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

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Date:   Tue, 7 Dec 2021 12:41:52 +0530
From:   Amit Kachhap <amit.kachhap@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     linux-kernel@...r.kernel.org,
        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 12/6/21 7:34 PM, Christoph Hellwig wrote:
> 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?

iov_iter seems to be a reasonable way. As a start I thought of adding
minimal changes.

>