[PATCH] can: etas_es58x: Replace 0-element raw_msg array

[PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Kees Cook]

While raw_msg isn't a fixed size, it does have a maximum size. Adjust the
struct to represent this and avoid the following warning when building
with -Wzero-length-bounds:

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:360:35: 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:231:6: note: while referencing 'raw_msg'
  231 |   u8 raw_msg[0];
      |      ^~~~~~~

Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Marc Kleine-Budde <mkl@pengutronix.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Arunachalam Santhanam <arunachalam.santhanam@in.bosch.com>
Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Cc: linux-can@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 drivers/net/can/usb/etas_es58x/es581_4.h  | 2 +-
 drivers/net/can/usb/etas_es58x/es58x_fd.h | 2 +-
 2 files changed, 2 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..af38c4938859 100644
--- a/drivers/net/can/usb/etas_es58x/es581_4.h
+++ b/drivers/net/can/usb/etas_es58x/es581_4.h
@@ -192,7 +192,7 @@ 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];
+		u8 raw_msg[USHRT_MAX];
 	} __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 ee18a87e40c0..e0319b8358ef 100644
--- a/drivers/net/can/usb/etas_es58x/es58x_fd.h
+++ b/drivers/net/can/usb/etas_es58x/es58x_fd.h
@@ -228,7 +228,7 @@ 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];
+		u8 raw_msg[USHRT_MAX];
 	} __packed;
 
 	__le16 reserved_for_crc16_do_not_use;
-- 
2.30.2

Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Vincent MAILHOL]

On Wed. 18 Aug 2021 at 12:40, Kees Cook <keescook@chromium.org> wrote:
> While raw_msg isn't a fixed size, it does have a maximum size. Adjust the
> struct to represent this and avoid the following warning when building
> with -Wzero-length-bounds:
>
> 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:360:35: 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:231:6: note: while referencing 'raw_msg'
>   231 |   u8 raw_msg[0];
>       |      ^~~~~~~
>
> Cc: Wolfgang Grandegger <wg@grandegger.com>
> Cc: Marc Kleine-Budde <mkl@pengutronix.de>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Arunachalam Santhanam <arunachalam.santhanam@in.bosch.com>
> Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> Cc: linux-can@vger.kernel.org
> Cc: netdev@vger.kernel.org
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  drivers/net/can/usb/etas_es58x/es581_4.h  | 2 +-
>  drivers/net/can/usb/etas_es58x/es58x_fd.h | 2 +-
>  2 files changed, 2 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..af38c4938859 100644
> --- a/drivers/net/can/usb/etas_es58x/es581_4.h
> +++ b/drivers/net/can/usb/etas_es58x/es581_4.h
> @@ -192,7 +192,7 @@ 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];
> +               u8 raw_msg[USHRT_MAX];
>         } __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 ee18a87e40c0..e0319b8358ef 100644
> --- a/drivers/net/can/usb/etas_es58x/es58x_fd.h
> +++ b/drivers/net/can/usb/etas_es58x/es58x_fd.h
> @@ -228,7 +228,7 @@ 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];
> +               u8 raw_msg[USHRT_MAX];
>         } __packed;
>
>         __le16 reserved_for_crc16_do_not_use;
> --
> 2.30.2

raw_msg is part of a union so its maximum size is implicitly the
biggest size of the other member of that union:

| struct es58x_fd_urb_cmd {
|     __le16 SOF;
|    u8 cmd_type;
|    u8 cmd_id;
|    u8 channel_idx;
|    __le16 msg_len;
|
|    union {
|        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;
|        u8 raw_msg[0];
|    } __packed;
|
|    __le16 reserved_for_crc16_do_not_use;
| } __packed;

ram_msg can then be used to manipulate the other fields at the byte level.
I am sorry but I fail to understand why this is an issue.

Also, the proposed fix drastically increases the size of the structure.


Yours sincerely,
Vincent

Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Kees Cook]

On Wed, Aug 18, 2021 at 02:13:51PM +0900, Vincent MAILHOL wrote:
> On Wed. 18 Aug 2021 at 12:40, Kees Cook <keescook@chromium.org> wrote:
> > While raw_msg isn't a fixed size, it does have a maximum size. Adjust the
> > struct to represent this and avoid the following warning when building
> > with -Wzero-length-bounds:
> >
> > 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:360:35: 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:231:6: note: while referencing 'raw_msg'
> >   231 |   u8 raw_msg[0];
> >       |      ^~~~~~~
> >
> > Cc: Wolfgang Grandegger <wg@grandegger.com>
> > Cc: Marc Kleine-Budde <mkl@pengutronix.de>
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Arunachalam Santhanam <arunachalam.santhanam@in.bosch.com>
> > Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> > Cc: linux-can@vger.kernel.org
> > Cc: netdev@vger.kernel.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> >  drivers/net/can/usb/etas_es58x/es581_4.h  | 2 +-
> >  drivers/net/can/usb/etas_es58x/es58x_fd.h | 2 +-
> >  2 files changed, 2 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..af38c4938859 100644
> > --- a/drivers/net/can/usb/etas_es58x/es581_4.h
> > +++ b/drivers/net/can/usb/etas_es58x/es581_4.h
> > @@ -192,7 +192,7 @@ 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];
> > +               u8 raw_msg[USHRT_MAX];
> >         } __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 ee18a87e40c0..e0319b8358ef 100644
> > --- a/drivers/net/can/usb/etas_es58x/es58x_fd.h
> > +++ b/drivers/net/can/usb/etas_es58x/es58x_fd.h
> > @@ -228,7 +228,7 @@ 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];
> > +               u8 raw_msg[USHRT_MAX];
> >         } __packed;
> >
> >         __le16 reserved_for_crc16_do_not_use;
> > --
> > 2.30.2
> 
> raw_msg is part of a union so its maximum size is implicitly the
> biggest size of the other member of that union:

Yup, understood. See below...

> 
> | struct es58x_fd_urb_cmd {
> |     __le16 SOF;
> |    u8 cmd_type;
> |    u8 cmd_id;
> |    u8 channel_idx;
> |    __le16 msg_len;
> |
> |    union {
> |        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;
> |        u8 raw_msg[0];
> |    } __packed;
> |
> |    __le16 reserved_for_crc16_do_not_use;
> | } __packed;
> 
> ram_msg can then be used to manipulate the other fields at the byte level.
> I am sorry but I fail to understand why this is an issue.

The issue is with using a 0-element array (these are being removed from
the kernel[1] so we can add -Warray-bounds). Normally in this situation I
would replace the 0-element array with a flexible array, but this
case is unusual in several ways:

- There is a trailing struct member (reserved_for_crc16_do_not_use),
  which is never accessed (good), and documented as "please never access
  this".

- struct es58x_fd_urb_cmd is statically allocated (it is written into
  from the URB handler).

- The message lengths coming from the USB device are stored in a u16,
  which looked like it was possible to overflow the buffer.

In taking a closer look, I see that the URB command length is checked,
and the in-data length is checked as well, so the overflow concern
appears to be addressed.

> Also, the proposed fix drastically increases the size of the structure.

Indeed. I will send a v2, now that I see that the overflow concern isn't
an issue.

Thanks!

-Kees

[1] https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

-- 
Kees Cook

Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Vincent MAILHOL]

On Wed. 18 Aug 2021 at 15:48, Kees Cook <keescook@chromium.org> wrote:
> On Wed, Aug 18, 2021 at 02:13:51PM +0900, Vincent MAILHOL wrote:
> > On Wed. 18 Aug 2021 at 12:40, Kees Cook <keescook@chromium.org> wrote:
> > > While raw_msg isn't a fixed size, it does have a maximum size. Adjust the
> > > struct to represent this and avoid the following warning when building
> > > with -Wzero-length-bounds:
> > >
> > > 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:360:35: 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:231:6: note: while referencing 'raw_msg'
> > >   231 |   u8 raw_msg[0];
> > >       |      ^~~~~~~
> > >
> > > Cc: Wolfgang Grandegger <wg@grandegger.com>
> > > Cc: Marc Kleine-Budde <mkl@pengutronix.de>
> > > Cc: "David S. Miller" <davem@davemloft.net>
> > > Cc: Jakub Kicinski <kuba@kernel.org>
> > > Cc: Arunachalam Santhanam <arunachalam.santhanam@in.bosch.com>
> > > Cc: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
> > > Cc: linux-can@vger.kernel.org
> > > Cc: netdev@vger.kernel.org
> > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > ---
> > >  drivers/net/can/usb/etas_es58x/es581_4.h  | 2 +-
> > >  drivers/net/can/usb/etas_es58x/es58x_fd.h | 2 +-
> > >  2 files changed, 2 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..af38c4938859 100644
> > > --- a/drivers/net/can/usb/etas_es58x/es581_4.h
> > > +++ b/drivers/net/can/usb/etas_es58x/es581_4.h
> > > @@ -192,7 +192,7 @@ 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];
> > > +               u8 raw_msg[USHRT_MAX];
> > >         } __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 ee18a87e40c0..e0319b8358ef 100644
> > > --- a/drivers/net/can/usb/etas_es58x/es58x_fd.h
> > > +++ b/drivers/net/can/usb/etas_es58x/es58x_fd.h
> > > @@ -228,7 +228,7 @@ 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];
> > > +               u8 raw_msg[USHRT_MAX];
> > >         } __packed;
> > >
> > >         __le16 reserved_for_crc16_do_not_use;
> > > --
> > > 2.30.2
> >
> > raw_msg is part of a union so its maximum size is implicitly the
> > biggest size of the other member of that union:
>
> Yup, understood. See below...
>
> >
> > | struct es58x_fd_urb_cmd {
> > |     __le16 SOF;
> > |    u8 cmd_type;
> > |    u8 cmd_id;
> > |    u8 channel_idx;
> > |    __le16 msg_len;
> > |
> > |    union {
> > |        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;
> > |        u8 raw_msg[0];
> > |    } __packed;
> > |
> > |    __le16 reserved_for_crc16_do_not_use;
> > | } __packed;
> >
> > ram_msg can then be used to manipulate the other fields at the byte level.
> > I am sorry but I fail to understand why this is an issue.
>
> The issue is with using a 0-element array (these are being removed from
> the kernel[1] so we can add -Warray-bounds). Normally in this situation I
> would replace the 0-element array with a flexible array, but this
> case is unusual in several ways:
>
> - There is a trailing struct member (reserved_for_crc16_do_not_use),
>   which is never accessed (good), and documented as "please never access
>   this".

Yes. And FYI, this field is here so that
| sizeof(struct es58x_fd_urb_cmd)
returns the correct maximum size.

And, of course, because this structure will be sent to the
device, there is no possibility to reorder those fields.

> - struct es58x_fd_urb_cmd is statically allocated (it is written into
>   from the URB handler).
>
> - The message lengths coming from the USB device are stored in a u16,
>   which looked like it was possible to overflow the buffer.
>
> In taking a closer look, I see that the URB command length is checked,
> and the in-data length is checked as well, so the overflow concern
> appears to be addressed.
>
> > Also, the proposed fix drastically increases the size of the structure.
>
> Indeed. I will send a v2, now that I see that the overflow concern isn't
> an issue.

Thanks for the explanation. That makes sense.

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?

Yours sincerely,
Vincent

Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Kees Cook]

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

Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Vincent MAILHOL]

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

Re: [PATCH] can: etas_es58x: Replace 0-element raw_msg array

[Kees Cook]

On Wed, Aug 18, 2021 at 06:33:39PM +0900, Vincent MAILHOL wrote:
> 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

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