Commit f1e79c7e authored May 07, 2020 by Gustavo A. R. Silva Committed by Chris Wilson May 09, 2020

drm/i915: Replace zero-length array with flexible-array

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]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

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 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200507185408.GA14561@embeddedor

parent 16dc224f

drivers/gpu/drm/i915/display/intel_vbt_defs.h

+2 −2

Original line number	Diff line number	Diff line
		@@ -462,7 +462,7 @@ struct bdb_general_definitions {
		* number = (block_size - sizeof(bdb_general_definitions))/
		* defs->child_dev_size;
		*/
		u8 devices[0];
		u8 devices[];
		} __packed;

		/*
		@@ -839,7 +839,7 @@ struct bdb_mipi_config {

		struct bdb_mipi_sequence {
		u8 version;
		u8 data[0]; /* up to 6 variable length blocks */
		u8 data[]; /* up to 6 variable length blocks */
		} __packed;

		/*

drivers/gpu/drm/i915/gt/intel_lrc.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -217,7 +217,7 @@ struct virtual_engine {

		/* And finally, which physical engines this virtual engine maps onto. */
		unsigned int num_siblings;
		struct intel_engine_cs *siblings[0];
		struct intel_engine_cs *siblings[];
		};

		static struct virtual_engine to_virtual_engine(struct intel_engine_cs engine)

drivers/gpu/drm/i915/i915_gpu_error.h

+1 −1

Original line number	Diff line number	Diff line
		@@ -42,7 +42,7 @@ struct i915_vma_coredump {
		int num_pages;
		int page_count;
		int unused;
		u32 *pages[0];
		u32 *pages[];
		};

		struct i915_request_coredump {

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