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

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Date:   Tue, 15 Jan 2019 11:13:26 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     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 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?

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