Subject: Re: [PATCH 5/5] sysctl: pass kernel pointers to ->proc_handler

From: Al Viro <[email-masked]>
Date: Tue, 21 Apr 2020 20:16:15 +0100

On Tue, Apr 21, 2020 at 07:15:39PM +0200, Christoph Hellwig wrote:
> Instead of having all the sysctl handlers deal with user pointers, which
> is rather hairy in terms of the BPF interaction, copy the input to and
> from  userspace in common code.  This also means that the strings are
> always NUL-terminated by the common code, making the API a little bit
> safer.
> 
> As most handler just pass through the data to one of the common handlers
> a lot of the changes are mechnical.

> @@ -564,27 +564,38 @@ static ssize_t proc_sys_call_handler(struct file *filp, void __user *buf,
>  	if (!table->proc_handler)
>  		goto out;
>  
> -	error = BPF_CGROUP_RUN_PROG_SYSCTL(head, table, write, buf, &count,
> -					   ppos, &new_buf);
> +	if (write) {
> +		kbuf = memdup_user_nul(ubuf, count);
> +		if (IS_ERR(kbuf)) {
> +			error = PTR_ERR(kbuf);
> +			goto out;
> +		}
> +	} else {
> +		error = -ENOMEM;
> +		kbuf = kzalloc(count, GFP_KERNEL);

Better allocate count + 1 bytes here, that way a lot of insanity in the
instances can be simply converted to snprintf().  Yes, I know it'll bring
the Church Of Avoiding The Abomination Of Sprintf out of the woodwork,
but...