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* [Intel-gfx] [PATCH][next] drm: i915_drm.h: Replace zero-length array with flexible-array member
@ 2020-03-03 22:06 Gustavo A. R. Silva
  2020-03-04  3:38 ` [Intel-gfx] ✗ Fi.CI.DOCS: warning for " Patchwork
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-03 22:06 UTC (permalink / raw)
  To: Jani Nikula, Joonas Lahtinen, Rodrigo Vivi, David Airlie,
	Daniel Vetter
  Cc: intel-gfx, linux-kernel, dri-devel, Gustavo A. R. Silva

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(-)

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 { \
-- 
2.25.0

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Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
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2020-03-03 22:06 [Intel-gfx] [PATCH][next] drm: i915_drm.h: Replace zero-length array with flexible-array member Gustavo A. R. Silva
2020-03-04  3:38 ` [Intel-gfx] ✗ Fi.CI.DOCS: warning for " Patchwork
2020-03-04  3:47 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork
2020-03-04  9:27 ` [Intel-gfx] [PATCH][next] " Jani Nikula
2020-03-04 20:14 ` [Intel-gfx] ✓ Fi.CI.IGT: success for " Patchwork

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