cat function calling read() infinite times

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.read function should also correctly process its len and off arguments. The simplest way to implement reading from memory-buffered file is to use simple_read_from_buffer helper:

static ssize_t dev_read(struct file *filp, char *buff, size_t len, loff_t *off)
{
    return simple_read_from_buffer(buff, len, off, msg, 100);
}

You can inspect code of that helper (defined in fs/libfs.c) for educational purposes.

BTW, for your .write method you could use simple_write_to_buffer helper.

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