dash: why am I getting the error "Syntax error: EOF in backquote substitution" when my script doesn't even contain a backquote?

Question

#!/bin/sh --

for set_trap_sig in HUP INT QUIT ALRM TERM; do
    trap -- '
        trap -- - '"${set_trap_sig:?}"' EXIT || exit "$?"
        kill -s '"${set_trap_sig:?}"' -- "$$" || exit "$?"' "$set_trap_sig"
done

sleep 15 || exit "$?"

Here's what happens when I send SIGINT to the script

user@hostname:/tmp$ ./script.sh
^C./script.sh: 3: ./script.sh: Syntax error: EOF in backquote substitution

This issue seems specific to dash. On ash, bash and ksh93 I do not get this error. This is particularly weird because my script does not even contain the backquote character.

If I remove the double quotes on the trailing $? on line 5 the error goes away.

Am I doing something stupid or is dash misbehaving? Please, no comments about the level of error-checking in my scripts.

We have now established that this a very serious bug that affects even modern versions of Ubuntu and Debian. Does anyone know of a workaround?

Answer 1

Here is a simplified testcase:

trap '
        trap    - HUP EXIT || exit "$?"
        kill -s HUP    "$$" || exit "$?" ' HUP INT
kill -HUP $$

$ dash dash-bug
dash-bug: 3: /home2/ahq/dash-bug: Syntax error: EOF in backquote substitution

Modifying the length or content of the quoted code may either "fix" the bug or produce different and interesting corruptions, with random bytes appearing out of nowhere.

This was caused by a use-after-free bug which was fixed since dash 0.5.9, but which is still present in the dash 0.5.8 from Debian 9.8 stable (stretch), Ubuntu 18.04 (bionic) and Ubuntu 18.10 (cosmic).

This is the commit which fixed it:

commit 6c3f73bc536082fec38bd36e6c8a121033c68835
Author: Herbert Xu <herbert@gondor.apana.org.au>
Date:   Thu Oct 2 08:26:06 2014 +0800

    [EVAL] Fix use-after-free in dotrap/evalstring

    The function dotrap calls evalstring using the stored trap string.
    If evalstring then unsets that exact trap string then we will end
    up using freed memory.

    This patch fixes it by making evalstring always duplicate the string
    before using it.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

---

A work-around is to put the trap handler in a function -- so that no commands run after the trap has been unset try to use the action string after it has been freed:

sighandler(){
    trap - "$sig" EXIT || exit "$?"
    kill -s "$sig" "$$" || exit "$?"
}
for sig in HUP INT QUIT ALRM TERM; do
    trap "sig=$sig; sighandler" "$sig"
done

kill -s HUP "$$"