Linux Kernel（21.1）- ID Allocation

如同ID Allocation的Overview提到的, kernel提供了對應的一些API, 用以產生與維護identifiers (IDs), 舉凡file descriptor, process IDs, device instance number等等. IDR主要多了ID與pointer的對用能力, 而IDA就是單純的分配ID, 本章透過簡單的程式碼讓大家能瞭解與使用IDA.

首先, 該範例是個簡易的kernel module, 透過DEFINE_IDA(my_ida)宣告一個my_ida變數, 這是我們, 並透過read file operation去取得一個新的ID(ida_simple_get), 在write file operation中透過寫入特定ID移除該ID(ida_simple_remove), 最後在移除kernel module時, 使用ida_destroy(struct ida * ida)把所有的IDA resource都釋放, 不然會造成memory leak.

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/idr.h>
#include <linux/moduleparam.h>
#include <linux/fs.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/uaccess.h> // Required for copy_from_user

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Brook");
MODULE_DESCRIPTION("Kernel module to demo IDA");
MODULE_VERSION("0.1");

static DEFINE_IDA(my_ida);

static ssize_t ida_demo_read(struct file *file, char *buf, size_t count, loff_t *ppos)
{
    int id, len;
    char tmp_buf[10];

    id = ida_simple_get(&my_ida, 0, 0, GFP_KERNEL);
    if (id >= 0) {
        printk(KERN_INFO "IDR Demo: Successfully allocated ID: %d\n", id);
    } else {
        printk(KERN_ERR "IDR Demo: Failed to allocate ID\n");
        return -ENOMEM;
    }

    len = snprintf(tmp_buf, sizeof(tmp_buf), "%d", id);
    if (len < 0) {
        return -EINVAL;
    }

    if (copy_to_user(buf, tmp_buf, len)) {
        return -EFAULT;
    }

    return 0;
}

static ssize_t
ida_demo_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
{
    char tmp_buf[20];
    int id;

    if (count >= sizeof(tmp_buf))
        return -EINVAL;

    if (copy_from_user(tmp_buf, buf, count))
        return -EFAULT;

    tmp_buf[count] = '\0';
    // Convert the input string to an integer
    if (kstrtoint(tmp_buf, 10, &id)) {
        printk(KERN_ERR "invalid ID: %s\n", tmp_buf);
        return -EINVAL;
    }

    printk(KERN_INFO "remove ID %d\n", id);
    ida_simple_remove(&my_ida, id);

    return count;
}

// Define a file operation structure for IDA access
static struct file_operations ida_fops = {
    .open = simple_open,
    .read = ida_demo_read,
    .write = ida_demo_write,
    .llseek = default_llseek,
};

static int __init ida_demo_init(void)
{
    struct proc_dir_entry *proc_entry;

    printk(KERN_INFO "IDR Demo: Initializing module\n");

    // Create a file entry to invoke 'ida'
    proc_entry = proc_create("ida", S_IRUGO, NULL, &ida_fops);
    if (!proc_entry) {
        printk(KERN_ERR "Failed to create sysfs entry for 'ida'\n");
        return -ENOMEM;
    }
    return 0;
}

static void __exit ida_demo_exit(void)
{
    printk(KERN_INFO "IDR Demo: Exiting module\n");
    ida_destroy(&my_ida);
}

module_init(ida_demo_init);
module_exit(ida_demo_exit);

簡易的Makefile

KDIR ?= /build/brook/Projects/qemu/linux/
# Modules which are included in the kernel are installed in the
# directory:
#       /lib/modules/$(KERNELRELEASE)/kernel/
# And external modules are installed in:
#       /lib/modules/$(KERNELRELEASE)/extra/
#
# INSTALL_MOD_PATH
# A prefix can be added to the
#       installation path using the variable INSTALL_MOD_PATH:
#
#       $ make INSTALL_MOD_PATH=/frodo modules_install
#       => Install dir: /frodo/lib/modules/$(KERNELRELEASE)/kernel/
export INSTALL_MOD_PATH=/build/brook/Projects/qemu/initrd-arm
obj-m := ida_demo.o
ida_demo-y := ida_main.o

modules modules_install clean:
 $(MAKE) -C $(KDIR) M=$$PWD $@

這是在QEMU下執行的結果, 會先產生ID 0/1/2, 然後移除1, 接著再產生的ID就會把1生出來, 再來就是3了, 所以透過IDA, 可以幫user管理ID(唯一的編號)

/ # uname -a
Linux (none) 5.4.0+ #6 SMP Tue Jan 3 08:39:24 CST 2023 armv7l GNU/Linux
/ # insmod /lib/modules/5.4.0+/extra/ida_demo.ko
ida_demo: loading out-of-tree module taints kernel.
IDR Demo: Initializing module
/ # cat /proc/ida
IDR Demo: Successfully allocated ID: 0
/ # cat /proc/ida
IDR Demo: Successfully allocated ID: 1
/ # cat /proc/ida
IDR Demo: Successfully allocated ID: 2
/ # echo 1 > /proc/ida
remove ID 1
/ # cat /proc/ida
IDR Demo: Successfully allocated ID: 1
/ # cat /proc/ida
IDR Demo: Successfully allocated ID: 3

• https://www.kernel.org/doc/html/v5.4/core-api/idr.html, ID Allocation