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Date:   Wed, 18 Aug 2021 02:03:16 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Vincent MAILHOL <mailhol.vincent@...adoo.fr>
Cc:     Wolfgang Grandegger <wg@...ndegger.com>,
        Marc Kleine-Budde <mkl@...gutronix.de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Arunachalam Santhanam <arunachalam.santhanam@...bosch.com>,
        linux-can <linux-can@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

On Wed, Aug 18, 2021 at 04:55:20PM +0900, Vincent MAILHOL wrote:
> At the end, the only goal of raw_msg[] is to have a tag pointing
> to the beginning of the union. It would be virtually identical to
> something like:
> |    u8 raw_msg[];
> |    union {
> |        /* ... */
> |    } __packed ;
> 
> I had a look at your work and especially at your struct_group() macro.
> Do you think it would make sense to introduce a union_group()?
> 
> Result would look like:
> 
> |    union_group_attr(urb_msg, __packed, /* raw_msg renamed to urb_msg */
> |        struct es58x_fd_tx_conf_msg tx_conf_msg;
> |        u8 tx_can_msg_buf[ES58X_FD_TX_BULK_MAX * ES58X_FD_CANFD_TX_LEN];
> |        u8 rx_can_msg_buf[ES58X_FD_RX_BULK_MAX * ES58X_FD_CANFD_RX_LEN];
> |        struct es58x_fd_echo_msg echo_msg[ES58X_FD_ECHO_BULK_MAX];
> |        struct es58x_fd_rx_event_msg rx_event_msg;
> |        struct es58x_fd_tx_ack_msg tx_ack_msg;
> |        __le64 timestamp;
> |        __le32 rx_cmd_ret_le32;
> |    );
> 
> And I can then use urb_msg in place of the old raw_msg (might
> need a bit of rework here and there but I can take care of it).
> 
> This is the most pretty way I can think of to remove this zero length array.
> Keeping the raw_msg[] but with another size seems odd to me.
> 
> Or maybe I would be the only one using this feature in the full
> tree? In that case, maybe it would make sense to keep the
> union_group_attr() macro local to the etas_es58x driver?

I actually ended up with something close to this idea, but more
generalized for other cases in the kernel. There was a sane way to
include a "real" flexible array in a union (or alone in a struct), so
I've proposed this flex_array() helper:
https://lore.kernel.org/lkml/20210818081118.1667663-2-keescook@chromium.org/

and then it's just a drop-in replacement for all the places that need
this fixed, including etas_es58x:
https://lore.kernel.org/lkml/20210818081118.1667663-3-keescook@chromium.org/#Z30drivers:net:can:usb:etas_es58x:es581_4.h

Hopefully this will work out; I think it's as clean as we can get for
now. :)

-- 
Kees Cook

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