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Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2 to F6 and over F7

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* William Burrow <wbkernel@gmail.com>:
> Hello, Camaleón and Salvatore.
> I have a similar problem with the TTYs and am very glad to have found
> this message.  Hopefully, my reply can make it to the list.
> .
> The offending commit was fairly easy to find as will be detailed
> later.  The git bisect on the linux-stable repo found the following
> commit as the problem:
> .
> ------------ First commit to cause the problem ------------
> commit 0998a6cb232674408a03e8561dc15aa266b2f53b
> Author: Junjie Cao <junjie.cao@intel.com>
> Date:   Mon Oct 20 21:47:01 2025 +0800
> 
>    fbdev: bitblit: bound-check glyph index in bit_putcs*
> 
>    commit 18c4ef4e765a798b47980555ed665d78b71aeadf upstream.
> ------------ First commit to cause the problem ------------
> .
> I leave it as an exercise to the reader to solve the problem created
> by this commit.  Hopefully, soon.

Can someone please try attached patch?
Alternatively (if then another char that the "copyright symbol" is
visible), we could try replacing it by:
	ch = 0x20;  /* use ASCII space character */

Overall, I'm still puzzled if the original patch is correct.
I always thought fonts have 256 or 512 characters, which means the
original patch isn't necessary at all.

Helge


>From 65c6bb3e7d06dba09f6b529a2478ee818c14e6ef Mon Sep 17 00:00:00 2001
From: Helge Deller <deller@gmx.de>
Date: Mon, 22 Dec 2025 10:19:55 +0100
Subject: [PATCH] fbdev: bitblit: fix bound-check of glyph index in bit_putcs*

The patch 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in
bit_putcs*") seems to trigger problems on virtual consoles, which
suddently show up with lots of copyright symbols.  This is e.g. reported
here:
https://www.reddit.com/r/debian/comments/1pnrm5t/problem_with_virtual_consoles_covered_with_a/

Try to narrow the issue down. Here we use the "last" char of a given
font (instead of char #0), if the previous char exceeds the actual
font's glyph count.

Reported-by: Gianluca Renzi <gianlucarenzi@eurek.it>
Signed-off-by: Helge Deller <deller@gmx.de>
Fixes: 18c4ef4e765a ("fbdev: bitblit: bound-check glyph index in bit_putcs*")

diff --git a/drivers/video/fbdev/core/bitblit.c b/drivers/video/fbdev/core/bitblit.c
index 085ffb44c51a..6b2f9748cd78 100644
--- a/drivers/video/fbdev/core/bitblit.c
+++ b/drivers/video/fbdev/core/bitblit.c
@@ -87,7 +87,7 @@ static inline void bit_putcs_aligned(struct vc_data *vc, struct fb_info *info,
 		u16 ch = scr_readw(s++) & charmask;
 
 		if (ch >= charcnt)
-			ch = 0;
+			ch = (charcnt - 1);
 		src = vc->vc_font.data + (unsigned int)ch * cellsize;
 
 		if (attr) {
@@ -126,7 +126,7 @@ static inline void bit_putcs_unaligned(struct vc_data *vc,
 		u16 ch = scr_readw(s++) & charmask;
 
 		if (ch >= charcnt)
-			ch = 0;
+			ch = (charcnt - 1);
 		src = vc->vc_font.data + (unsigned int)ch * cellsize;
 
 		if (attr) {

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Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2  to F6 and over F7 William Burrow <wbkernel@gmail.com> - 2025-12-22 03:20 +0100
  Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2  to F6 and over F7 Helge Deller <deller@kernel.org> - 2025-12-22 11:10 +0100
    Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2  to F6 and over F7 Gianluca Renzi <icjtqr@gmail.com> - 2025-12-22 11:10 +0100
      Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2  to F6 and over F7 Gianluca Renzi <gianlucarenzi@eurek.it> - 2025-12-22 12:20 +0100
        Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2  to F6 and over F7 Camaleón <noelamac@gmail.com> - 2025-12-22 16:30 +0100
          Re: Weird characters in virtual terminal consoles from (CTRL-ALT) F2  to F6 and over F7 William Burrow <wbkernel@gmail.com> - 2025-12-23 19:00 +0100

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