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

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Message-ID: <20200213151409.GA30541@embeddedor>
Date:   Thu, 13 Feb 2020 09:14:09 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] kernel: module: 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>
---
 kernel/module.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 33569a01d6e1..b88ec9cd2a7f 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1515,7 +1515,7 @@ struct module_sect_attr {
 struct module_sect_attrs {
 	struct attribute_group grp;
 	unsigned int nsections;
-	struct module_sect_attr attrs[0];
+	struct module_sect_attr attrs[];
 };
 
 static ssize_t module_sect_show(struct module_attribute *mattr,
@@ -1608,7 +1608,7 @@ static void remove_sect_attrs(struct module *mod)
 struct module_notes_attrs {
 	struct kobject *dir;
 	unsigned int notes;
-	struct bin_attribute attrs[0];
+	struct bin_attribute attrs[];
 };
 
 static ssize_t module_notes_read(struct file *filp, struct kobject *kobj,
-- 
2.25.0