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 <[email protected]>
---
include/drm/bridge/mhl.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/include/drm/bridge/mhl.h b/include/drm/bridge/mhl.h
index 1cc77bf38324..d96626a0e3fa 100644
--- a/include/drm/bridge/mhl.h
+++ b/include/drm/bridge/mhl.h
@@ -327,13 +327,13 @@ struct mhl_burst_bits_per_pixel_fmt {
struct {
u8 stream_id;
u8 pixel_format;
- } __packed desc[0];
+ } __packed desc[];
} __packed;
struct mhl_burst_emsc_support {
struct mhl3_burst_header hdr;
u8 num_entries;
- __be16 burst_id[0];
+ __be16 burst_id[];
} __packed;
struct mhl_burst_audio_descr {
--
2.25.0
On Thu, Mar 05, 2020 at 05:00:11AM -0600, Gustavo A. R. Silva 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 <[email protected]>
Applied to drm-misc-next, thanks for your patch.
-Daniel
> ---
> include/drm/bridge/mhl.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/drm/bridge/mhl.h b/include/drm/bridge/mhl.h
> index 1cc77bf38324..d96626a0e3fa 100644
> --- a/include/drm/bridge/mhl.h
> +++ b/include/drm/bridge/mhl.h
> @@ -327,13 +327,13 @@ struct mhl_burst_bits_per_pixel_fmt {
> struct {
> u8 stream_id;
> u8 pixel_format;
> - } __packed desc[0];
> + } __packed desc[];
> } __packed;
>
> struct mhl_burst_emsc_support {
> struct mhl3_burst_header hdr;
> u8 num_entries;
> - __be16 burst_id[0];
> + __be16 burst_id[];
> } __packed;
>
> struct mhl_burst_audio_descr {
> --
> 2.25.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch