TASK4: Implement sysfs attributes example

04_basic_struct/Makefile

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+KDIR ?= ~/work/buildroot-2021.02.7/output/build/linux-5.10.7
+CHECKPATCH := $(KDIR)/scripts/checkpatch.pl
+
+SRC := sysfs_list.c
+OBJS := $(SRC:.c=.o)
+
+obj-m += $(OBJS)
+
+all:
+	$(CHECKPATCH) -f $(SRC) || exit 1
+	$(MAKE) -C $(KDIR) M=$(PWD) modules
+
+default: all
+
+clean:
+	$(MAKE) -C $(KDIR) M=$(PWD) clean
+

04_basic_struct/build_log.txt

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+victor@orion5491  ~/work/gl_kernel_procamp_2021/04_basic_struct   task04  make
+~/work/buildroot-2021.02.7/output/build/linux-5.10.7/scripts/checkpatch.pl -f sysfs_list.c || exit 1
+total: 0 errors, 0 warnings, 176 lines checked
+
+sysfs_list.c has no obvious style problems and is ready for submission.
+make -C ~/work/buildroot-2021.02.7/output/build/linux-5.10.7 M=/home/victor/work/gl_kernel_procamp_2021/04_basic_struct modules
+make[1]: Entering directory '/home/victor/work/buildroot-2021.02.7/output/build/linux-5.10.7'
+  CC [M]  /home/victor/work/gl_kernel_procamp_2021/04_basic_struct/sysfs_list.o
+  MODPOST /home/victor/work/gl_kernel_procamp_2021/04_basic_struct/Module.symvers
+WARNING: modpost: Symbol info of vmlinux is missing. Unresolved symbol check will be entirely skipped.
+  CC [M]  /home/victor/work/gl_kernel_procamp_2021/04_basic_struct/sysfs_list.mod.o
+  LD [M]  /home/victor/work/gl_kernel_procamp_2021/04_basic_struct/sysfs_list.ko
+make[1]: Leaving directory '/home/victor/work/buildroot-2021.02.7/output/build/linux-5.10.7'

04_basic_struct/console_log.txt

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+...
+OK
+Starting dropbear sshd: OK
+
+Welcome to Buildroot
+buildroot login: root
+# cd /home/user
+# ls
+sysfs_list.ko
+# dmesg -c
+Linux version 5.10.7 (victor@orion5491) (x86_64-buildroot-linux-uclibc-gcc.br_real (Buildroot 2021.02.7) 9.4.0, GNU ld (GNU Binutils) 2.35.2) #1 SMP Sun Nov 28 01:28:29 EET 2021
+Command line: rootwait root=/dev/vda console=tty1 console=ttyS0
+x86/fpu: x87 FPU will use FXSAVE
+...
+# insmod sysfs_list.ko 
+sysfs_list: loading out-of-tree module taints kernel.
+# echo "Foo!" > /sys/kernel/mod_sysfs/list 
+Node added: Foo!
+# echo "Bar!" > /sys/kernel/mod_sysfs/list 
+Node added: Bar!
+# echo "12345" > /sys/kernel/mod_sysfs/list 
+Node added: 12345
+# echo "%3@*&%#@" > /sys/kernel/mod_sysfs/list 
+Node added: %3@*&%#@
+# cat /sys/kernel/mod_sysfs/list 
+Foo!
+Bar!
+12345
+%3@*&%#@
+# dmesg
+sysfs_list: loading out-of-tree module taints kernel.
+Node added: Foo!
+Node added: Bar!
+Node added: 12345
+Node added: %3@*&%#@
+# rmmod sysfs_list.ko 
+4 nodes freed, the list is empty
+# exit
+
+Welcome to Buildroot
+buildroot login: qemu-system-x86_64: terminating on signal 2

04_basic_struct/sysfs_list.c

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+// SPDX-License-Identifier: GPL-2.0-only
+
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/kobject.h>
+#include <linux/list.h>
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/string.h>
+#include <linux/sysfs.h>
+#include <linux/types.h>
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Victor Krasnoshchok <ct3da21164@protonmail.ch>");
+MODULE_DESCRIPTION("Interface to a linked list of strings through sysfs.");
+MODULE_VERSION("0.1");
+
+/* ------------------------------------------ */
+/* List-related logic and data manipulation.  */
+
+struct data_item {
+	struct list_head list_node;
+	char *payload;
+	size_t payload_len;
+};
+
+static struct list_head root_node = { NULL, NULL };
+
+static ssize_t add_node(struct list_head *parent, const char *node_data,
+			size_t node_data_len)
+{
+	struct data_item *new_item;
+	size_t payload_len;
+
+	if (parent == NULL || node_data == NULL)
+		return -EINVAL;
+
+	new_item = kmalloc(sizeof(struct data_item), GFP_KERNEL);
+	if (new_item == NULL)
+		return -ENOMEM;
+
+	payload_len = node_data_len + 1; /* Including \0 */
+	new_item->payload = kzalloc(payload_len, GFP_KERNEL);
+	if (new_item->payload == NULL) {
+		pr_err("Failed to alloc. %lu bytes for payload.", payload_len);
+		kfree(new_item);
+		return -ENOMEM;
+	}
+
+	strncpy(new_item->payload, node_data, node_data_len);
+	new_item->payload_len = node_data_len;
+
+	list_add_tail(&new_item->list_node, parent);
+
+	/* No extra newline needed here - echo appends NLs by itself */
+	pr_info("Node added: %s", new_item->payload);
+	return node_data_len;
+}
+
+static ssize_t serialize_list(struct list_head *list_root, char *output)
+{
+	ssize_t total_len = 0;
+	char *wr_ptr = output;
+	struct list_head *curr_node;
+	struct data_item *curr_data;
+
+	if (output == NULL)
+		return -EINVAL;
+
+	list_for_each(curr_node, list_root) {
+		curr_data = list_entry(curr_node, struct data_item, list_node);
+		strncpy(wr_ptr, curr_data->payload, curr_data->payload_len);
+		wr_ptr += curr_data->payload_len;
+		total_len += curr_data->payload_len;
+	}
+
+	return total_len;
+}
+
+static void destroy_list(struct list_head *list_root)
+{
+	struct data_item *curr_data_item, *placeholder;
+	size_t nodes_cnt = 0;
+
+	list_for_each_entry_safe(curr_data_item, placeholder, list_root,
+				  list_node) {
+		kfree(curr_data_item->payload);
+		list_del(&curr_data_item->list_node);
+		kfree(curr_data_item);
+		++nodes_cnt;
+	}
+
+	pr_info("%lu nodes freed, the list is%sempty\n", nodes_cnt,
+			(list_empty_careful(list_root) ? " " : " NOT "));
+}
+
+/* ------------------------------------------ */
+/* Generic module context.                    */
+
+static ssize_t mod_sysfs_show(struct kobject *kobj, struct kobj_attribute *attr,
+			      char *buf)
+{
+	(void)kobj;
+	(void)attr;
+
+	return serialize_list(&root_node, buf);
+}
+
+static ssize_t mod_sysfs_store(struct kobject *kobj,
+			       struct kobj_attribute *attr, const char *buf,
+			       size_t count)
+{
+	(void)kobj;
+	(void)attr;
+
+	return add_node(&root_node, buf, count);
+}
+
+static struct module_context {
+	struct kobj_attribute mod_attr;
+	struct kobject *mod_kobj;
+} mod_sysfs_ctx = { __ATTR(list, 0644, mod_sysfs_show, mod_sysfs_store), NULL };
+
+static int module_context_init(struct module_context *ctx)
+{
+	int ret;
+
+	if (ctx == NULL)
+		return -EINVAL;
+
+	if (ctx->mod_kobj != NULL) {
+		pr_err("Context has been already initialized.");
+		return -EEXIST;
+	}
+
+	ctx->mod_kobj = kobject_create_and_add("mod_sysfs", kernel_kobj);
+	if (ctx->mod_kobj == NULL)
+		return -ENOMEM;
+
+	ret = sysfs_create_file(ctx->mod_kobj, &ctx->mod_attr.attr);
+	if (ret) {
+		kobject_put(ctx->mod_kobj);
+		ctx->mod_kobj = NULL;
+	}
+
+	return ret;
+}
+
+static void module_context_cleanup(struct module_context *ctx)
+{
+	if (ctx->mod_kobj != NULL)
+		kobject_put(ctx->mod_kobj);
+}
+
+static int __init mod_sysfs_init(void)
+{
+	int ret = module_context_init(&mod_sysfs_ctx);
+
+	if (ret != 0) {
+		pr_err("Ctx init. failed: %d", ret);
+		return ret;
+	}
+
+	INIT_LIST_HEAD(&root_node);
+	return 0;
+}
+
+static void __exit mod_sysfs_exit(void)
+{
+	destroy_list(&root_node);
+	module_context_cleanup(&mod_sysfs_ctx);
+}
+
+module_init(mod_sysfs_init);
+module_exit(mod_sysfs_exit);
+