linux-kernel - [PATCH][next] dlm: dlm_internal: Replace zero-length array with flexible-array member

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Date:   Mon, 9 Mar 2020 10:56:08 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Christine Caulfield <ccaulfie@...hat.com>,
        David Teigland <teigland@...hat.com>
Cc:     cluster-devel@...hat.com, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] dlm: dlm_internal: 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>
---
 fs/dlm/dlm_internal.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/dlm/dlm_internal.h b/fs/dlm/dlm_internal.h
index 416d9de35679..d231ae5d2c65 100644
--- a/fs/dlm/dlm_internal.h
+++ b/fs/dlm/dlm_internal.h
@@ -421,7 +421,7 @@ struct dlm_message {
 	int			m_bastmode;
 	int			m_asts;
 	int			m_result;	/* 0 or -EXXX */
-	char			m_extra[0];	/* name or lvb */
+	char			m_extra[];	/* name or lvb */
 };
 
 
@@ -450,7 +450,7 @@ struct dlm_rcom {
 	uint64_t		rc_id;		/* match reply with request */
 	uint64_t		rc_seq;		/* sender's ls_recover_seq */
 	uint64_t		rc_seq_reply;	/* remote ls_recover_seq */
-	char			rc_buf[0];
+	char			rc_buf[];
 };
 
 union dlm_packet {
@@ -506,7 +506,7 @@ struct rcom_lock {
 	__le16			rl_wait_type;
 	__le16			rl_namelen;
 	char			rl_name[DLM_RESNAME_MAXLEN];
-	char			rl_lvb[0];
+	char			rl_lvb[];
 };
 
 /*
-- 
2.25.0