Re: [PATCH v3 2/3] fbcon: Rebuild rotated font buffer on font change
From: Thomas Zimmermann
Date: Fri Oct 16 2026 - 04:02:06 EST
Hi
Am 16.10.26 um 09:30 schrieb Yao Yiqi:
From: Yao Yiqi <yaoyiqi3@xxxxxxxxxx>
fbcon_rotate_font() caches a rotated copy of the console font in
par->rotated.buf. The buffer is sized from the font's width, height
and charcount, and is rebuilt in fbcon_init(), fbcon_switch() and
fbcon_modechanged(), but not in fbcon_do_set_font().
When the console font is changed through KDFONTOP/KD_FONT_OP_SET, the
new font data and geometry are installed and vc_resize() is called.
The redraw that follows reaches the rotated bitops while the buffer
still holds the old font: the rotated putcs/cursor ops index
par->rotated.buf with the new cell size and character codes, so a
stale, too-small buffer is read out of bounds.
KASAN reports, for example:
BUG: KASAN: slab-out-of-bounds in soft_cursor
Read of size 64 at addr ...
memcpy
soft_cursor
cw_cursor
fbcon_cursor
hide_cursor
redraw_screen
vc_do_resize
fbcon_do_set_font
fbcon_set_font
con_font_op
vt_ioctl
Rebuild the buffer for the visible console right after the new font
geometry has been installed and before any redraw. Select the
console's blitting state with set_blitting_type() first, as
fbcon_init(), fbcon_switch() and fbcon_modechanged() do before their
rotate_font() calls: fbcon_rotate_font() reads the source font
through par->p, which may otherwise still refer to another console of
this framebuffer. On tile-blitting hardware, the same call uploads
the new font's map to the text engine, which previously happened only
when the console was switched. Background consoles keep their rebuild
at fbcon_switch() time, as the buffer is shared per framebuffer and
must not be rebuilt for a console that is not being displayed.
If vc_resize() fails, err_out restores the previous font. The rotated
buffer that was rebuilt for the new font earlier in the function
cannot be restored with it: font_data_rotate() has already reused or
freed the buffer that belonged to the old font. Select the blitting
type and rebuild the buffer again after the restore: the upload to
tile-blitting hardware and the rotated cache would otherwise keep
describing the rejected font, and par->rotated.fontdata would keep
referencing the font data whose last reference the error path drops.
A failing rebuild leaves par->rotated.buf NULL, so the rotated ops
skip drawing instead of reading a mismatched buffer; the next patch
retries the allocation on the following call.
Unlike the other callers, this one does not fall back to unrotated
rendering on failure. The "par->rotate = FB_ROTATE_UR;
set_blitting_type(vc, info);" fallback they use does not actually
switch to unrotated rendering: set_blitting_type() calls
fbcon_set_rotation(), which immediately re-derives par->rotate from
par->p->con_rotate. Replicating that pattern here would not change
the outcome; repairing the fallback is left to separate work.
It's way too long and boring to read. Can you please try to shorten the AI-generated commit description?
Fixes: 6cc50e1c5b57 ("[PATCH] fbcon: Console Rotation - Add support to rotate font bitmap")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Yao Yiqi <yaoyiqi3@xxxxxxxxxx>
---
Changes in v3:
- Also select the blitting type again in the error path of
fbcon_do_set_font(). On tile-blitting hardware, set_blitting_type()
uploads the font map to the text engine; without the second call, a
failed vc_resize() left the engine with the rejected font (found by
the Sashiko review).
The problem is that rotate_font can fail as well. Handling that in the error path is not possible; hence results might be worse than keeping the rejected font. Can you try to keep the original rotated font and reinstall that directly?
Best regards
Thomas
Changes in v2:
- Rebuild the buffer again in the error path of fbcon_do_set_font().
Without this, a failed vc_resize() left the rotated cache describing
the rejected font while the console rendered with the restored one,
and par->rotated.fontdata referenced freed font data (found by the
Sashiko review).
- Select the console's blitting state with set_blitting_type() before
rebuilding, like the other rotate_font() callers, instead of writing
par->p directly.
- Reworded the note about the unrotated-rendering fallback, which is
now left to separate work.
---
drivers/video/fbdev/core/fbcon.c | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 01715873ea49..d90a2ad45053 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2432,6 +2432,17 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
else if (!vc->vc_hi_font_mask && charcount == 512)
set_vc_hi_font(vc, true);
+ /*
+ * Select the console's blitting state and update par->rotated to
+ * match the new font, so that the rendering below does not use a
+ * cache that was built for the old font.
+ */
+ if (con_is_visible(vc)) {
+ set_blitting_type(vc, info);
+ if (par->bitops->rotate_font)
+ par->bitops->rotate_font(info, vc);
+ }
+
if (resize) {
int cols, rows;
@@ -2466,6 +2477,18 @@ static int fbcon_do_set_font(struct vc_data *vc, int w, int h, int charcount,
else if (!old_hi_font_mask && vc->vc_hi_font_mask)
set_vc_hi_font(vc, false);
+ /*
+ * Restore the rendering state for the old font: re-select the
+ * blitting type, which re-uploads the font map on tile-blitting
+ * hardware, and overwrite par->rotated with the restored font's
+ * state.
+ */
+ if (con_is_visible(vc)) {
+ set_blitting_type(vc, info);
+ if (par->bitops->rotate_font)
+ par->bitops->rotate_font(info, vc);
+ }
+
font_data_put(data);
return ret;
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Stefan Gaiser, Jochen Jaser, Abhinav Puri, (HRB 36809, AG Nürnberg)