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From:   Vincent MAILHOL <mailhol.vincent@wanadoo.fr>
Date:   Wed, 18 Aug 2021 18:33:39 +0900
Message-ID: <CAMZ6RqK=Q3mvV5gyPVhBsFxE+JPANHNrgFqs=bvTgkbXjwT4Eg@mail.gmail.com>
Subject: Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array
To:     Kees Cook <keescook@chromium.org>
Cc:     Wolfgang Grandegger <wg@grandegger.com>,
        Marc Kleine-Budde <mkl@pengutronix.de>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Arunachalam Santhanam <arunachalam.santhanam@in.bosch.com>,
        linux-can <linux-can@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        linux-hardening@vger.kernel.org
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On Wed. 18 Aug 2021 at 18:03, Kees Cook <keescook@chromium.org> wrote:
> 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. :)

The __flex_array itself is a nasty hack :D but the rest is clean.

Is this compliant to the C standard? Well, I guess that as long
as both GCC and LLVM supports it, it is safe to add it to the
kernel.

I like the final result. I will do a bit more testing and give my
acknowledgement if everything goes well.


Yours sincerely,
Vincent