linux-kernel - [PATCH] ima: Replace zero-length array with flexible-array

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Message-ID: <20200507192159.GA16250@embeddedor>
Date:   Thu, 7 May 2020 14:21:59 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>
Cc:     linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] ima: Replace zero-length array with flexible-array

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 <gustavoars@...nel.org>
---
 security/integrity/integrity.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index 298b73794d8b..16c1894c29bb 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -107,7 +107,7 @@ struct ima_digest_data {
 		} ng;
 		u8 data[2];
 	} xattr;
-	u8 digest[0];
+	u8 digest[];
 } __packed;

 /*
@@ -119,7 +119,7 @@ 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 */