| Message ID | 20200303220636.GA2788@embeddedor (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [next] drm: i915_drm.h: Replace zero-length array with flexible-array member | expand |
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 { \
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 { \
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/uapi/drm/i915_drm.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)