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Date: Mon, 18 Mar 2024 16:53:32 -0700
From: Kees Cook <keescook@chromium.org>
To: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Subject: Re: [PATCH][next] stddef: Introduce struct_group_tagged_attr()
 helper macro
Message-ID: <202403181652.B8787FB288@keescook>
References: <ZfjHkXJWg12rARKU@neat>
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In-Reply-To: <ZfjHkXJWg12rARKU@neat>

On Mon, Mar 18, 2024 at 05:00:33PM -0600, Gustavo A. R. Silva wrote:
> We need a new `struct_group()` helper that allows for both having the
> struct be tagged, and specifying struct attributes like `__packed`
> or `__align(x)`.
> 
> This new helper will initially be used to address
> -Wflex-array-member-not-at-end warnings, where a tagged struct is used
> to separate the flexible-array member from the rest of the members in
> the flexible structure [1]. There are some scenarios in which those
> members need to be packed, as well.
> 
> So, `struct_group_tagged_attr()` is introduced for this.
> 
> Link: https://lore.kernel.org/linux-hardening/ZeIgeZ5Sb0IZTOyt@neat/ [1]
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
>  include/linux/stddef.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
> 
> diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> index 929d67710cc5..919df9453257 100644
> --- a/include/linux/stddef.h
> +++ b/include/linux/stddef.h
> @@ -80,6 +80,26 @@ enum {
>  #define struct_group_tagged(TAG, NAME, MEMBERS...) \
>  	__struct_group(TAG, NAME, /* no attrs */, MEMBERS)
>  
> +/**
> + * struct_group_tagged_attr() - Create a struct_group with a reusable
> + * tag and trailing attributes.
> + *
> + * @TAG: The tag name for the named sub-struct
> + * @NAME: The identifier name of the mirrored sub-struct
> + * @ATTRS: Any struct attributes to apply
> + * @MEMBERS: The member declarations for the mirrored structs
> + *
> + * Used to create an anonymous union of two structs with identical
> + * layout and size: one anonymous and one named. The former can be
> + * used normally without sub-struct naming, and the latter can be
> + * used to reason about the start, end, and size of the group of
> + * struct members. Includes struct tag argument for the named copy,
> + * so the specified layout can be reused later. Also includes
> + * structure attributes argument.
> + */
> +#define struct_group_tagged_attr(TAG, NAME, ATTRS, MEMBERS...) \
> +	__struct_group(TAG, NAME, ATTRS, MEMBERS)

This is the same as __struct_group() only with a longer name? Why not
just use __struct_group() directly?

-- 
Kees Cook