t0034: add negative tests and allow git init to mostly work under sudo

Add a support library that provides one function that can be used
to run a "scriplet" of commands through sudo and that helps invoking
sudo in the slightly awkward way that is required to ensure it doesn't
block the call (if shell was allowed as tested in the prerequisite)
and it doesn't run the command through a different shell than the one
we intended.

Add additional negative tests as suggested by Junio and that use a
new workspace that is owned by root.

Document a regression that was introduced by previous commits where
root won't be able anymore to access directories they own unless
SUDO_UID is removed from their environment.

The tests document additional ways that this new restriction could
be worked around and the documentation explains why it might be instead
considered a feature, but a "fix" is planned for a future change.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Phillip Wood <phillip.wood123@gmail.com>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-sudo.sh

@@ -0,0 +1,15 @@
+# Helpers for running git commands under sudo.
+
+# Runs a scriplet passed through stdin under sudo.
+run_with_sudo () {
+	local ret
+	local RUN="$TEST_DIRECTORY/$$.sh"
+	write_script "$RUN" "$TEST_SHELL_PATH"
+	# avoid calling "$RUN" directly so sudo doesn't get a chance to
+	# override the shell, add aditional restrictions or even reject
+	# running the script because its security policy deem it unsafe
+	sudo "$TEST_SHELL_PATH" -c "\"$RUN\""
+	ret=$?
+	rm -f "$RUN"
+	return $ret
+}

t/t0034-root-safe-directory.sh

@@ -3,6 +3,7 @@
 test_description='verify safe.directory checks while running as root'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-sudo.sh
 
 if [ "$GIT_TEST_ALLOW_SUDO" != "YES" ]
 then
@@ -10,6 +11,12 @@ then
 	test_done
 fi
 
+if ! test_have_prereq NOT_ROOT
+then
+	skip_all="These tests do not support running as root"
+	test_done
+fi
+
 test_lazy_prereq SUDO '
 	sudo -n id -u >u &&
 	id -u root >r &&
@@ -19,6 +26,12 @@ test_lazy_prereq SUDO '
 	test_cmp u r
 '
 
+if ! test_have_prereq SUDO
+then
+	skip_all="Your sudo/system configuration is either too strict or unsupported"
+	test_done
+fi
+
 test_expect_success SUDO 'setup' '
 	sudo rm -rf root &&
 	mkdir -p root/r &&
@@ -36,6 +49,55 @@ test_expect_success SUDO 'sudo git status as original owner' '
 	)
 '
 
+test_expect_success SUDO 'setup root owned repository' '
+	sudo mkdir -p root/p &&
+	sudo git init root/p
+'
+
+test_expect_success 'cannot access if owned by root' '
+	(
+		cd root/p &&
+		test_must_fail git status
+	)
+'
+
+test_expect_success 'can access if addressed explicitly' '
+	(
+		cd root/p &&
+		GIT_DIR=.git GIT_WORK_TREE=. git status
+	)
+'
+
+test_expect_failure SUDO 'can access with sudo if root' '
+	(
+		cd root/p &&
+		sudo git status
+	)
+'
+
+test_expect_success SUDO 'can access with sudo if root by removing SUDO_UID' '
+	(
+		cd root/p &&
+		run_with_sudo <<-END
+			unset SUDO_UID &&
+			git status
+		END
+	)
+'
+
+test_lazy_prereq SUDO_SUDO '
+	sudo sudo id -u >u &&
+	id -u root >r &&
+	test_cmp u r
+'
+
+test_expect_success SUDO_SUDO 'can access with sudo abusing SUDO_UID' '
+	(
+		cd root/p &&
+		sudo sudo git status
+	)
+'
+
 # this MUST be always the last test
 test_expect_success SUDO 'cleanup' '
 	sudo rm -rf root