[PATCH] drm/i915/gt: Replace zero-length array with flexible-array

Gustavo A. R. Silva
2020-05-07T18:49:43Z

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>
---
drivers/gpu/drm/i915/display/intel_vbt_defs.h | 4 ++--
drivers/gpu/drm/i915/gt/intel_lrc.c | 2 +-
drivers/gpu/drm/i915/i915_gpu_error.h | 2 +-
3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_vbt_defs.h b/drivers/gpu/drm/i915/display/intel_vbt_defs.h
index 05c7cbe32eb4..aef7fe932d1a 100644
--- 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 683014e7bc51..f42c99da2580 100644
--- a/drivers/gpu/drm/i915/gt/intel_lrc.c
+++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
@@ -216,7 +216,7 @@ 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 0d1f6c8ff355..5a6561f7a210 100644
--- 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 {