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Date: Wed, 11 May 2022 14:58:56 +0700
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Subject: Re: [PATCH v5] mm/ksm: introduce ksm_force for each process
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To: Andrew Morton <akpm@linux-foundation.org>
Cc: cgel.zte@gmail.com, Kees Cook <keescook@chromium.org>,
 Matthew Wilcox <willy@infradead.org>, Yang Yang <yang.yang29@zte.com.cn>,
 Ran Xiaokai <ran.xiaokai@zte.com.cn>, Yunkai Zhang
 <zhang.yunkai@zte.com.cn>, xu xin <xu.xin16@zte.com.cn>,
 wangyong <wang.yong12@zte.com.cn>, Linux MM Mailing List
 <linux-mm@kvack.org>,
 Linux fsdevel Mailing List <linux-fsdevel@vger.kernel.org>,
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References: <20220507105926.d4423601230f698b0f5228d1@linux-foundation.org>
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From: Ammar Faizi <ammarfaizi2@gnuweeb.org>
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On 5/11/22 3:30 AM, Andrew Morton wrote:
> On Wed, 11 May 2022 03:10:31 +0700 Ammar Faizi <ammarfaizi2@gnuweeb.org> wrote:
> 
>> On 5/8/22 4:27 PM, cgel.zte@gmail.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';
>> ```
> 
> Use strncpy_from_user()?

Sounds better.

> Can this code use proc_dointvec_minmax() or similar?

Not familiar with that API at all. Leaving it to other participants...

-- 
Ammar Faizi