
+
+module_init(myinit);
+module_exit(myexit);
+MODULE_LICENSE("GPL");
diff --git a/rootfs_overlay/sysfs.sh b/rootfs_overlay/sysfs.sh
new file mode 100755
index 0000000..73a1d63
--- /dev/null
+++ b/rootfs_overlay/sysfs.sh
@@ -0,0 +1,11 @@
+#!/bin/sh
+
+set -x
+insmod /sysfs.ko
+cd /sys/kernel/lkmc_sysfs
+printf 12345 >foo
+cat foo
+# => 1234
+dd if=foo bs=1 count=2 skip=1 status=none
+# => 23
+rmmod sysfs