linux-kernel - Re: [PATCH 1/5] linux/kernel.h: Add sizeof

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Message-ID: <201909240920.AE3CD67E87@keescook>
Date:   Tue, 24 Sep 2019 09:22:10 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>
Cc:     pankaj.bharadiya@...il.com, andriy.shevchenko@...ux.intel.com,
        kernel-hardening@...ts.openwall.com, akpm@...ux-foundation.org,
        mayhs11saini@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] linux/kernel.h: Add sizeof_member macro

On Tue, Sep 24, 2019 at 04:28:35PM +0530, Pankaj Bharadiya wrote:
> At present we have 3 different macros to calculate the size of a
> member of a struct:
>   - SIZEOF_FIELD
>   - FIELD_SIZEOF
>   - sizeof_field
> 
> To bring uniformity in entire kernel source tree let's add
> sizeof_member macro.
> 
> Replace all occurrences of above 3 macro's with sizeof_member in
> future patches.
> 
> Signed-off-by: Pankaj Bharadiya <pankaj.laxminarayan.bharadiya@...el.com>
> ---
>  include/linux/kernel.h | 9 +++++++++
>  1 file changed, 9 insertions(+)

Since stddef.h ends up needing this macro, and kernel.h includes
stddef.h, why not put this macro in stddef.h instead? Then the
open-coded version of it in stddef (your last patch) can use
sizeof_member()?

Otherwise, yes, looks good. (Though I might re-order the patches so the
last patch is the tree-wide swap -- then you don't need the exclusions,
I think?)

-Kees

> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 4fa360a13c1e..0b80d8bb3978 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -79,6 +79,15 @@
>   */
>  #define round_down(x, y) ((x) & ~__round_mask(x, y))
>  
> +/**
> + * sizeof_member - get the size of a struct's member
> + * @T: the target struct
> + * @m: the target struct's member
> + * Return: the size of @m in the struct definition without having a
> + * declared instance of @T.
> + */
> +#define sizeof_member(T, m) (sizeof(((T *)0)->m))
> +
>  /**
>   * FIELD_SIZEOF - get the size of a struct's field
>   * @t: the target struct
> -- 
> 2.17.1
> 

-- 
Kees Cook