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

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Date:   Thu, 7 May 2020 13:51:45 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Ayush Sawal <ayush.sawal@...lsio.com>,
        Vinay Kumar Yadav <vinay.yadav@...lsio.com>,
        Rohit Maheshwari <rohitm@...lsio.com>
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] crypto: 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>
---
 drivers/crypto/chelsio/chcr_crypto.h |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/crypto/chelsio/chcr_crypto.h b/drivers/crypto/chelsio/chcr_crypto.h
index 542bebae001f..1cc522a07aac 100644
--- a/drivers/crypto/chelsio/chcr_crypto.h
+++ b/drivers/crypto/chelsio/chcr_crypto.h
@@ -223,7 +223,7 @@ struct chcr_authenc_ctx {

 struct __aead_ctx {
 	struct chcr_gcm_ctx gcm[0];
-	struct chcr_authenc_ctx authenc[0];
+	struct chcr_authenc_ctx authenc[];
 };

 struct chcr_aead_ctx {
@@ -235,7 +235,7 @@ struct chcr_aead_ctx {
 	u8 nonce[4];
 	u16 hmac_ctrl;
 	u16 mayverify;
-	struct	__aead_ctx ctx[0];
+	struct	__aead_ctx ctx[];
 };

 struct hmac_ctx {
@@ -247,7 +247,7 @@ struct hmac_ctx {
 struct __crypto_ctx {
 	struct hmac_ctx hmacctx[0];
 	struct ablk_ctx ablkctx[0];
-	struct chcr_aead_ctx aeadctx[0];
+	struct chcr_aead_ctx aeadctx[];
 };

 struct chcr_context {
@@ -257,7 +257,7 @@ struct chcr_context {
 	unsigned int  ntxq;
 	unsigned int  nrxq;
 	struct completion cbc_aes_aio_done;
-	struct __crypto_ctx crypto_ctx[0];
+	struct __crypto_ctx crypto_ctx[];
 };

 struct chcr_hctx_per_wr {