[PATCH for-next 08/25] drm/mga/mga_ioc32: Use struct_group() for memcpy() region

[Kees Cook <keescook@chromium.org>]

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memcpy(), memmove(), and memset(), avoid
intentionally writing across neighboring fields.

Use struct_group() in struct drm32_mga_init around members chipset, sgram,
maccess, fb_cpp, front_offset, front_pitch, back_offset, back_pitch,
depth_cpp, depth_offset, depth_pitch, texture_offset, and texture_size,
so they can be referenced together. This will allow memcpy() and sizeof()
to more easily reason about sizes, improve readability, and avoid future
warnings about writing beyond the end of chipset.

"pahole" shows no size nor member offset changes to struct drm32_mga_init.
"objdump -d" shows no meaningful object code changes (i.e. only source
line number induced differences and optimizations).

Note that since this is a UAPI header, __struct_group() is used
directly.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Daniel Vetter <daniel@ffwll.ch>
Link: https://lore.kernel.org/lkml/YQKa76A6XuFqgM03@phenom.ffwll.local
---
 drivers/gpu/drm/mga/mga_ioc32.c | 27 ++++++++++++++-------------
 include/uapi/drm/mga_drm.h      | 22 ++++++++++++----------
 2 files changed, 26 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/mga/mga_ioc32.c b/drivers/gpu/drm/mga/mga_ioc32.c
index 4fd4de16cd32..894472921c30 100644
--- a/drivers/gpu/drm/mga/mga_ioc32.c
+++ b/drivers/gpu/drm/mga/mga_ioc32.c
@@ -38,16 +38,18 @@
 typedef struct drm32_mga_init {
 	int func;
 	u32 sarea_priv_offset;
-	int chipset;
-	int sgram;
-	unsigned int maccess;
-	unsigned int fb_cpp;
-	unsigned int front_offset, front_pitch;
-	unsigned int back_offset, back_pitch;
-	unsigned int depth_cpp;
-	unsigned int depth_offset, depth_pitch;
-	unsigned int texture_offset[MGA_NR_TEX_HEAPS];
-	unsigned int texture_size[MGA_NR_TEX_HEAPS];
+	struct_group(always32bit,
+		int chipset;
+		int sgram;
+		unsigned int maccess;
+		unsigned int fb_cpp;
+		unsigned int front_offset, front_pitch;
+		unsigned int back_offset, back_pitch;
+		unsigned int depth_cpp;
+		unsigned int depth_offset, depth_pitch;
+		unsigned int texture_offset[MGA_NR_TEX_HEAPS];
+		unsigned int texture_size[MGA_NR_TEX_HEAPS];
+	);
 	u32 fb_offset;
 	u32 mmio_offset;
 	u32 status_offset;
@@ -67,9 +69,8 @@ static int compat_mga_init(struct file *file, unsigned int cmd,
 
 	init.func = init32.func;
 	init.sarea_priv_offset = init32.sarea_priv_offset;
-	memcpy(&init.chipset, &init32.chipset,
-		offsetof(drm_mga_init_t, fb_offset) -
-		offsetof(drm_mga_init_t, chipset));
+	memcpy(&init.always32bit, &init32.always32bit,
+	       sizeof(init32.always32bit));
 	init.fb_offset = init32.fb_offset;
 	init.mmio_offset = init32.mmio_offset;
 	init.status_offset = init32.status_offset;
diff --git a/include/uapi/drm/mga_drm.h b/include/uapi/drm/mga_drm.h
index 8c4337548ab5..bb31567e66c0 100644
--- a/include/uapi/drm/mga_drm.h
+++ b/include/uapi/drm/mga_drm.h
@@ -279,20 +279,22 @@ typedef struct drm_mga_init {
 
 	unsigned long sarea_priv_offset;
 
-	int chipset;
-	int sgram;
+	__struct_group(/* no tag */, always32bit, /* no attrs */,
+		int chipset;
+		int sgram;
 
-	unsigned int maccess;
+		unsigned int maccess;
 
-	unsigned int fb_cpp;
-	unsigned int front_offset, front_pitch;
-	unsigned int back_offset, back_pitch;
+		unsigned int fb_cpp;
+		unsigned int front_offset, front_pitch;
+		unsigned int back_offset, back_pitch;
 
-	unsigned int depth_cpp;
-	unsigned int depth_offset, depth_pitch;
+		unsigned int depth_cpp;
+		unsigned int depth_offset, depth_pitch;
 
-	unsigned int texture_offset[MGA_NR_TEX_HEAPS];
-	unsigned int texture_size[MGA_NR_TEX_HEAPS];
+		unsigned int texture_offset[MGA_NR_TEX_HEAPS];
+		unsigned int texture_size[MGA_NR_TEX_HEAPS];
+	);
 
 	unsigned long fb_offset;
 	unsigned long mmio_offset;
-- 
2.30.2