Re: [RFC PATCH 01/14] fs/proc/vmcore: Update read_from_oldmem() for user pointer

[Christoph Hellwig]

On Fri, Dec 03, 2021 at 04:12:18PM +0530, Amit Daniel Kachhap wrote:
> +	return read_from_oldmem_to_kernel(buf, count, ppos,
> +					  cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT));

Overly long line.

> +ssize_t read_from_oldmem(char __user *ubuf, char *kbuf, size_t count,
> +			 u64 *ppos, bool encrypted)
>  {
>  	unsigned long pfn, offset;
>  	size_t nr_bytes;
> @@ -156,19 +163,27 @@ ssize_t read_from_oldmem(char *buf, size_t count,
>  		/* If pfn is not ram, return zeros for sparse dump files */
>  		if (!pfn_is_ram(pfn)) {
>  			tmp = 0;
> -			if (!userbuf)
> -				memset(buf, 0, nr_bytes);
> -			else if (clear_user(buf, nr_bytes))
> +			if (kbuf)
> +				memset(kbuf, 0, nr_bytes);
> +			else if (clear_user(ubuf, nr_bytes))
>  				tmp = -EFAULT;

This looks like a huge mess.  What speak against using an iov_iter
here?