linux-kernel - Re: [PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

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Date:   Tue, 15 Jan 2019 11:21:44 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Thomas Gleixner <tglx@...utronix.de>, peng.hao2@....com.cn
Cc:     bp@...en8.de, dave.hansen@...ux.intel.com, peterz@...radead.org,
        luto@...nel.org, x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm/mem_encrypt_identity : fix error useage to sizeof

On 15/01/2019 11:13, Thomas Gleixner wrote:
> On Mon, 7 Jan 2019, peng.hao2@....com.cn wrote:
> 
>>>> Fix error usage to sizeof. It should not use sizeof to pointer.
>>>
>>> .... because?
>>>
>>> The commit message needs to explain what the potential issue could be
>>> and why it doesn't matter in this case.
>> I see the definition of pte_t may be more than sizeof(unsigned long).
>> So I think sizeof(pte_t) is safer.
> 
> What exactly is the difference between:
> 
> 	pte_t	*p;
> 
> 	sizeof(*p)
> 
> and
> 
> 	sizeof(pte_t)
> 
> and what is safer about the latter?

Please note that the current code is using sizeof(p) instead of sizeof(*p).

And this is really different for X86_32 PAE.


Juergen

> 
> Answer: No difference and nothing is safer because it's exactly the same.
> 
> In general we use sizeof(*p) simply because when the data type of p changes
> you don't have to update the code, it just works and stays correct.
> 
> Thanks,
> 
> 	tglx
>