[PATCH master] font: fbconsole: fix underflow when printing non-ASCII characters

[PATCH master] font: fbconsole: fix underflow when printing non-ASCII characters

[Ahmad Fatoum]

With signed characters, non-ASCII characters (i.e. valued >= 128)
are negative and remain so when casted to int.

find_font_index will thus multiply the index with a negative number and
look for font data before the actual start of the font.

A cast to unsigned fixes this, because all fonts that trigger this if
clause's branch have 256 characters.

Fixes: 27f79c05ab42 ("video: implement framebuffer console")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 lib/fonts/fonts.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/fonts/fonts.c b/lib/fonts/fonts.c
index b3bdaa21a2aa..6047b4cbe223 100644
--- a/lib/fonts/fonts.c
+++ b/lib/fonts/fonts.c
@@ -37,7 +37,7 @@ int find_font_index(const struct font_desc *font, int ch)
 	if (font->index == NULL) {
 		index  = DIV_ROUND_UP(font->width, 8);
 		index *= font->height;
-		index *= ch;
+		index *= (unsigned char)ch;
 	} else {
 		/*
 		* FIXME: use binary search instead!
-- 
2.39.5

Re: [PATCH master] font: fbconsole: fix underflow when printing non-ASCII characters

[Sascha Hauer]

On Tue, 22 Apr 2025 07:36:41 +0200, Ahmad Fatoum wrote:
> With signed characters, non-ASCII characters (i.e. valued >= 128)
> are negative and remain so when casted to int.
> 
> find_font_index will thus multiply the index with a negative number and
> look for font data before the actual start of the font.
> 
> A cast to unsigned fixes this, because all fonts that trigger this if
> clause's branch have 256 characters.
> 
> [...]

Applied, thanks!

[1/1] font: fbconsole: fix underflow when printing non-ASCII characters
      https://git.pengutronix.de/cgit/barebox/commit/?id=fbf4d8e13976 (link may not be stable)

Best regards,
-- 
Sascha Hauer <s.hauer@pengutronix.de>