linux-kernel - Re: [PATCH 00/12] treewide: Remove unnecessary kmalloc casts

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Message-ID: <AANLkTimSgiMP8EZ57Eba00vGsVTA3QKRZbh1bNYavISv@mail.gmail.com>
Date:	Wed, 9 Jun 2010 13:06:59 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Milton Miller <miltonm@....com>
Cc:	Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 00/12] treewide: Remove unnecessary kmalloc casts

Hi Milton,

On Wed, Jun 9, 2010 at 10:20 AM, Milton Miller <miltonm@....com> wrote:
> However, in this case you are removing casts that, while not necessary
> for C, are indeed there for a reason.
>
> Specifically, they are of the form
>   type *p;
>   <code>
>   p = (type *)kmalloc(sizeof(type), ...);
>
> For example, from the powerpc patch:
>> goto out;
>> }
>> - tmp_part = (struct nvram_partition *)
>> - kmalloc(sizeof(struct nvram_partition), GFP_KERNEL);
>> + tmp_part = kmalloc(sizeof(struct nvram_partition), GFP_KERNEL);
>> err = -ENOMEM;
>
> The reason they casts are present is to guard against someone changing
> the type of p at the top of the function and not changing the type at
> the kmalloc.

If you're worried about that...

[snip]

> There may have been discussion of doing the above vs
>   p = kmalloc(sizeof(*p), ...);

...it's better to use this form. There's actually a mention of this in
"Chapter 14: Allocating memory" of Documentation/CodingStyle. The
guard is not really a guard as someone can still change the "sizeof"
part to something else and the cast from "void *" will silently ignore
it.

                        Pekka
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