selftests/lkdtm: Avoid needing explicit sub-shell

Some environments do not set $SHELL when running tests. There's no
need to use $SHELL here anyway, since "cat" can be used to receive any
delivered signals from the kernel. Additionally avoid using bash-isms
in the command, and record stderr for posterity.

Fixes: 46d1a0f0 ("selftests/lkdtm: Add tests for LKDTM targets")
Cc: stable@vger.kernel.org
Suggested-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210623203936.3151093-2-keescook@chromium.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

tools/testing/selftests/lkdtm/run.sh

@@ -76,10 +76,14 @@ fi
 # Save existing dmesg so we can detect new content below
 dmesg > "$DMESG"
 
-# Most shells yell about signals and we're expecting the "cat" process
-# to usually be killed by the kernel. So we have to run it in a sub-shell
-# and silence errors.
-($SHELL -c 'cat <(echo '"$test"') >'"$TRIGGER" 2>/dev/null) || true
+# Since the kernel is likely killing the process writing to the trigger
+# file, it must not be the script's shell itself. i.e. we cannot do:
+#     echo "$test" >"$TRIGGER"
+# Instead, use "cat" to take the signal. Since the shell will yell about
+# the signal that killed the subprocess, we must ignore the failure and
+# continue. However we don't silence stderr since there might be other
+# useful details reported there in the case of other unexpected conditions.
+echo "$test" | cat >"$TRIGGER" || true
 
 # Record and dump the results
 dmesg | comm --nocheck-order -13 "$DMESG" - > "$LOG" || true