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

[PATCH] zorro: 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>
---
 drivers/zorro/zorro.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/zorro/zorro.c b/drivers/zorro/zorro.c
index 8eeb84c239db..47c733817903 100644
--- a/drivers/zorro/zorro.c
+++ b/drivers/zorro/zorro.c
@@ -41,7 +41,7 @@ struct zorro_dev *zorro_autocon;
 
 struct zorro_bus {
 	struct device dev;
-	struct zorro_dev devices[0];
+	struct zorro_dev devices[];
 };
 
 
-- 
2.25.0

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

[Geert Uytterhoeven]

Hi Gustavo,

Thanks for your patch!

On Thu, Feb 27, 2020 at 7:35 PM Gustavo A. R. Silva
<gustavo@embeddedor.com> wrote:
> 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")

Note that in practice, this "undefined behavior" may lead to corrupting
the next member, when the zero-length array is written to.

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

Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
i.e. will queue in the m68k for-v5.7 branch.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds