linux-kernel - Re: [PATCH 1/1] virtio_console: fix missing byte order handling for cols and rows

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Message-ID: <3920c0f0da1b6e324d6367cbff22d313d6981742.camel@linux.ibm.com>
Date: Mon, 24 Mar 2025 18:56:03 +0100
From: Maximilian Immanuel Brandtner <maxbr@...ux.ibm.com>
To: Halil Pasic <pasic@...ux.ibm.com>, Amit Shah <amit@...nel.org>,
        Arnd
 Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>
Cc: stable@...r.kernel.org
Subject: Re: [PATCH 1/1] virtio_console: fix missing byte order handling for
 cols and rows

On Sat, 2025-03-22 at 01:29 +0100, Halil Pasic wrote:
> As per virtio spec the fields cols and rows are specified as little
> endian. Although there is no legacy interface requirement that would
> state that cols and rows need to be handled as native endian when
> legacy
> interface is used, unlike for the fields of the adjacent struct
> virtio_console_control, I decided to err on the side of caution based
> on some non-conclusive virtio spec repo archaeology and opt for using
> virtio16_to_cpu() much like for virtio_console_control.event.
> Strictly
> by the letter of the spec virtio_le_to_cpu() would have been
> sufficient.
> But when the legacy interface is not used, it boils down to the same.
> 
> And when using the legacy interface, the device formatting these as
> little endian when the guest is big endian would surprise me more
> than
> it using guest native byte order (which would make it compatible with
> the current implementation). Nevertheless somebody trying to
> implement
> the spec following it to the letter could end up forcing little
> endian
> byte order when the legacy interface is in use. So IMHO this
> ultimately
> needs a judgement call by the maintainers.
> 
> Fixes: 8345adbf96fc1 ("virtio: console: Accept console size along
> with resize control message")
> Signed-off-by: Halil Pasic <pasic@...ux.ibm.com>
> Cc: stable@...r.kernel.org # v2.6.35+
> ---
> 
> @Michael: I think it would be nice to add a clarification on the byte
> order to be used for cols and rows when the legacy interface is used
> to
> the spec, regardless of what we decide the right byte order is. If
> it is native endian that shall be stated much like it is stated for
> virtio_console_control. If it is little endian, I would like to add
> a sentence that states that unlike for the fields of
> virtio_console_control
> the byte order of the fields of struct virtio_console_resize is
> little
> endian also when the legacy interface is used.
> 
> @Maximilian: Would you mind giving this a spin with your
> implementation
> on the device side of things in QEMU?
> ---
>  drivers/char/virtio_console.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/virtio_console.c
> b/drivers/char/virtio_console.c
> index 18f92dd44d45..fc698e2b1da1 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1579,8 +1579,8 @@ static void handle_control_message(struct
> virtio_device *vdev,
>  		break;
>  	case VIRTIO_CONSOLE_RESIZE: {
>  		struct {
> -			__u16 rows;
> -			__u16 cols;
> +			__virtio16 rows;
> +			__virtio16 cols;
>  		} size;
>  
>  		if (!is_console_port(port))
> @@ -1588,7 +1588,8 @@ static void handle_control_message(struct
> virtio_device *vdev,
>  
>  		memcpy(&size, buf->buf + buf->offset +
> sizeof(*cpkt),
>  		       sizeof(size));
> -		set_console_size(port, size.rows, size.cols);
> +		set_console_size(port, virtio16_to_cpu(vdev,
> size.rows),
> +				 virtio16_to_cpu(vdev, size.cols));
>  
>  		port->cons.hvc->irq_requested = 1;
>  		resize_console(port);
> 
> base-commit: b3ee1e4609512dfff642a96b34d7e5dfcdc92d05

It took me a while to recompile the kernel, but now that it has
compiled it works! Unfortunately, images don't lend themselves well to
mailing lists, but here is tmux running at 18x55(you'll just have to
trust me that it's over a virtio serial console)

                           │top - 12:54:04 up 4 min,  1
~                          │Tasks: 222 total,   1 runni
~                          │%Cpu(s):  0.0 us,  0.0 sy, 
~                          │MiB Mem :  15987.2 total,  
~                          │MiB Swap:   8192.0 total,  
~                          │
~                          │    PID USER      PR  NI 
~                          │      1 root      20   0 
~                          │      2 root      20   0 
~                          │      3 root      20   0 
~                          │      4 root       0 -20 
~                          │      5 root       0 -20 
~                          │      6 root       0 -20 
~                          │      7 root       0 -20 
~                          │      8 root       0 -20 
[No Name]     0,0-1     All│      9 root      20   0 
                           │     10 root       0 -20 
[0] 0:zsh*                     "fedora" 12:53 24-Mar-25

Cheers,
Max