linux-kernel - [PATCH][next] ext2: xattr.h: Replace zero-length array with flexible-array member

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Date:   Mon, 9 Mar 2020 13:04:41 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Jan Kara <jack@...e.com>
Cc:     linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH][next] ext2: xattr.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/ext2/xattr.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext2/xattr.h b/fs/ext2/xattr.h
index cee888cdc235..16272e6ddcf4 100644
--- a/fs/ext2/xattr.h
+++ b/fs/ext2/xattr.h
@@ -39,7 +39,7 @@ struct ext2_xattr_entry {
 	__le32	e_value_block;	/* disk block attribute is stored on (n/i) */
 	__le32	e_value_size;	/* size of attribute value */
 	__le32	e_hash;		/* hash value of name and value */
-	char	e_name[0];	/* attribute name */
+	char	e_name[];	/* attribute name */
 };
 
 #define EXT2_XATTR_PAD_BITS		2
-- 
2.25.0