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[next] atmel: at76c50x-usb.h: Replace zero-length array with flexible-array member

Message ID 20200319225133.GA29672@embeddedor.com (mailing list archive)
State Accepted
Commit b0c40588161dafdafdbac537b4441be02c15aff2
Delegated to: Kalle Valo
Headers show
Series [next] atmel: at76c50x-usb.h: Replace zero-length array with flexible-array member | expand

Commit Message

Gustavo A. R. Silva March 19, 2020, 10:51 p.m. UTC
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/net/wireless/atmel/at76c50x-usb.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Kalle Valo March 23, 2020, 5:17 p.m. UTC | #1
"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")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

Patch applied to wireless-drivers-next.git, thanks.

b0c40588161d atmel: at76c50x: Replace zero-length array with flexible-array member
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Patch

diff --git a/drivers/net/wireless/atmel/at76c50x-usb.h b/drivers/net/wireless/atmel/at76c50x-usb.h
index f56863403b05..746e64dfd8aa 100644
--- a/drivers/net/wireless/atmel/at76c50x-usb.h
+++ b/drivers/net/wireless/atmel/at76c50x-usb.h
@@ -151,7 +151,7 @@  struct at76_command {
 	u8 cmd;
 	u8 reserved;
 	__le16 size;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /* Length of Atmel-specific Rx header before 802.11 frame */