On Thu, 05 Mar 2020, "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

Reviewed-by: Jani Nikula jani.nikula@intel.com

---

include/drm/drm_displayid.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/drm/drm_displayid.h b/include/drm/drm_displayid.h index 9d3b745c3107..94b4390bf990 100644 --- a/include/drm/drm_displayid.h +++ b/include/drm/drm_displayid.h @@ -89,7 +89,7 @@ struct displayid_detailed_timings_1 {

struct displayid_detailed_timing_block { struct displayid_block base;

• struct displayid_detailed_timings_1 timings[0];

• struct displayid_detailed_timings_1 timings[];

};

#define for_each_displayid_db(displayid, block, idx, length) \