kernel module write to proc - c

Question

I have made the following kernel module to create a process "hello_proc" in /proc directory:
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
static int hello_proc_show(struct seq_file *m, void *v) {
seq_printf(m, "P5 : Hello proc!\n");
return 0;
}
static int hello_proc_open(struct inode *inode, struct file *file) {
return single_open(file, hello_proc_show, NULL);
}
static const struct file_operations hello_proc_fops = {
.owner = THIS_MODULE,
.open = hello_proc_open,
.read = seq_read,
.write = seq_write,
.llseek = seq_lseek,
.release = single_release,
};
static int hello_proc_init(void) {
proc_create("hello_proc", 0, NULL, &hello_proc_fops);
printk("P5 : Process hello proc created");
return 0;
}
static void hello_proc_exit(void) {
remove_proc_entry("hello_proc", NULL);
}
MODULE_LICENSE("GPL");
module_init(hello_proc_init);
module_exit(hello_proc_exit);
I inserted the module and a proc file "hello_proc" was created successfully in directory /proc. The next thing I want to do is to write the output of command:
ls -l -t /proc | head -21 > /proc/hello_proc
to file "hello_proc" followed by reading as well. When I do (as root):
root#anubhav-Inspiron-3421:~$ ls -l -t /proc | head -21 > /proc/hello_proc
the execution just stops.
Now, I checked a lot of codes and resources on internet, but could not find one that explains how to write to a proc file. No resource on youtube either.
The best thing I found for writing to proc file were codes that were creating proc files using function "create_proc_entry", which looked fairly simple but for an older kernel version, different than mine. Any suggestions/directions to move ahead.

Answer 1

seq_write doesn't do what you might think. It's actually like seq_printf except it just writes a fixed number of bytes (rather than formatted outpuut). The seq_xxx API doesn't support writing to the device. You have to implement that separately.
For a fairly simple model of how to create a writable device using single_open on the read side, take a look at proc_pid_set_comm_operations, which implements /proc/<pid>/comm and also supports writing.
Also, note that create_proc_entry was deprecated but it's pretty straight-forward to change create_proc_entry to proc_create. As described in Documentation/filesystems/seq_file.txt:
- entry = create_proc_entry("sequence", 0, NULL);
- if (entry)
- entry->proc_fops = &ct_file_ops;
+ entry = proc_create("sequence", 0, NULL, &ct_file_ops);