linux-kernel - [PATCH][next] hfs: btree.h: Replace zero-length array with flexible-array member

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Message-ID: <20200309181218.GA3726@embeddedor>
Date:   Mon, 9 Mar 2020 13:12:18 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] hfs: btree.h: 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/hfs/btree.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/hfs/btree.h b/fs/hfs/btree.h
index dcc2aab1b2c4..4ba45caf5939 100644
--- a/fs/hfs/btree.h
+++ b/fs/hfs/btree.h
@@ -60,7 +60,7 @@ struct hfs_bnode {
 	wait_queue_head_t lock_wq;
 	atomic_t refcnt;
 	unsigned int page_offset;
-	struct page *page[0];
+	struct page *page[];
 };

 #define HFS_BNODE_ERROR		0
-- 
2.25.0