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From:   Arnd Bergmann <arnd@kernel.org>
To:     Vincent Mailhol <mailhol.vincent@wanadoo.fr>,
        Wolfgang Grandegger <wg@grandegger.com>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     Arnd Bergmann <arnd@arndb.de>,
        Arunachalam Santhanam <arunachalam.santhanam@in.bosch.com>,
        linux-can@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: [PATCH] can: etas_es58x: avoid -Wzero-length-bounds warning
Date:   Mon, 20 Sep 2021 14:30:29 +0200
Message-Id: <20210920123045.795228-1-arnd@kernel.org>
MIME-Version: 1.0
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From: Arnd Bergmann <arnd@arndb.de>

gcc complains when writing into a zero-length array:

drivers/net/can/usb/etas_es58x/es581_4.c: In function 'es581_4_tx_can_msg':
drivers/net/can/usb/etas_es58x/es581_4.c:374:42: warning: array subscript 65535 is outside the bounds of an interior zero-length array 'u8[0]' {aka 'unsigned char[]'} [-Wzero-length-bounds]
  374 |         tx_can_msg = (typeof(tx_can_msg))&es581_4_urb_cmd->raw_msg[msg_len];
      |                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/net/can/usb/etas_es58x/es58x_core.h:21,
                 from drivers/net/can/usb/etas_es58x/es581_4.c:15:
drivers/net/can/usb/etas_es58x/es581_4.h:195:20: note: while referencing 'raw_msg'
  195 |                 u8 raw_msg[0];
      |                    ^~~~~~~
  CC [M]  drivers/net/can/usb/etas_es58x/es58x_fd.o
drivers/net/can/usb/etas_es58x/es58x_fd.c: In function 'es58x_fd_tx_can_msg':
drivers/net/can/usb/etas_es58x/es58x_fd.c:360:42: warning: array subscript 65535 is outside the bounds of an interior zero-length array 'u8[0]' {aka 'unsigned char[]'} [-Wzero-length-bounds]
  360 |         tx_can_msg = (typeof(tx_can_msg))&es58x_fd_urb_cmd->raw_msg[msg_len];
      |                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from drivers/net/can/usb/etas_es58x/es58x_core.h:22,
                 from drivers/net/can/usb/etas_es58x/es58x_fd.c:17:
drivers/net/can/usb/etas_es58x/es58x_fd.h:222:20: note: while referencing 'raw_msg'
  222 |                 u8 raw_msg[0];
      |                    ^~~~~~~

The solution is usually to use a flexible-array member the struct, but
we can't directly have that inside of a union, nor can it be the only
member of a struct, so add a dummy struct with another zero-length
member to get the intended behavior.

If someone has a better workaround, let me know and I can send a new
patch, as this version is rather ugly.

Fixes: f4f5247daa45 ("can: etas_es58x: rewrite the message cast in es58{1,_fd}_tx_can_msg to increase readability")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/net/can/usb/etas_es58x/es581_4.h  | 5 ++++-
 drivers/net/can/usb/etas_es58x/es58x_fd.h | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/net/can/usb/etas_es58x/es581_4.h b/drivers/net/can/usb/etas_es58x/es581_4.h
index 4bc60a6df697..ac5ef88db565 100644
--- a/drivers/net/can/usb/etas_es58x/es581_4.h
+++ b/drivers/net/can/usb/etas_es58x/es581_4.h
@@ -192,7 +192,10 @@ struct es581_4_urb_cmd {
 		struct es581_4_rx_cmd_ret rx_cmd_ret;
 		__le64 timestamp;
 		u8 rx_cmd_ret_u8;
-		u8 raw_msg[0];
+		struct {
+			u8 __pad[0];
+			u8 raw_msg[];
+		};
 	} __packed;
 
 	__le16 reserved_for_crc16_do_not_use;
diff --git a/drivers/net/can/usb/etas_es58x/es58x_fd.h b/drivers/net/can/usb/etas_es58x/es58x_fd.h
index a191891b8777..253e7bafd0b6 100644
--- a/drivers/net/can/usb/etas_es58x/es58x_fd.h
+++ b/drivers/net/can/usb/etas_es58x/es58x_fd.h
@@ -219,7 +219,10 @@ struct es58x_fd_urb_cmd {
 		struct es58x_fd_tx_ack_msg tx_ack_msg;
 		__le64 timestamp;
 		__le32 rx_cmd_ret_le32;
-		u8 raw_msg[0];
+		struct {
+			u8 __pad[0];
+			u8 raw_msg[];
+		};
 	} __packed;
 
 	__le16 reserved_for_crc16_do_not_use;
-- 
2.29.2