firewire: core: add __counted_by_ptr attribute to struct fw_device

The GCC and Clang attribute '__counted_by_ptr' associates a pointer
field with an integer field holding its element count. This enables
runtime bounds checking by KASAN and UBSAN to prevent out-of-bounds
accesses to the pointer.

In 'struct fw_device' (defined in 'include/linux/firewire.h'), the
field 'config_rom' is a pointer to the device's Configuration ROM data,
and its associated element count is stored in 'config_rom_length'.

Additionally, update the existing KUnit test in
'drivers/firewire/device-attribute-test.c' where 'config_rom_length'
was incorrectly initialized with the byte size
('sizeof(simple_avc_config_rom)') instead of the element count
('ARRAY_SIZE(simple_avc_config_rom)'). This ensures the
'__counted_by_ptr' bounds-checking annotation does not trigger any
false-positive panics or compile/runtime checks.

Cc: codemender-patching+linux@google.com
Assisted-by: LLM
Signed-off-by: Bill Wendling <morbo@google.com>
Link: https://lore.kernel.org/r/20260925212552.125652-1-morbo@google.com
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

drivers/firewire/device-attribute-test.c

@@ -86,7 +86,7 @@ static void device_attr_simple_avc(struct kunit *test)
 			.type = &fw_device_type,
 		},
 		.config_rom = simple_avc_config_rom,
-		.config_rom_length = sizeof(simple_avc_config_rom),
+		.config_rom_length = ARRAY_SIZE(simple_avc_config_rom),
 	};
 	static const struct fw_unit unit0 = {
 		.device = {
@@ -168,7 +168,7 @@ static void device_attr_legacy_avc(struct kunit *test)
 			.type = &fw_device_type,
 		},
 		.config_rom = legacy_avc_config_rom,
-		.config_rom_length = sizeof(legacy_avc_config_rom),
+		.config_rom_length = ARRAY_SIZE(legacy_avc_config_rom),
 	};
 	static const struct fw_unit unit0 = {
 		.device = {

include/linux/firewire.h

@@ -224,7 +224,7 @@ struct fw_device {
 	struct mutex client_list_mutex;
 	struct list_head client_list;
 
-	const u32 *config_rom;
+	const u32 *config_rom __counted_by_ptr(config_rom_length);
 	size_t config_rom_length;
 	int config_rom_retries;
 	unsigned is_local:1;