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

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Message-Id: <20220510133016.9feff1aeec1a7a9ae137a8c3@linux-foundation.org>
Date:   Tue, 10 May 2022 13:30:16 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     cgel.zte@...il.com, 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 Wed, 11 May 2022 03:10:31 +0700 Ammar Faizi <ammarfaizi2@...weeb.org> wrote:

> 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';
> ```

Use strncpy_from_user()?

Can this code use proc_dointvec_minmax() or similar?