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

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Message-ID: <fe36683b-ec78-4f83-b4ad-8dca35e4fa09@embeddedor.com>
Date: Wed, 20 Mar 2024 21:39:32 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Mimi Zohar <zohar@...ux.ibm.com>,
 "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



On 20/03/24 19:19, Mimi Zohar wrote:
> Hi Gustavo,
> 
> Sorry for the delay...

No worries. :)

> 
> 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?

Oh, I missed that one. I'll include it and send v2, shortly.

Thanks!
--
Gustavo