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

From: Jessica Yu
Date: Mon Feb 17 2020 - 11:36:18 EST

+++ Gustavo A. R. Silva [13/02/20 09:14 -0600]:
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.

[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@xxxxxxxxxxxxxx>

Applied, thanks Gustavo!


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,