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

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Message-ID: <202401101557.87634A6A@keescook>
Date: Wed, 10 Jan 2024 15:58:18 -0800
From: Kees Cook <keescook@...omium.org>
To: "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc: Vinod Koul <vkoul@...nel.org>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
	Tudor Ambarus <tudor.ambarus@...aro.org>, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
	Claudiu Beznea <claudiu.beznea.uj@...renesas.com>,
	Lad Prabhakar <prabhakar.mahadev-lad.rj@...renesas.com>
Subject: Re: [PATCH] dmaengine: usb-dmac: Avoid format-overflow warning

On Wed, Jan 10, 2024 at 10:46:02PM +0000, Lad, Prabhakar wrote:
> Hi Kees,
> 
> Thank you for the review.
> 
> On Wed, Jan 10, 2024 at 10:41 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Wed, Jan 10, 2024 at 10:22:10PM +0000, Prabhakar wrote:
> > > From: Lad Prabhakar <prabhakar.mahadev-lad.rj@...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@...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?)
> >
> Good point, I oversighted it by just looking at the loop indices. I
> will send a v2 with that change.

I think you'll need a bounce variable in usb_dmac_chan_probe() since it
looks like it's reading a 32-bit value from DT, but otherwise, it should
be okay.

-Kees

-- 
Kees Cook