[PATCH 2/2] can: mcp251xfd: decorate mcp251xfd_rx_ring.obj with __counted_by()

[Vincent Mailhol <mailhol.vincent@wanadoo.fr>]

A new __counted_by() attribute was introduced in [1]. It makes the
compiler's sanitizer aware of the actual size of a flexible array
member, allowing for additional runtime checks.

Apply the __counted_by() attribute to the obj flexible array member of
struct mcp251xfd_rx_ring.

Note that the mcp251xfd_rx_ring.obj member is polymorphic: it can be
either of:

  * an array of struct mcp251xfd_hw_rx_obj_can
  * an array of struct mcp251xfd_hw_rx_obj_canfd

The canfd type was chosen in the declaration by the original author to
reflect the upper bound. We pursue the same logic here: the sanitizer
will only see the accurate size of canfd frames. For classical can
frames, it will see a size bigger than the reality, making the check
incorrect but silent (false negative).

[1] commit dd06e72e68bc ("Compiler Attributes: Add __counted_by macro")
Link: https://git.kernel.org/torvalds/c/dd06e72e68bc

CC: Kees Cook <kees@kernel.org>
Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
---
 drivers/net/can/spi/mcp251xfd/mcp251xfd.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd.h b/drivers/net/can/spi/mcp251xfd/mcp251xfd.h
index 24510b3b8020..b7579fba9457 100644
--- a/drivers/net/can/spi/mcp251xfd/mcp251xfd.h
+++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd.h
@@ -565,7 +565,7 @@ struct mcp251xfd_rx_ring {
 	union mcp251xfd_write_reg_buf uinc_buf;
 	union mcp251xfd_write_reg_buf uinc_irq_disable_buf;
 	struct spi_transfer uinc_xfer[MCP251XFD_FIFO_DEPTH];
-	struct mcp251xfd_hw_rx_obj_canfd obj[];
+	struct mcp251xfd_hw_rx_obj_canfd obj[] __counted_by(obj_num);
 };
 
 struct __packed mcp251xfd_map_buf_nocrc {
-- 
2.43.0