[PATCH] nouveau/gsp: Avoid addressing beyond end of rpc->entries

[Kees Cook]

Using the end of rpc->entries[] for addressing runs into both compile-time
and run-time detection of accessing beyond the end of the array. Use the
base pointer instead, since was allocated with the additional bytes for
storing the strings. Avoids the following warning in future GCC releases
with support for __counted_by:

In function 'fortify_memcpy_chk',
    inlined from 'r535_gsp_rpc_set_registry' at ../drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c:1123:3:
../include/linux/fortify-string.h:553:25: error: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror=attribute-warning]
  553 |                         __write_overflow_field(p_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

for this code:

	strings = (char *)&rpc->entries[NV_GSP_REG_NUM_ENTRIES];
	...
                memcpy(strings, r535_registry_entries[i].name, name_len);

Signed-off-by: Kees Cook <keescook at chromium.org>
---
Cc: Karol Herbst <kherbst at redhat.com>
Cc: Lyude Paul <lyude at redhat.com>
Cc: Danilo Krummrich <dakr at redhat.com>
Cc: David Airlie <airlied at gmail.com>
Cc: Daniel Vetter <daniel at ffwll.ch>
Cc: Dave Airlie <airlied at redhat.com>
Cc: Ben Skeggs <bskeggs at redhat.com>
Cc: Timur Tabi <ttabi at nvidia.com>
Cc: dri-devel at lists.freedesktop.org
Cc: nouveau at lists.freedesktop.org
---
 drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c b/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c
index 9994cbd6f1c4..9858c1438aa7 100644
--- a/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c
+++ b/drivers/gpu/drm/nouveau/nvkm/subdev/gsp/r535.c
@@ -1112,7 +1112,7 @@ r535_gsp_rpc_set_registry(struct nvkm_gsp *gsp)
 	rpc->numEntries = NV_GSP_REG_NUM_ENTRIES;
 
 	str_offset = offsetof(typeof(*rpc), entries[NV_GSP_REG_NUM_ENTRIES]);
-	strings = (char *)&rpc->entries[NV_GSP_REG_NUM_ENTRIES];
+	strings = (char *)rpc + str_offset;
 	for (i = 0; i < NV_GSP_REG_NUM_ENTRIES; i++) {
 		int name_len = strlen(r535_registry_entries[i].name) + 1;
 
-- 
2.34.1