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

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Message-ID: <20200213160244.GA6088@embeddedor>
Date:   Thu, 13 Feb 2020 10:02:44 -0600
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] ocfs2: 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/journal.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ocfs2/journal.c b/fs/ocfs2/journal.c
index 68ba354cf361..b425f0b01dce 100644
--- a/fs/ocfs2/journal.c
+++ b/fs/ocfs2/journal.c
@@ -91,7 +91,7 @@ enum ocfs2_replay_state {
 struct ocfs2_replay_map {
 	unsigned int rm_slots;
 	enum ocfs2_replay_state rm_state;
-	unsigned char rm_replay_slots[0];
+	unsigned char rm_replay_slots[];
 };
 
 static void ocfs2_replay_map_set_state(struct ocfs2_super *osb, int state)
-- 
2.25.0