The UTF-8 part of the vt driver suffers from the following issues which are
addressed in my patch:

1) If there's no glyph found for a particular valid UTF-8 character, we try
   to display U+FFFD. However if this one is not found either, here's what
   the current kernel does:

   - First, if the Unicode value is less than the number of glyphs, use the
     glyph directly from that position of the glyph table. While it may be a
     good idea in the 8-bit world, it has absolutely no sense with Unicode
     in mind. For example, if a Latin-2 font is loaded and an application
     prints U+00FB ("u with circumflex", not present in Latin-2) then as a
     fallback solution the glyph from the 0xFB position of the Latin-2
     fontset (which is an "u with double accent" - a different character) is
     displayed.

   - Second, if this fallback fails too, a simple ASCII question mark is
     printed, which is visually undistinguishable from a real question mark.

   I changed the code to skip the first step (except if in non-UTF-8 mode),
   and changed the second step to print the question mark with inverse color
   attributes, so it is visually clear that it's not a real question mark,
   and resembles more to the common glyph of U+FFFD.

2) The UTF-8 decoder is buggy in many ways:

   - Lone continuation bytes (section 3.1 of Markus Kuhn's UTF-8 stress
     test) are not caught, they are displayed as some "random" (taken
     directly form the font table, see above) glyphs instead the replacement
     character.

   - Incomplete sequences (sections 3.2 and 3.3 of the stress test) emit no
     replacement character, but rather cause the subsequent valid character
     to be displayed more times(!).

   - The decoder is not safe: overlong sequences are not caught currently,
     they are displayed as if these were valid representations. This may
     even have security impacts.

   - The decoder does not handle D800..DFFF and FFFE..FFFF specially, it
     just emits these code points and lets it be looked up in the glyph
     table. Since these are invalid code points, I replace them by U+FFFD
     and hence give no chance for them to be looked up in the glyph table.
     (Assuming no font ships glyphs for these code points, this change is
     not visible to the users since the glyph shown will be the same.)

   With my fixes to the decoder it now behaves exactly as Markus Kuhn's
   stress test recommends.

3) It has no concept of double-width (CJK) characters. It's way beyond the
   scope of my patch to try to display them, but at least I think it's
   important for the cursor to jump two positions when printing such
   characters, since this is what applications (such as text editors)
   expect. Currently the cursor only jumps one position, and hence
   applications suffer from displaying and refreshing problems, and editing
   some English letters that are preceded by some CJK characters in the same
   line is a nightmare. With my patch an additional space is inserted after
   the CJK character has been printed (which usually means a replacement
   symbol of course). (If U+FFFD isn't availble and hence an inverse
   question mark is displayed in the first cell, I keep the inverted state
   for the space in the 2nd column so it's quite easy to see that they are
   tied together.)

4) There is a small built-in table of zero-width spaces that are not to be
   printed but silently skipped. U+200A is included there, but it's not a
   zero-width character, so I remove it from there.
Signed-off-by: Egmont Koblinger <egmont@uhulinux.hu>
Cc: Jan Engelhardt <jengelh@linux01.gwdg.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>