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Message-ID: <b3956fd36307d33ebbf0e4633e0d2389860cf720.camel@linux.ibm.com>
Subject: Re: [PATCH v5][next] integrity: Avoid
 -Wflex-array-member-not-at-end warnings
From: Mimi Zohar <zohar@linux.ibm.com>
To: "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Roberto Sassu
 <roberto.sassu@huawei.com>,
        Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
        Eric Snowberg <eric.snowberg@oracle.com>,
        Paul Moore <paul@paul-moore.com>, James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-integrity@vger.kernel.org, linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org
Date: Thu, 04 Apr 2024 16:11:54 -0400
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On Thu, 2024-04-04 at 12:49 -0400, Mimi Zohar wrote:
> Hi Gustavo,
> 
> On Thu, 2024-04-04 at 09:00 -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 a struct, we use the `__struct_group()` helper to separate
> > the flexible array from the rest of the members in the flexible
> > structure:
> > 
> > struct ima_digest_data {
> >         __struct_group(ima_digest_data_hdr, hdr, __packed,
> > 
> >         ... the rest of the members
> > 
> >         );
> >         u8 digest[];
> > } __packed;
> > 
> > And similarly for `struct evm_ima_xattr_data`.
> > 
> > With the change described above, we can now declare an object of the
> > type of the tagged `struct ima_digest_data_hdr`, 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;
> > 
> > And similarly for `struct evm_digest` and `struct evm_xattr`.
> > 
> > We also use `container_of()` whenever we need to retrieve a pointer to
> > the flexible structure.
> > 
> > So, with these changes, fix the following warnings:
> > 
> > security/integrity/evm/evm.h:64:32: warning: structure containing a flexible
> > array member is not at the end of another structure [-Wflex-array-member-
> > not-
> > at-end]
> > security/integrity/evm/../integrity.h:40:35: warning: structure containing a
> > flexible array member is not at the end of another structure [-Wflex-array-
> > member-not-at-end]
> > security/integrity/evm/../integrity.h:68:32: warning: structure containing a
> > flexible array member is not at the end of another structure [-Wflex-array-
> > member-not-at-end]
> > security/integrity/ima/../integrity.h:40:35: warning: structure containing a
> > flexible array member is not at the end of another structure [-Wflex-array-
> > member-not-at-end]
> > security/integrity/ima/../integrity.h:68:32: warning: structure containing a
> > flexible array member is not at the end of another structure [-Wflex-array-
> > member-not-at-end]
> > security/integrity/integrity.h:40:35: warning: structure containing a
> > flexible
> > array member is not at the end of another structure [-Wflex-array-member-
> > not-
> > at-end]
> > security/integrity/integrity.h:68:32: warning: structure containing a
> > flexible
> > array member is not at the end of another structure [-Wflex-array-member-
> > not-
> > at-end]
> > security/integrity/platform_certs/../integrity.h:40:35: warning: structure
> > containing a flexible array member is not at the end of another structure [-
> > Wflex-array-member-not-at-end]
> > security/integrity/platform_certs/../integrity.h:68:32: warning: structure
> > containing a flexible array member is not at the end of another structure [-
> > Wflex-array-member-not-at-end]
> > 
> > Link: https://github.com/KSPP/linux/issues/202
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> 
> Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
> 
> Thanks, Gustavo.  I already applied and tested v4, but was trying to actually
> see the errors with before pushing it out.

With GCC-14, I'm seeing the "warning: structure containing a flexible array
member is not at the end of another structure [-Wflex-array-member-not-at-end]"
messages.  As expected, with the patch no warnings.

"checkpatch.pl --strict" complains "CHECK: Alignment should match open
parenthesis".  I'll queue the patch, but how about teaching checkpatch.pl to
ignore __struct_group()?

thanks,

Mimi