linux-kernel - Re: [PATCH] x86/microcode/intel: Fix allocation size of struct ucode

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Date:   Fri, 6 Jan 2017 00:14:13 +0000
From:   Junichi Nomura <j-nomura@...jp.nec.com>
To:     Borislav Petkov <bp@...en8.de>
CC:     Andy Shevchenko <andy.shevchenko@...il.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH] x86/microcode/intel: Fix allocation size of struct
 ucode_patch

On 01/06/17 09:02, Borislav Petkov wrote:
> On Thu, Jan 05, 2017 at 11:52:07PM +0000, Junichi Nomura wrote:
>>>> +       p = kzalloc(sizeof(struct ucode_patch), GFP_KERNEL);
>>>
>>> Perhaps sizeof(*p) ?
>>
>> Yeah, that might be preferred.
> 
> No, those things are never preferred because
> 
> 	sizeof(struct <type>)
> 
> tells you exactly the size of what kind of object you're getting vs
> 
> 	sizeof(*p)
> 
> which tells you you're getting the size of what p points to.
> 
> Now you have to go look at p and what type it is. In the current case, p
> is defined not far away from the use site but in a larger function, you
> most likely need to eyeball up to its type when reading the code. Which
> makes the whole thing less readable.

Personally I have same opinion as yours. :)

But according to Documentation/process/coding-style.rst, it seems
"sizeof(*p)" is preferred style and the reason there makes some
sense.

Quote from coding-style.rst:
  > The preferred form for passing a size of a struct is the following:
  > 
  >         p = kmalloc(sizeof(*p), ...);
  > 
  > The alternative form where struct name is spelled out hurts readability and
  > introduces an opportunity for a bug when the pointer variable type is changed
  > but the corresponding sizeof that is passed to a memory allocator is not.

I'm fine with either way.

-- 
Jun'ichi Nomura, NEC Corporation / NEC Solution Innovators, Ltd.