failed with:

Running test ["sort", "-f"]
	Input: ["-f"]
	Rust stdout: (IÃ7Ã7LWPcp8(pzZ9Ã
	0Y5iRJ63yz
	3ÃM
	H7ÃJn8Cx1L(C
	nr4ÃYzÃ
	R(cHY1E
	U
	w
	Z
	zI2muK
	
	GNU stdout: 0Y5iRJ63yz
	3ÃM
	H7ÃJn8Cx1L(C
	(IÃ7Ã7LWPcp8(pzZ9Ã
	nr4ÃYzÃ
	R(cHY1E
	U
	w
	Z
	zI2muK
	
	Diff=
	-(IÃ7Ã7LWPcp8(pzZ9Ã
	 0Y5iRJ63yz
	 3ÃM
	 H7ÃJn8Cx1L(C
	+(IÃ7Ã7LWPcp8(pzZ9Ã
	 nr4ÃYzÃ
	 R(cHY1E
	 U
	 w
	 Z
	 zI2muK
	
	Discrepancy detected: stdout differs
	

You can simplify it to:

$ printf "A\n(C\nb\n" | sort -f
A
b
(C
$ printf "A\n(C\nb\n" | cargo run sort -f
(C
A
b

I took a brief look at this, admittedly without actually running any code from this crate, and I believe this is a locale issue. I did read through the code and believe issue lies in how cmp_chars determines sort order. Looking at the reference docs for the GNU version there is a footnote about collation that states

If you use a non-POSIX locale (e.g., by setting LC_ALL to ‘en_US’), then sort may produce output that is sorted differently than you’re accustomed to. In that case, set the LC_ALL environment variable to ‘C’.

I tested this with GNU sort on my machine and was able to produce both sorting orders with GNU sort just by changing locale:

echo -e "a\n(c\nb\nd" | LC_ALL=C sort
(c
a
b
d

vs

echo -e "a\n(c\nb\nd" | LC_ALL=en_US.UTF-8 sort
a
b
(c
d

Based on this (very limited character set) it looks like rust is using the C locale and the GNU test is not? I did a quick search on locales in rust and it seems there has been some recent discussion on on it, although the discussion was more focused on international comparisons, which I am unclear of if it is in scope or not. I did find a crate that in theory can load locales since a quick search indicated that the stdlib does not respect them, which a quick playground test seemed to confirm.

Hopefully this is helpful :)

Yes, you are right, uutils sort uses the C locale (there is no localization support yet) whereas GNU sort uses whatever locale is defined. Though I don't understand why the fuzzer doesn't use the C locale because we set env::set_var("LC_COLLATE", "C"); before running GNU sort. In #5889 I changed it to LC_ALL, maybe it will fix the issue.

OSS-Fuzz has closed this bug. Please see https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=65860 for details.