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Bug#1123750: [5.10] regression: virtual consoles 2-12 unusable

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Hello stable maintainers,

Several Debian users reported a regression after updating to kernel
version 5.10.247.

Commit f0982400648a ("fbdev: Add bounds checking in bit_putcs to fix
vmalloc-out-of-bounds"), a backport of upstream commit 3637d34b35b2,
depends on vc_data::vc_font.charcount being initialised correctly.

However, before commit a1ac250a82a5 ("fbcon: Avoid using FNTCHARCNT()
and hard-coded built-in font charcount") in 5.11, this member was set
to 256 for VTs initially created with a built-in font and 0 for VTs
initially created with a user font.

Since Debian normally sets a user font before creating VTs 2 and up,
those additional VTs became unusable.  VT 1 also doesn't work correctly
if the user font has > 256 characters, and the bounds check is
ineffective if it has < 256 characters.

This can be fixed by backporting the following commits from 5.11:

7a089ec7d77f console: Delete unused con_font_copy() callback implementations
259a252c1f4e console: Delete dummy con_font_set() and con_font_default() callback implementations
4ee573086bd8 Fonts: Add charcount field to font_desc
4497364e5f61 parisc/sticore: Avoid hard-coding built-in font charcount
a1ac250a82a5 fbcon: Avoid using FNTCHARCNT() and hard-coded built-in font charcount

These all apply without fuzz and builds cleanly for x86_64 and parisc64.

I tested on x86_64 that:

- VT 2 works again
- bit_putcs_aligned() is setting charcnt = 256
- After loading a font with 512 characters, bit_putcs_aligned() sets
  charcnt = 512 and is able to display characters at positions >= 256

Ben.

-- 
Ben Hutchings
Man invented language to satisfy his deep need to complain.
                                                          - Lily Tomlin

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Bug#1123750: linux: regression: virtual consoles 2-12 unusable Salvatore Bonaccorso <carnil@debian.org> - 2025-12-21 08:00 +0100
  Processed: Re: Bug#1123750: linux: regression: virtual consoles  2-12 unusable "Debian Bug Tracking System" <owner@bugs.debian.org> - 2025-12-21 08:00 +0100
  Bug#1123750: linux: regression: virtual consoles 2-12 unusable Salvatore Bonaccorso <carnil@debian.org> - 2025-12-21 09:10 +0100
  Bug#1123750: linux: regression: virtual consoles 2-12 unusable Thorsten Glaser <tg@mirbsd.de> - 2025-12-21 17:40 +0100
    Bug#1123750: linux: regression: virtual consoles 2-12 unusable Salvatore Bonaccorso <carnil@debian.org> - 2025-12-26 18:40 +0100
      Processed: Re: Bug#1123750: linux: regression: virtual consoles  2-12 unusable "Debian Bug Tracking System" <owner@bugs.debian.org> - 2025-12-26 18:40 +0100
      Bug#1123750: linux: regression: virtual consoles 2-12 unusable William Burrow <wbkernel@gmail.com> - 2025-12-26 20:30 +0100
        Bug#1123750: linux: regression: virtual consoles 2-12 unusable William Burrow <wbkernel@gmail.com> - 2025-12-27 03:10 +0100
          Bug#1123750: linux: regression: virtual consoles 2-12 unusable William Burrow <wbkernel@gmail.com> - 2025-12-27 03:30 +0100
        Bug#1123750: [5.10] regression: virtual consoles 2-12 unusable Ben Hutchings <ben@decadent.org.uk> - 2026-01-02 17:30 +0100
          Bug#1123750: [5.10] regression: virtual consoles 2-12 unusable Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2026-01-08 14:40 +0100
            Bug#1123750: [5.10] regression: virtual consoles 2-12 unusable Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2026-01-28 15:30 +0100
      Bug#1123750: linux: regression: virtual consoles 2-12 unusable Thorsten Glaser <tg@debian.org> - 2025-12-26 23:10 +0100
      Bug#1123750: linux: regression: virtual consoles 2-12 unusable Thorsten Glaser <tg@debian.org> - 2025-12-26 23:10 +0100

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