Re: [PATCH v2] drm/log: Fix division by zero when scale module parameter is 0

[Jocelyn Falempe <jfalempe@redhat.com>]

On 29/07/2026 12:58, oushixiong1025@163.com wrote:
> From: Shixiong Ou <oushixiong@kylinos.cn>
> 
> The scale module parameter can be set to 0 via kernel command line or
> sysfs. When scale is 0, scaled_font_h and scaled_font_w become 0,
> causing a division by zero in the rows/columns calculation.
> 
> Introduce a drm_log_scale() helper that returns scale ?: 1, and use it
> at all read sites. This avoids a race that a setter-based clamp would
> have between param_set_uint() and the subsequent check, where another
> CPU could observe scale == 0.

The scale module parameter is read only (that's the meaning of 0444 in 
module_param()) so it can't be set by sysfs, or change at runtime.

So this check can be done only once in drm_log_register().

Best regards,

-- 

Jocelyn
> 
> Signed-off-by: Shixiong Ou <oushixiong@kylinos.cn>
> ---
> v1->v2:
>     Introduce a drm_log_scale() helper.
> 
>   drivers/gpu/drm/clients/drm_log.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/clients/drm_log.c b/drivers/gpu/drm/clients/drm_log.c
> index 294b3be1a6b3..9522c1344123 100644
> --- a/drivers/gpu/drm/clients/drm_log.c
> +++ b/drivers/gpu/drm/clients/drm_log.c
> @@ -29,6 +29,11 @@ static unsigned int scale = 1;
>   module_param(scale, uint, 0444);
>   MODULE_PARM_DESC(scale, "Integer scaling factor for drm_log, default is 1");
>   
> +static inline unsigned int drm_log_scale(void)
> +{
> +	return scale ?: 1;
> +}
> +
>   /**
>    * DOC: overview
>    *
> @@ -76,13 +81,13 @@ static void drm_log_blit(struct iosys_map *dst, unsigned int dst_pitch,
>   {
>   	switch (px_width) {
>   	case 2:
> -		drm_draw_blit16(dst, dst_pitch, src, src_pitch, height, width, scale, color);
> +		drm_draw_blit16(dst, dst_pitch, src, src_pitch, height, width, drm_log_scale(), color);
>   		break;
>   	case 3:
> -		drm_draw_blit24(dst, dst_pitch, src, src_pitch, height, width, scale, color);
> +		drm_draw_blit24(dst, dst_pitch, src, src_pitch, height, width, drm_log_scale(), color);
>   		break;
>   	case 4:
> -		drm_draw_blit32(dst, dst_pitch, src, src_pitch, height, width, scale, color);
> +		drm_draw_blit32(dst, dst_pitch, src, src_pitch, height, width, drm_log_scale(), color);
>   		break;
>   	default:
>   		WARN_ONCE(1, "Can't blit with pixel width %d\n", px_width);
> @@ -216,8 +221,8 @@ static int drm_log_setup_modeset(struct drm_client_dev *client,
>   		return -ENOMEM;
>   	}
>   	mode_set->fb = scanout->buffer->fb;
> -	scanout->scaled_font_h = scanout->font->height * scale;
> -	scanout->scaled_font_w = scanout->font->width * scale;
> +	scanout->scaled_font_h = scanout->font->height * drm_log_scale();
> +	scanout->scaled_font_w = scanout->font->width * drm_log_scale();
>   	scanout->rows = height / scanout->scaled_font_h;
>   	scanout->columns = width / scanout->scaled_font_w;
>   	scanout->front_color = drm_draw_color_from_xrgb8888(0xffffff, format);