linux-kernel - [PATCH] crypto: img-hash - Replace zero-length array with flexible-array member

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Message-ID: <20200213165054.GA11109@embeddedor>
Date:   Thu, 13 Feb 2020 10:50:54 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] crypto: img-hash - 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>
---
 drivers/crypto/img-hash.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/crypto/img-hash.c b/drivers/crypto/img-hash.c
index 25d5227f74a1..0e25fc3087f3 100644
--- a/drivers/crypto/img-hash.c
+++ b/drivers/crypto/img-hash.c
@@ -103,7 +103,7 @@ struct img_hash_request_ctx {
 	struct ahash_request	fallback_req;
 
 	/* Zero length buffer must remain last member of struct */
-	u8 buffer[0] __aligned(sizeof(u32));
+	u8 buffer[] __aligned(sizeof(u32));
 };
 
 struct img_hash_ctx {
-- 
2.25.0