linux-kernel - Re: [PATCH v5] mm/ksm: introduce ksm

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Message-ID: <435b5f7a-fcbd-f7ae-b66f-670e5997aa1b@gnuweeb.org>
Date:   Wed, 11 May 2022 03:10:31 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     cgel.zte@...il.com
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yang Yang <yang.yang29@....com.cn>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        Yunkai Zhang <zhang.yunkai@....com.cn>,
        xu xin <xu.xin16@....com.cn>,
        wangyong <wang.yong12@....com.cn>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Linux fsdevel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] mm/ksm: introduce ksm_force for each process

On 5/8/22 4:27 PM, cgel.zte@...il.com wrote:
> +static ssize_t ksm_force_write(struct file *file, const char __user *buf,
> +				size_t count, loff_t *ppos)
> +{
> +	struct task_struct *task;
> +	struct mm_struct *mm;
> +	char buffer[PROC_NUMBUF];
> +	int force;
> +	int err = 0;
> +
> +	memset(buffer, 0, sizeof(buffer));
> +	if (count > sizeof(buffer) - 1)
> +		count = sizeof(buffer) - 1;
> +	if (copy_from_user(buffer, buf, count)) {
> +		err = -EFAULT;
> +		goto out_return;
> +	}

This one looks like over-zeroing to me. You don't need to zero
all elements in the array. You're going to overwrite it with
`copy_from_user()` anyway.

Just zero the last potentially useful element by using @count
as the index. It can be like this:

```
	char buffer[PROC_NUMBUF];

	if (count > sizeof(buffer) - 1)
		count = sizeof(buffer) - 1;
	if (copy_from_user(buffer, buf, count))
		return -EFAULT;
	buffer[count] = '\0';
```

-- 
Ammar Faizi