fortify: Add compile-time FORTIFY_SOURCE tests

While the run-time testing of FORTIFY_SOURCE is already present in
LKDTM, there is no testing of the expected compile-time detections. In
preparation for correctly supporting FORTIFY_SOURCE under Clang, adding
additional FORTIFY_SOURCE defenses, and making sure FORTIFY_SOURCE
doesn't silently regress with GCC, introduce a build-time test suite that
checks each expected compile-time failure condition.

As this is relatively backwards from standard build rules in the
sense that a successful test is actually a compile _failure_, create
a wrapper script to check for the correct errors, and wire it up as
a dummy dependency to lib/string.o, collecting the results into a log
file artifact.

Signed-off-by: Kees Cook <keescook@chromium.org>

scripts/test_fortify.sh

@@ -0,0 +1,62 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only
+set -e
+
+# Argument 1: Source file to build.
+IN="$1"
+shift
+# Extract just the filename for error messages below.
+FILE="${IN##*/}"
+# Extract the function name for error messages below.
+FUNC="${FILE#*-}"
+FUNC="${FUNC%%-*}"
+FUNC="${FUNC%%.*}"
+# Extract the symbol to test for in build/symbol test below.
+WANT="__${FILE%%-*}"
+
+# Argument 2: Where to write the build log.
+OUT="$1"
+shift
+TMP="${OUT}.tmp"
+
+# Argument 3: Path to "nm" tool.
+NM="$1"
+shift
+
+# Remaining arguments are: $(CC) $(c_flags)
+
+# Clean up temporary file at exit.
+__cleanup() {
+	rm -f "$TMP"
+}
+trap __cleanup EXIT
+
+# Function names in warnings are wrapped in backticks under UTF-8 locales.
+# Run the commands with LANG=C so that grep output will not change.
+export LANG=C
+
+status=
+# Attempt to build a source that is expected to fail with a specific warning.
+if "$@" -Werror -c "$IN" -o "$OUT".o 2> "$TMP" ; then
+	# If the build succeeds, either the test has failed or the
+	# warning may only happen at link time (Clang). In that case,
+	# make sure the expected symbol is unresolved in the symbol list.
+	# If so, FORTIFY is working for this case.
+	if ! $NM -A "$OUT".o | grep -m1 "\bU ${WANT}$" >>"$TMP" ; then
+		status="warning: unsafe ${FUNC}() usage lacked '$WANT' symbol in $IN"
+	fi
+else
+	# If the build failed, check for the warning in the stderr (gcc).
+	if ! grep -q -m1 "error: call to .\b${WANT}\b." "$TMP" ; then
+		status="warning: unsafe ${FUNC}() usage lacked '$WANT' warning in $IN"
+	fi
+fi
+
+if [ -n "$status" ]; then
+	# Report on failure results, including compilation warnings.
+	echo "$status" | tee "$OUT" >&2
+else
+	# Report on good results, and save any compilation output to log.
+	echo "ok: unsafe ${FUNC}() usage correctly detected with '$WANT' in $IN" >"$OUT"
+fi
+cat "$TMP" >>"$OUT"