linux-kernel - [PATCH] USB: serial: garmin_gps: Use flexible-array member

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Message-ID: <20200116220327.GA12537@embeddedor.com>
Date:   Thu, 16 Jan 2020 16:03:27 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] USB: serial: garmin_gps: Use flexible-array member

Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
presence of a "variable length array":

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

There is also 0-byte arrays. Both cases pose confusion for things like
sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
to declare variable-length types such as the one above is a flexible array
member[2] which need to be the last member of a structure and empty-sized:

struct something {
        int stuff;
        u8 data[];
};

Also, by making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
unadvertenly introduced[3] to the codebase from now on.

[1] https://github.com/KSPP/linux/issues/21
[2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
---
 drivers/usb/serial/garmin_gps.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/serial/garmin_gps.c b/drivers/usb/serial/garmin_gps.c
index 633550ec3025..ffd984142171 100644
--- a/drivers/usb/serial/garmin_gps.c
+++ b/drivers/usb/serial/garmin_gps.c
@@ -104,7 +104,7 @@ struct garmin_packet {
 	int               seq;
 	/* the real size of the data array, always > 0 */
 	int               size;
-	__u8              data[1];
+	__u8              data[];
 };

 /* structure used to keep the current state of the driver */
-- 
2.23.0