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

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

[Gustavo A. R. Silva]

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@embeddedor.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

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

[Jessica Yu]

+++ 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.
>
>[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@embeddedor.com>

Applied, thanks Gustavo!

Jessica

>---
> 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
>