Re: [PATCH 3/3] mm,page_owner: Filter out stacks by a threshold counter

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

On 9/1/22 11:42 AM, Oscar Salvador wrote:> +static ssize_t read_page_owner_threshold(struct file *file, char __user *buf,
> +					 size_t count, loff_t *pos)
> +{
> +	char *kbuf;
> +	int ret = 0;
> +
> +	count = min_t(size_t, count, PAGE_SIZE);
> +
> +	if (*pos >= count)
> +		return 0;
> +
> +	kbuf = kmalloc(count, GFP_KERNEL);
> +	if (!kbuf)
> +		return ENOMEM;

Missing a negative sign, return -ENOMEM.

> +	ret = scnprintf(kbuf, count, "%lu\n", threshold_count);
> +	if (copy_to_user(buf, kbuf, ret))
> +		ret = -EFAULT;
> +
> +	*pos += count;
> +	kfree(kbuf);
> +
> +	return ret;
> +}
> +
> +static ssize_t write_page_owner_threshold(struct file *file, const char __user *buf,
> +					  size_t count, loff_t *pos)
> +{
> +	char *kbuf;
> +	int ret = 0;
> +
> +	count = min_t(size_t, count, PAGE_SIZE);
> +	kbuf = kmalloc(count, GFP_KERNEL);

This looks overestimating to me. For unsigned long, on a 64-bit system
has max val 18446744073709551615 (20 chars).

You can use stack a allocated local variable with length 21. No need
to use kmalloc(). The same way with the read() op.

> +	if (!kbuf)
> +		return -ENOMEM;
> +
> +	if (copy_from_user(kbuf, buf, count)) {
> +		ret = -EFAULT;
> +		goto out;
> +	}
> +
> +	kbuf[count - 1] = '\0';
> +
> +	ret = kstrtoul(kbuf, 10, &threshold_count);
> +
> +out:
> +	kfree(kbuf);
> +	return ret ? ret : count;
> +}
> +
> +static const struct file_operations proc_page_owner_threshold = {
> +	.read = read_page_owner_threshold,
> +	.write = write_page_owner_threshold,
> +};

-- 
Ammar Faizi