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://urldefense.com/v3/__https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html__;!!GqivPVa7Brio!I_pkpK-fMbcZGGP9iPUPG8LvqoSenH6iuL5O5nTqw9zX-e4A8Mjur9Xx4xl8U41yd3A-vA$
[2] https://urldefense.com/v3/__https://github.com/KSPP/linux/issues/21__;!!GqivPVa7Brio!I_pkpK-fMbcZGGP9iPUPG8LvqoSenH6iuL5O5nTqw9zX-e4A8Mjur9Xx4xl8U42h1JbMjw$
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 fs/ocfs2/dlm/dlmcommon.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

fs/ocfs2/dlm/dlmcommon.h +4 -4

index 0463dce65bb2..c8a444622faa 100644
@@ -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)