linux-kernel - Re: [PATCH][next] stddef: Introduce struct_group_tagged

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Message-ID: <1c05cba3-41c9-4862-b126-4ec6a79876d6@embeddedor.com>
Date: Mon, 18 Mar 2024 17:59:38 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Kees Cook <keescook@...omium.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] stddef: Introduce struct_group_tagged_attr() helper
 macro



On 3/18/24 17:53, Kees Cook wrote:
> 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@...nel.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?
> 

Mmmh, the rest of the helpers in the struct_group() family fall in this
same category, can we use __struct_group() indistinctly?

--
Gustavo