| Message ID | 20200305110105.GA21188@embeddedor (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [next] drm/drm_displayid.h: Replace zero-length array with flexible-array member | expand |
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) \
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) \
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> --- include/drm/drm_displayid.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)