Troubleshoot Module Dependencies with depmod
In this final step, you will learn how to use the depmod command to troubleshoot issues related to kernel module dependencies.
Let's start by creating a custom kernel module that has a dependency on another module. We'll use the hello_world module as an example:
## Create a new directory for the module
mkdir ~/project/hello_world
cd ~/project/hello_world
## Create the module source file
cat << EOF > hello_world.c
#include <linux/module.h>
#include <linux/kernel.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A simple hello world kernel module");
static int __init hello_world_init(void)
{
printk(KERN_INFO "Hello, World!\n");
return 0;
}
static void __exit hello_world_exit(void)
{
printk(KERN_INFO "Goodbye, World!\n");
}
module_init(hello_world_init);
module_exit(hello_world_exit);
EOF
## Compile the module
make -C /lib/modules/$(uname -r)/build M=$(pwd) modules
Now, let's create a second module that depends on the hello_world module:
## Create the dependent module source file
cat << EOF > hello_dependent.c
#include <linux/module.h>
#include <linux/kernel.h>
MODULE_LICENSE("GPL");
MODULE_AUTHOR("Your Name");
MODULE_DESCRIPTION("A module that depends on the hello_world module");
MODULE_DEPENDS_ON("hello_world");
static int __init hello_dependent_init(void)
{
printk(KERN_INFO "Hello Dependent Module!\n");
return 0;
}
static void __exit hello_dependent_exit(void)
{
printk(KERN_INFO "Goodbye Dependent Module!\n");
}
module_init(hello_dependent_init);
module_exit(hello_dependent_exit);
EOF
## Compile the dependent module
make -C /lib/modules/$(uname -r)/build M=$(pwd) modules
Now, let's try to load the hello_dependent module:
sudo insmod hello_dependent.ko
Example output:
insmod: ERROR: could not insert module hello_dependent.ko: Unknown symbol in module
The error message indicates that the hello_dependent module is unable to find the hello_world module, which it depends on.
To troubleshoot this issue, we can use the depmod command to update the module dependency information:
sudo depmod -a
Now, let's try loading the hello_dependent module again:
sudo insmod hello_dependent.ko
This time, the module should load successfully, and you should see the "Hello Dependent Module!" message in the kernel logs.
In this step, you learned how to use the depmod command to troubleshoot issues related to kernel module dependencies. By updating the module dependency information, you can ensure that all required modules are loaded correctly, allowing your custom modules to function as expected.
