Re: [PATCH] dmaengine: usb-dmac: Avoid format-overflow warning

[Kees Cook <keescook@chromium.org>]

On Wed, Jan 10, 2024 at 10:22:10PM +0000, Prabhakar wrote:
> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
> 
> gcc points out that the fix-byte buffer might be too small:
> drivers/dma/sh/usb-dmac.c: In function 'usb_dmac_probe':
> drivers/dma/sh/usb-dmac.c:720:34: warning: '%u' directive writing between 1 and 10 bytes into a region of size 3 [-Wformat-overflow=]
>   720 |         sprintf(pdev_irqname, "ch%u", index);
>       |                                  ^~
> In function 'usb_dmac_chan_probe',
>     inlined from 'usb_dmac_probe' at drivers/dma/sh/usb-dmac.c:814:9:
> drivers/dma/sh/usb-dmac.c:720:31: note: directive argument in the range [0, 4294967294]
>   720 |         sprintf(pdev_irqname, "ch%u", index);
>       |                               ^~~~~~
> drivers/dma/sh/usb-dmac.c:720:9: note: 'sprintf' output between 4 and 13 bytes into a destination of size 5
>   720 |         sprintf(pdev_irqname, "ch%u", index);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Maximum number of channels for USB-DMAC as per the driver is 1-99 so use
> u8 instead of unsigned int/int for DMAC channel indexing and make the
> pdev_irqname string long enough to avoid the warning.
> 
> While at it use scnprintf() instead of sprintf() to make the code more
> robust.
> 
> Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>

This looks like good fixes; thanks! I see n_channels is sanity checked
during the probe in usb_dmac_chan_probe(), so this looks good.

(Is there a reason not to also change n_channels to a u8?)

-Kees

> ---
>  drivers/dma/sh/usb-dmac.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/dma/sh/usb-dmac.c b/drivers/dma/sh/usb-dmac.c
> index a9b4302f6050..f7cd0cad056c 100644
> --- a/drivers/dma/sh/usb-dmac.c
> +++ b/drivers/dma/sh/usb-dmac.c
> @@ -706,10 +706,10 @@ static const struct dev_pm_ops usb_dmac_pm = {
>  
>  static int usb_dmac_chan_probe(struct usb_dmac *dmac,
>  			       struct usb_dmac_chan *uchan,
> -			       unsigned int index)
> +			       u8 index)
>  {
>  	struct platform_device *pdev = to_platform_device(dmac->dev);
> -	char pdev_irqname[5];
> +	char pdev_irqname[6];
>  	char *irqname;
>  	int ret;
>  
> @@ -717,7 +717,7 @@ static int usb_dmac_chan_probe(struct usb_dmac *dmac,
>  	uchan->iomem = dmac->iomem + USB_DMAC_CHAN_OFFSET(index);
>  
>  	/* Request the channel interrupt. */
> -	sprintf(pdev_irqname, "ch%u", index);
> +	scnprintf(pdev_irqname, sizeof(pdev_irqname), "ch%u", index);
>  	uchan->irq = platform_get_irq_byname(pdev, pdev_irqname);
>  	if (uchan->irq < 0)
>  		return -ENODEV;
> @@ -768,8 +768,8 @@ static int usb_dmac_probe(struct platform_device *pdev)
>  	const enum dma_slave_buswidth widths = USB_DMAC_SLAVE_BUSWIDTH;
>  	struct dma_device *engine;
>  	struct usb_dmac *dmac;
> -	unsigned int i;
>  	int ret;
> +	u8 i;
>  
>  	dmac = devm_kzalloc(&pdev->dev, sizeof(*dmac), GFP_KERNEL);
>  	if (!dmac)
> @@ -869,7 +869,7 @@ static void usb_dmac_chan_remove(struct usb_dmac *dmac,
>  static void usb_dmac_remove(struct platform_device *pdev)
>  {
>  	struct usb_dmac *dmac = platform_get_drvdata(pdev);
> -	int i;
> +	u8 i;
>  	for (i = 0; i < dmac->n_channels; ++i)
>  		usb_dmac_chan_remove(dmac, &dmac->channels[i]);
> -- 
> 2.34.1
> 

-- 
Kees Cook