linux-kernel - [PATCH] ext4: namei: Replace zero-length array with flexible-array member

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Date:   Thu, 13 Feb 2020 10:06:48 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] ext4: namei: 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/ext4/namei.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 129d2ebae00d..77c327d904bf 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -233,13 +233,13 @@ struct dx_root
 		u8 unused_flags;
 	}
 	info;
-	struct dx_entry	entries[0];
+	struct dx_entry	entries[];
 };
 
 struct dx_node
 {
 	struct fake_dirent fake;
-	struct dx_entry	entries[0];
+	struct dx_entry	entries[];
 };
 
 
-- 
2.25.0