copy string in kernel using kstrcpy

Creating copy of existing strings would require allocation of new memory and then assign the old string to new one. The multiple steps required to create a string copy can be done with the help of a single function in the kernel.

char kstrdup (const char * s,gfp_t gfp); Where : const char * s: the string to duplicate gfp_t gfp: the GFP mask used in the kmalloc call when allocating memory

Ref: Kernel API

In the example below, we have created a proc entry to show the usage of the function. A call to read the proc entry creates a copy of the existing string in temp allcates it to the string called copy which we send as output of the proc entry.

	/*Tested on kernel 5.3*/
	#include <linux/module.h>
	#include <linux/kernel.h>
	#include <linux/proc_fs.h>
	#include<linux/slab.h>
	ssize_t size;
	ssize_t read_proc(struct file *filp,char *buf,size_t count,loff_t *offp )
	{
	int ret;
	char temp[6],*copy;
	sprintf(temp,"hello");
	copy=kstrdup(temp,GFP_KERNEL);
	printk(KERN_INFO "%s",copy);
	size=sizeof(char)*strlen(copy);
	ret = simple_read_from_buffer(buf,count,offp,copy,size);
	return ret;
	}
	struct file_operations proc_fops = {
	read: read_proc
	};
	void create_new_proc_entry(void)
	{
	proc_create("copy",0,NULL,&proc_fops);
	}
	int proc_init (void) {
	create_new_proc_entry();
	return 0;
	}
	void proc_cleanup(void) {
	remove_proc_entry("copy",NULL);
	}
	MODULE_LICENSE("GPL");
	module_init(proc_init);
	module_exit(proc_cleanup);

view raw proc_kstrcpy.c hosted with ❤ by GitHub

Save the file as proc_read_copy.c and compile it using the following makefile.

ifneq ($(KERNELRELEASE),) obj-m := proc_read_copy.o else KERNELDIR ?= /lib/modules/$(shell uname -r)/build PWD := $(shell pwd) default: $(MAKE) -C $(KERNELDIR) M=$(PWD) modules clean: $(MAKE) -C $(KERNELDIR) M=$(PWD) clean endif

Compile and load the module into kernel using.

$ make $ sudo insmod proc_read_copy.ko

To see the output, read the proc entry created by the module.

$cat /proc/copy hello