On Tue, 03 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:

My earlier question about making the change in uapi was really about potentially bumping up userspace compiler requirements, though I did not actually say so. :)

I guess effectively uapi already requires C99 to build? And we (i915_drm.h) have both [0] and []. So go for it.

What's your baseline? I think you've missed one instance of struct i915_engine_class_instance engines[0];

BR, Jani.

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/uapi/drm/i915_drm.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h index 2813e579b480..413d923b332a 100644 --- a/include/uapi/drm/i915_drm.h +++ b/include/uapi/drm/i915_drm.h @@ -1729,7 +1729,7 @@ struct i915_context_engines_load_balance {

__u64 mbz64; /* reserved for future use; must be zero */

• struct i915_engine_class_instance engines[0];

• struct i915_engine_class_instance engines[];

} __attribute__((packed));

#define I915_DEFINE_CONTEXT_ENGINES_LOAD_BALANCE(name__, N__) struct { \ @@ -1767,7 +1767,7 @@ struct i915_context_engines_bond { __u64 flags; /* all undefined flags must be zero */ __u64 mbz64[4]; /* reserved for future use; must be zero */

• struct i915_engine_class_instance engines[0];

• struct i915_engine_class_instance engines[];

} __attribute__((packed));

#define I915_DEFINE_CONTEXT_ENGINES_BOND(name__, N__) struct { \