[Bug glob/31075] fnmatch("??") matches on 2-byte single characters (as well as 2 any-length characters)

["stephane+sourceware at chazelas dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31075

--- Comment #1 from Stephane Chazelas <stephane+sourceware at chazelas dot org> ---
(In reply to Stephane Chazelas from comment #0)
> Regression introduced in 2.34 by commit
> a79328c745219dcb395070cdcd3be065a8347f24  reproduced on Ubuntu 22.04, Debian
> sid libc6:amd64 2.37-12, and current git HEAD
> (dae3cf4134d476a4b4ef86fd7012231d6436c15e) built on that sid system.
> 
> find . -name '??'
[...]

To clarify, "find" is used here to demonstrate the behaviour of the libc's
fnmatch(). Here with GNU find.

$ LD_DEBUG=bindings find é -name '??' |& grep fnmatch
    323895:     binding file /lib/x86_64-linux-gnu/libselinux.so.1 [0] to
/lib/x86_64-linux-gnu/libc.so.6 [0]: normal symbol `fnmatch' [GLIBC_2.2.5]
    323895:     binding file find [0] to /lib/x86_64-linux-gnu/libc.so.6 [0]:
normal symbol `fnmatch' [GLIBC_2.2.5]

$ ltrace -e 'fnmatch' find é éé $'\U10FFFF\U10FFFF' -name '??'
find->fnmatch("foo", "foo", 0)                                                 
                                      = 0
find->fnmatch("Foo", "foo", 0)                                                 
                                      = 1
find->fnmatch("Foo", "foo", 16)                                                
                                      = 0
find->fnmatch("??", "\303\251", 0)                                             
                                      = 0
é
find->fnmatch("??", "\303\251\303\251", 0)                                     
                                      = 0
éé
find->fnmatch("??", "\364\217\277\277\364\217\277\277", 0)                     
                                      = 0
??
+++ exited (status 0) +++

(here showing ?? matching one 2-byte character, two 2-byte characters and two
4-byte characters).

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