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 76497732932f ("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

diff --git a/drivers/gpu/drm/i915/display/intel_vbt_defs.h b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
index 05c7cbe..aef7fe9 100644 (file)
--- a/drivers/gpu/drm/i915/display/intel_vbt_defs.h
+++ b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
@@ -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;
 
 /*

diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
index 28b477f..8e254f6 100644 (file)
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -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)

diff --git a/drivers/gpu/drm/i915/i915_gpu_error.h b/drivers/gpu/drm/i915/i915_gpu_error.h
index fa2d82a..76b80fb 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gpu_error.h
+++ b/drivers/gpu/drm/i915/i915_gpu_error.h
@@ -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 {