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

[Liu Ying]

On Wed, 2021-03-24 at 17:47 +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> 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@xxxxxxxx>

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

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__);