Get Character String Written to Custom /dev/ file

Question

I'm currently working with a custom device driver that I installed. I'm very new to this and having trouble understanding how to get the data that I write into it from the command line.

For example, I write data to the file like so:

echo -n "test" > /dev/custom

And then I merely want to get the string here, so I can use the characters within:

static ssize_t custom_write (struct file *f, const char *buf, size_t len, loff_t *offset)
{
  printk(KERN_ALERT "Test\n");
    return len;
}

That's my first thought, but I'm unsure how to access it properly. Most times I've tried I've received some sort of permission error. — YenBenLemon
– YenBenLemon, Commented Nov 13, 2020 at 22:10

Barmar · Accepted Answer · 2020-11-16 17:23:26Z

1

The string is in buf, but it's in user memory. You need to use copy_from_user() to copy to kernel memory.

static ssize_t custom_write (struct file *f, const char *buf, size_t len, loff_t *offset)
{
    char custom_buffer[MAX_BUFFER_SIZE];
    size_t custom_buffer_size = len;
    if (custom_buffer_size > MAX_BUFFER_SIZE) {
        custom_buffer_size = MAX_BUFFER_SIZE;
    }
    if (copy_from_user(custom_buffer, buf, custom_buffer_size) != 0) {
        return -EFAULT;
    }
    printk(KERN_ALERT "Test %.*s\n", (int)custom_buffer_size, custom_buffer);
    return custom_buffer_size;
}

edited Nov 16, 2020 at 17:23

answered Nov 13, 2020 at 22:21

Barmar

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1 Comment

Ian Abbott Over a year ago

For printk(KERN_ALERT "Test %s\n", custom_buffer); the string in custom_buffer[] is not properly null terminated. printk(KERN_ALERT, "Test %.*s\n", (int)custom_buffer_size, custom_buffer); would be better.

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