Re: [PATCH] overflow: Support sub-union members in struct_size()

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

On 7/28/26 05:49, Bence Csokas wrote:
> Original definition of struct_size() assumed that the total size of an
> object with a flex-array member can be calculated simply by adding the
> flex-array size to what sizeof() returns. This is however not the case
> if the struct has a union inside it, and `member` refers to a field
> inside that sub-union:
> 
>    struct my_inner {
>      int a;
>      char b[];
>    };
> 
>    struct my_outer {
>      char c;
>      union {
>        struct my_inner inner;
>        long l;
>      };
>    };
> 
>    /* This will give the wrong result */
>    struct_size_t(struct my_outer, inner.b, n_elems);
> 
> Replacing sizeof() by the offsetof() of the member fixes this.
> 
> Existing users specifying non-union members should not be affected.

This actually affects users of flexible structs that have tail padding[1].

-Gustavo

[1] https://godbolt.org/z/KPq59EYej

> 
> Cc: Mark Rutland <mark.rutland@arm.com>
> 
> Signed-off-by: Bence Csokas <bence.csokas@arm.com>
> ---
>   include/linux/overflow.h | 4 ++--
>   include/linux/stddef.h   | 8 ++++++++
>   2 files changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/overflow.h b/include/linux/overflow.h
> index a8cb6319b4fb..a11c1135f617 100644
> --- a/include/linux/overflow.h
> +++ b/include/linux/overflow.h
> @@ -433,8 +433,8 @@ static __always_inline size_t __must_check size_sub(size_t minuend, size_t subtr
>    */
>   #define struct_size(p, member, count)					\
>   	__builtin_choose_expr(__is_constexpr(count),			\
> -		sizeof(*(p)) + flex_array_size(p, member, count),	\
> -		size_add(sizeof(*(p)), flex_array_size(p, member, count)))
> +		offsetofp((p), member) + flex_array_size(p, member, count),	\
> +		size_add(offsetofp((p), member), flex_array_size(p, member, count)))
>   
>   /**
>    * struct_size_t() - Calculate size of structure with trailing flexible array
> diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> index e1851c50c89b..b8f0dbd2de55 100644
> --- a/include/linux/stddef.h
> +++ b/include/linux/stddef.h
> @@ -32,6 +32,14 @@ enum {
>   #define offsetofend(TYPE, MEMBER) \
>   	(offsetof(TYPE, MEMBER)	+ sizeof_field(TYPE, MEMBER))
>   
> +/**
> + * offsetofp() - Get the offset of a struct field from a pointer to the struct
> + *
> + * @P: Pointer to the structure
> + * @MEMBER: The member within the structure to get the end offset of
> + */
> +#define offsetofp(P, MEMBER) offsetof(typeof(*(P)), MEMBER)
> +
>   /**
>    * struct_group() - Wrap a set of declarations in a mirrored struct
>    *
> 
> base-commit: 62cc90241548d5570ee68e01aaba6506964e9811