In a non-unicode locale, such as LANG=C, running createProcess with an env that contains a unicode character, such as '¡' results in it being stripped out of the value that is seen by the child proccess; only the ascii characters remain.

A program demonstrating the bug is:

import System.Process
import System.Environment
main = do
v <- lookupEnv "FOO"
case v of
	Just foo -> print $ "FOO is set to: " ++ foo
	Nothing -> do
		let e = [("FOO", "¡foo!")]
		print $ "running child process with environment " ++ show e
		self <- getExecutablePath
		let p = (proc self []) { env = Just e }
		(_, _, _, pid) <- createProcess p
		_ <- waitForProcess pid
		return ()

This program execs itself, so needs to be compiled, not run in ghci. On linux, built with process-1.6.5.0 and ghc-8.6.5, it behaves like this:

# LANG=en_US.utf8 ./foo
"running child process with environment [(\"FOO\",\"\\161foo!\")]"
"FOO is set to: \56514\56481foo!"
# LANG=C ./foo
"running child process with environment [(\"FOO\",\"\\161foo!\")]"
"FOO is set to: foo!"

Using strace -vf foo shows that the non-ascii characters do not make it to exec:

[pid 4335] execve("/home/joey/foo", ["/home/joey/foo"], ["FOO=foo!"] <unfinished ...>

This was discovered affecting a program that sets GIT_INDEX_FILE to a path when running git; if the path happens to contain unicode, this results in an corrupted path being passed to git.