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Editorial Team
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Asked: 2026-05-22T18:03:04+00:00

ssize_t probchar_write(struct file *filp, const char __user *data, size_t s, loff_t *off) { printk(KERN_DEBUG

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ssize_t probchar_write(struct file *filp, 
    const char __user *data, size_t s, loff_t *off) {

    printk(KERN_DEBUG "Data> |%s|\n", data); // only for debug
    char chars[MAX_LENGHT];
    if(s > MAX_LENGHT)
        s = MAX_LENGHT;
    if (copy_from_user(chars, data, s)) {
        return -EFAULT;
    }
    printk(KERN_DEBUG "Chars> |%s|\n", chars);
    return s;
}

This is dmesg output

[66777.956582] Data> |45
[66777.956596] |
[66777.956634] Chars> |45
[66777.956636] �    Ҩ�H��   H�� |

Why the copied chain has more characters in the end?

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1 Answer

  1. Editorial Team

    This seems to be a write function from a device driver.

    First

    printk(KERN_DEBUG "Data> |%s|\n", data);

    Do not do this! Do not ever directly access user data ever!

    Second

    char chars[s];

    I doubt this is legal C. You need to either specify a size at compile time, or use kmalloc.

    The copy_from_user usage is good. You should check for error and return -EFAULT. This is ok.

    So just try and allocating the chars and it should work. You might also want to look at the offset, though for academic purposes that can be initially skipped.

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