[PATCH][next] stddef: Introduce struct_group_tagged_attr() helper macro

[PATCH][next] stddef: Introduce struct_group_tagged_attr() helper macro

[Gustavo A. R. Silva]

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)
+
 /**
  * DECLARE_FLEX_ARRAY() - Declare a flexible array usable in a union
  *
-- 
2.34.1

Re: [PATCH][next] stddef: Introduce struct_group_tagged_attr() helper macro

[Kees Cook]

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

Re: [PATCH][next] stddef: Introduce struct_group_tagged_attr() helper macro

[Gustavo A. R. Silva]

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@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?
> 

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

--
Gustavo