linux-kernel - Re: [PATCH] [v3] drm/imx: imx-ldb: fix out of bounds array access warning

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Date:   Thu, 25 Mar 2021 10:03:23 +0800
From:   Liu Ying <victor.liu@....com>
To:     Arnd Bergmann <arnd@...nel.org>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     Arnd Bergmann <arnd@...db.de>, Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>,
        Marco Felsch <m.felsch@...gutronix.de>,
        Sam Ravnborg <sam@...nborg.org>, Joe Perches <joe@...ches.com>,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [v3] drm/imx: imx-ldb: fix out of bounds array access
 warning

On Wed, 2021-03-24 at 17:47 +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> When CONFIG_OF is disabled, building with 'make W=1' produces warnings
> about out of bounds array access:
> 
> drivers/gpu/drm/imx/imx-ldb.c: In function 'imx_ldb_set_clock.constprop':
> drivers/gpu/drm/imx/imx-ldb.c:186:8: error: array subscript -22 is below array bounds of 'struct clk *[4]' [-Werror=array-bounds]
> 
> Add an error check before the index is used, which helps with the
> warning, as well as any possible other error condition that may be
> triggered at runtime.
> 
> The warning could be fixed by adding a Kconfig depedency on CONFIG_OF,
> but Liu Ying points out that the driver may hit the out-of-bounds
> problem at runtime anyway.

Almost impossible to hit the out-of-bounds problem at runtime, unless
something wrong happens and makes unexpected parameters(node and/or
encoder) be handed over to drm_of_encoder_active_port_id(). Anyway, an
error check on return value from drm_of_encoder_active_port_id() looks
ok to me.

> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Liu Ying <victor.liu@....com>

Thanks,
Liu Ying

> ---
> v3: fix build regression from v2
> v2: fix subject line
>     expand patch description
>     print mux number
>     check upper bound as well
> ---
>  drivers/gpu/drm/imx/imx-ldb.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/gpu/drm/imx/imx-ldb.c b/drivers/gpu/drm/imx/imx-ldb.c
> index dbfe39e2f7f6..565482e2b816 100644
> --- a/drivers/gpu/drm/imx/imx-ldb.c
> +++ b/drivers/gpu/drm/imx/imx-ldb.c
> @@ -197,6 +197,11 @@ static void imx_ldb_encoder_enable(struct drm_encoder *encoder)
>  	int dual = ldb->ldb_ctrl & LDB_SPLIT_MODE_EN;
>  	int mux = drm_of_encoder_active_port_id(imx_ldb_ch->child, encoder);
>  
> +	if (mux < 0 || mux >= ARRAY_SIZE(ldb->clk_sel)) {
> +		dev_warn(ldb->dev, "%s: invalid mux %d\n", __func__, mux);
> +		return;
> +	}
> +
>  	drm_panel_prepare(imx_ldb_ch->panel);
>  
>  	if (dual) {
> @@ -255,6 +260,11 @@ imx_ldb_encoder_atomic_mode_set(struct drm_encoder *encoder,
>  	int mux = drm_of_encoder_active_port_id(imx_ldb_ch->child, encoder);
>  	u32 bus_format = imx_ldb_ch->bus_format;
>  
> +	if (mux < 0 || mux >= ARRAY_SIZE(ldb->clk_sel)) {
> +		dev_warn(ldb->dev, "%s: invalid mux %d\n", __func__, mux);
> +		return;
> +	}
> +
>  	if (mode->clock > 170000) {
>  		dev_warn(ldb->dev,
>  			 "%s: mode exceeds 170 MHz pixel clock\n", __func__);