linux-kernel - Re: [PATCH v2] usb: r8a66597-hcd: host: fix port index underflow and UBSAN complains

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Date:   Sat, 1 Jul 2023 19:16:48 +0200
From:   Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To:     Zhang Shurong <zhang_shurong@...mail.com>
Cc:     gregkh@...uxfoundation.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] usb: r8a66597-hcd: host: fix port index underflow and
 UBSAN complains

On Sun, Jul 02, 2023 at 12:39:20AM +0800, Zhang Shurong wrote:
> If wIndex is 0 (and it often is), these calculations underflow and
> UBSAN complains, here resolve this by not decrementing the index when
> it is equal to 0.
> 
> Similar commit 85e3990bea49 ("USB: EHCI: avoid undefined pointer
> arithmetic and placate UBSAN")
> 
> The changes in this version:
> - fix some compile error
> 
> Signed-off-by: Zhang Shurong <zhang_shurong@...mail.com>
> ---
>  drivers/usb/host/r8a66597-hcd.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/usb/host/r8a66597-hcd.c b/drivers/usb/host/r8a66597-hcd.c
> index 9f4bf8c5f8a5..6c597c668364 100644
> --- a/drivers/usb/host/r8a66597-hcd.c
> +++ b/drivers/usb/host/r8a66597-hcd.c
> @@ -2141,10 +2141,12 @@ static int r8a66597_hub_control(struct usb_hcd *hcd, u16 typeReq, u16 wValue,
>  {
>  	struct r8a66597 *r8a66597 = hcd_to_r8a66597(hcd);
>  	int ret;
> -	int port = (wIndex & 0x00FF) - 1;
> -	struct r8a66597_root_hub *rh = &r8a66597->root_hub[port];
>  	unsigned long flags;
> +	struct r8a66597_root_hub *rh;
> +	u32 port = wIndex & 0xFF;
>  
> +	port -= (port > 0);

I have no idea about this hardware, but it seems strange to me that
calling r8a66597_hub_control with wIndex = 1 should have the same effect
as with wIndex = 0. Is you changed backed by knowledge about the
hardware, or is that just the most obvious way to get rid of the UB
warning?

Having said that, I think

	port -= (port > 0);

is hard to read compared to:

	if (port > 0)
		port--;

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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