linux-kernel - [PATCH][next] ocfs2: dlm: Replace zero-length array with flexible-array member

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Date:   Mon, 9 Mar 2020 15:20:16 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>
Cc:     ocfs2-devel@....oracle.com, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] ocfs2: dlm: 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/ocfs2/dlm/dlmcommon.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/ocfs2/dlm/dlmcommon.h b/fs/ocfs2/dlm/dlmcommon.h
index 0463dce65bb2..c8a444622faa 100644
--- a/fs/ocfs2/dlm/dlmcommon.h
+++ b/fs/ocfs2/dlm/dlmcommon.h
@@ -564,7 +564,7 @@ struct dlm_migratable_lockres
 	// 48 bytes
 	u8 lvb[DLM_LVB_LEN];
 	// 112 bytes
-	struct dlm_migratable_lock ml[0];  // 16 bytes each, begins at byte 112
+	struct dlm_migratable_lock ml[];  // 16 bytes each, begins at byte 112
 };
 #define DLM_MIG_LOCKRES_MAX_LEN  \
 	(sizeof(struct dlm_migratable_lockres) + \
@@ -601,7 +601,7 @@ struct dlm_convert_lock
 
 	u8 name[O2NM_MAX_NAME_LEN];
 
-	s8 lvb[0];
+	s8 lvb[];
 };
 #define DLM_CONVERT_LOCK_MAX_LEN  (sizeof(struct dlm_convert_lock)+DLM_LVB_LEN)
 
@@ -616,7 +616,7 @@ struct dlm_unlock_lock
 
 	u8 name[O2NM_MAX_NAME_LEN];
 
-	s8 lvb[0];
+	s8 lvb[];
 };
 #define DLM_UNLOCK_LOCK_MAX_LEN  (sizeof(struct dlm_unlock_lock)+DLM_LVB_LEN)
 
@@ -632,7 +632,7 @@ struct dlm_proxy_ast
 
 	u8 name[O2NM_MAX_NAME_LEN];
 
-	s8 lvb[0];
+	s8 lvb[];
 };
 #define DLM_PROXY_AST_MAX_LEN  (sizeof(struct dlm_proxy_ast)+DLM_LVB_LEN)
 
-- 
2.25.0