linux-hardening - Re: [PATCH][next] integrity: Avoid -Wflex-array-member-not-at-end warnings

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Date: Wed, 20 Mar 2024 21:19:36 -0400
From: Mimi Zohar <zohar@...ux.ibm.com>
To: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Roberto Sassu
 <roberto.sassu@...wei.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Eric Snowberg <eric.snowberg@...cle.com>,
        Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Cc: linux-integrity@...r.kernel.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        Kees Cook
	 <keescook@...omium.org>
Subject: Re: [PATCH][next] integrity: Avoid -Wflex-array-member-not-at-end
 warnings

Hi Gustavo,

Sorry for the delay...

On Mon, 2024-03-04 at 11:52 -0600, Gustavo A. R. Silva wrote:
> -Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
> ready to enable it globally.
> 
> There is currently an object (`hdr)` in `struct ima_max_digest_data`
> that contains a flexible structure (`struct ima_digest_data`):
> 
>  struct ima_max_digest_data {
> 	struct ima_digest_data hdr;
>         u8 digest[HASH_MAX_DIGESTSIZE];
>  } __packed;
> 
> So, in order to avoid ending up with a flexible-array member in the
> middle of another struct, we use the `struct_group_tagged()` helper to
> separate the flexible array from the rest of the members in the flexible
> structure:
> 
> struct ima_digest_data {
>         struct_group_tagged(ima_digest_data_hdr, hdr,
> 
> 	... the rest of the members
> 
>         );
>         u8 digest[];
> } __packed;
> 
> With the change described above, we can now declare an object of the
> type of the tagged struct, without embedding the flexible array in the
> middle of another struct:
> 
>  struct ima_max_digest_data {
>         struct ima_digest_data_hdr hdr;
>         u8 digest[HASH_MAX_DIGESTSIZE];
>  } __packed;
> 
> We also use `container_of()` whenever we need to retrieve a pointer to
> the flexible structure.

Nice!

> 
> So, with these changes, fix the following warnings:
> 
> security/integrity/evm/evm.h:45:32: warning: structure containing a flexible
> array member is not at the end of another structure [-Wflex-array-member-not-
> at-end]
> security/integrity/evm/evm.h:45:32: warning: structure containing a flexible
> array member is not at the end of another structure [-Wflex-array-member-not-
> at-end]
> security/integrity/evm/evm.h:45:32: warning: structure containing a flexible
> array member is not at the end of another structure [-Wflex-array-member-not-
> at-end]

A similar change would need to be made to struct evm_digest:

struct evm_digest {
        struct ima_digest_data hdr;
        char digest[IMA_MAX_DIGEST_SIZE];
} __packed;

Is there are another patch?

Mimi

> 
> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
>