linux-kernel - Re: [RFC PATCH 01/14] fs/proc/vmcore: Update read_from

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Message-ID: <20211206140451.GA4936@lst.de>
Date:   Mon, 6 Dec 2021 15:04:51 +0100
From:   Christoph Hellwig <hch@....de>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Vincenzo Frascino <Vincenzo.Frascino@....com>,
        Kevin Brodsky <kevin.brodsky@....com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        kexec <kexec@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Young <dyoung@...hat.com>, Baoquan He <bhe@...hat.com>,
        Vivek Goyal <vgoyal@...hat.com>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 01/14] fs/proc/vmcore: Update read_from_oldmem()
 for user pointer

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?