linux-kernel - [PATCH] security: integrity: Replace zero-length array with flexible-array member

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Message-ID: <20200227125841.GA23703@embeddedor>
Date:   Thu, 27 Feb 2020 06:58:41 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Mimi Zohar <zohar@...ux.ibm.com>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.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,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] security: integrity: Replace zero-length array with
 flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 security/integrity/ima/ima.h   | 2 +-
 security/integrity/integrity.h | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index 64317d95363e..da4246ee7e35 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -95,7 +95,7 @@ struct ima_template_entry {
 	u8 digest[TPM_DIGEST_SIZE];	/* sha1 or md5 measurement hash */
 	struct ima_template_desc *template_desc; /* template descriptor */
 	u32 template_data_len;
-	struct ima_field_data template_data[0];	/* template related data */
+	struct ima_field_data template_data[];	/* template related data */
 };
 
 struct ima_queue_entry {
diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index 543d277c7e48..3c7e8b902256 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -103,7 +103,7 @@ struct ima_digest_data {
 		} ng;
 		u8 data[2];
 	} xattr;
-	u8 digest[0];
+	u8 digest[];
 } __packed;
 
 /*
@@ -115,7 +115,7 @@ struct signature_v2_hdr {
 	uint8_t	hash_algo;	/* Digest algorithm [enum hash_algo] */
 	__be32 keyid;		/* IMA key identifier - not X509/PGP specific */
 	__be16 sig_size;	/* signature size */
-	uint8_t sig[0];		/* signature payload */
+	uint8_t sig[];		/* signature payload */
 } __packed;
 
 /* integrity data associated with an inode */
-- 
2.25.0