linux-kernel - Re: [PATCH] [next] HID: hyperv: Replace one-element array with flexible-array member

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Date:   Mon, 17 Oct 2022 10:30:54 +0200
From:   Benjamin Tissoires <benjamin.tissoires@...hat.com>
To:     Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
Cc:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Jiri Kosina <jikos@...nel.org>, linux-hyperv@...r.kernel.org,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [next] HID: hyperv: Replace one-element array with
 flexible-array member

On Mon, Oct 17, 2022 at 9:51 AM Paulo Miguel Almeida
<paulo.miguel.almeida.rodenas@...il.com> wrote:
>
> One-element arrays are deprecated, and we are replacing them with
> flexible array members instead. So, replace one-element array with
> flexible-array member in structs synthhid_msg, synthhid_input_report,
> pipe_prt_msg and refactor the rest of the code accordingly.
>
> This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> routines on memcpy() and help us make progress towards globally
> enabling -fstrict-flex-arrays=3 [1].
>
> Link: https://github.com/KSPP/linux/issues/79
> Link: https://github.com/KSPP/linux/issues/210
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
>
> Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
> ---

FWIW, this is
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@...hat.com>

Couple of questions though:
- do the hyperv want to take this patch through their tree or should I
(I plan on sending some fixes for 6.1-rc2 this week, so I can piggy
back this one)
- on the driver itself, please see inline:

>  drivers/hid/hid-hyperv.c | 17 +++++++----------
>  1 file changed, 7 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/hid/hid-hyperv.c b/drivers/hid/hid-hyperv.c
> index e0bc73124196..208cf8d981a5 100644
> --- a/drivers/hid/hid-hyperv.c
> +++ b/drivers/hid/hid-hyperv.c
> @@ -61,7 +61,7 @@ struct synthhid_msg_hdr {
>
>  struct synthhid_msg {
>         struct synthhid_msg_hdr header;
> -       char data[1]; /* Enclosed message */
> +       char data[]; /* Enclosed message */
>  };

IMO that struct has no real use. We just use it in
mousevsc_on_receive() to dereference the first field only, the header.
So how about we have a followup cleanup patch that just removes it and
in mousevsc_on_receive() we convert those usages directly to struct
synthhid_msg_hdr?

Cheers,
Benjamin

>
>  union synthhid_version {
> @@ -99,7 +99,7 @@ struct synthhid_device_info_ack {
>
>  struct synthhid_input_report {
>         struct synthhid_msg_hdr header;
> -       char buffer[1];
> +       char buffer[];
>  };
>
>  #pragma pack(pop)
> @@ -118,7 +118,7 @@ enum pipe_prot_msg_type {
>  struct pipe_prt_msg {
>         enum pipe_prot_msg_type type;
>         u32 size;
> -       char data[1];
> +       char data[];
>  };
>
>  struct  mousevsc_prt_msg {
> @@ -232,7 +232,7 @@ static void mousevsc_on_receive_device_info(struct mousevsc_dev *input_device,
>
>         ret = vmbus_sendpacket(input_device->device->channel,
>                         &ack,
> -                       sizeof(struct pipe_prt_msg) - sizeof(unsigned char) +
> +                       sizeof(struct pipe_prt_msg) +
>                         sizeof(struct synthhid_device_info_ack),
>                         (unsigned long)&ack,
>                         VM_PKT_DATA_INBAND,
> @@ -271,16 +271,14 @@ static void mousevsc_on_receive(struct hv_device *device,
>                  * malicious/buggy hypervisor/host, add a check here to
>                  * ensure we don't corrupt memory.
>                  */
> -               if ((pipe_msg->size + sizeof(struct pipe_prt_msg)
> -                       - sizeof(unsigned char))
> +               if (struct_size(pipe_msg, data, pipe_msg->size)
>                         > sizeof(struct mousevsc_prt_msg)) {
>                         WARN_ON(1);
>                         break;
>                 }
>
>                 memcpy(&input_dev->protocol_resp, pipe_msg,
> -                      pipe_msg->size + sizeof(struct pipe_prt_msg) -
> -                      sizeof(unsigned char));
> +                               struct_size(pipe_msg, data, pipe_msg->size));
>                 complete(&input_dev->wait_event);
>                 break;
>
> @@ -359,8 +357,7 @@ static int mousevsc_connect_to_vsp(struct hv_device *device)
>         request->request.version_requested.version = SYNTHHID_INPUT_VERSION;
>
>         ret = vmbus_sendpacket(device->channel, request,
> -                               sizeof(struct pipe_prt_msg) -
> -                               sizeof(unsigned char) +
> +                               sizeof(struct pipe_prt_msg) +
>                                 sizeof(struct synthhid_protocol_request),
>                                 (unsigned long)request,
>                                 VM_PKT_DATA_INBAND,
> --
> 2.37.3
>