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Date:   Wed, 18 Aug 2021 17:02:10 -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 06:33:39PM +0900, Vincent MAILHOL wrote:
> On Wed. 18 Aug 2021 at 18:03, Kees Cook <keescook@...omium.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

Indeed. ;)

> but the rest is clean.

Thanks!

> 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.

The kernel already uses a bunch of compiler extensions, none of which
were legal under the C standard to begin with. :) So, really, this is
about normalizing what we're already doing and finding a single hack
that helps the code base for readability and robustness.

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

Great; thank you!

-- 
Kees Cook

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