[PATCH][next] integrity: Use static_assert() to check struct sizes

[Roberto Sassu]

On Thu, 2024-08-08 at 16:04 -0600, Gustavo A. R. Silva wrote:
> Commit 38aa3f5ac6d2 ("integrity: Avoid -Wflex-array-member-not-at-end
> warnings") introduced tagged `struct evm_ima_xattr_data_hdr` and
> `struct ima_digest_data_hdr`. We want to ensure that when new members
> need to be added to the flexible structures, they are always included
> within these tagged structs.
> 
> So, we use `static_assert()` to ensure that the memory layout for
> both the flexible structure and the tagged struct is the same after
> any changes.

Looks good to me.

Tested-by: Roberto Sassu <roberto.sassu at huawei.com>
Reviewed-by: Roberto Sassu <roberto.sassu at huawei.com>

Thanks

Roberto

> Signed-off-by: Gustavo A. R. Silva <gustavoars at kernel.org>
> ---
>  security/integrity/integrity.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
> index 660f76cb69d3..c2c2da691123 100644
> --- a/security/integrity/integrity.h
> +++ b/security/integrity/integrity.h
> @@ -37,6 +37,8 @@ struct evm_ima_xattr_data {
>  	);
>  	u8 data[];
>  } __packed;
> +static_assert(offsetof(struct evm_ima_xattr_data, data) == sizeof(struct evm_ima_xattr_data_hdr),
> +	      "struct member likely outside of __struct_group()");
>  
>  /* Only used in the EVM HMAC code. */
>  struct evm_xattr {
> @@ -65,6 +67,8 @@ struct ima_digest_data {
>  	);
>  	u8 digest[];
>  } __packed;
> +static_assert(offsetof(struct ima_digest_data, digest) == sizeof(struct ima_digest_data_hdr),
> +	      "struct member likely outside of __struct_group()");
>  
>  /*
>   * Instead of wrapping the ima_digest_data struct inside a local structure