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

[Shixiong Ou]

Adding to Cc Jani Nikula

On 2026/7/29 18:58, oushixiong1...@163.com wrote:

From: Shixiong Ou <oushixi...@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.

Signed-off-by: Shixiong Ou <oushixi...@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);