safe.directory: use git_protected_config()

Use git_protected_config() to read `safe.directory` instead of
read_very_early_config(), making it 'protected configuration only'.

As a result, `safe.directory` now respects "-c", so update the tests and
docs accordingly. It used to ignore "-c" due to how it was implemented,
not because of security or correctness concerns [1].

[1] https://lore.kernel.org/git/xmqqlevabcsu.fsf@gitster.g/

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/safe.txt

@@ -12,9 +12,9 @@ via `git config --add`. To reset the list of safe directories (e.g. to
 override any such directories specified in the system config), add a
 `safe.directory` entry with an empty value.
 +
-This config setting is only respected when specified in a system or global
-config, not when it is specified in a repository config, via the command
-line option `-c safe.directory=<path>`, or in environment variables.
+This config setting is only respected in protected configuration (see
+<<SCOPES>>). This prevents the untrusted repository from tampering with this
+value.
 +
 The value of this setting is interpolated, i.e. `~/<path>` expands to a
 path relative to the home directory and `%(prefix)/<path>` expands to a

setup.c

@@ -1155,7 +1155,7 @@ static int ensure_valid_ownership(const char *gitfile,
 	 * constant regardless of what failed above. data.is_safe should be
 	 * initialized to false, and might be changed by the callback.
 	 */
-	read_very_early_config(safe_directory_cb, &data);
+	git_protected_config(safe_directory_cb, &data);
 
 	return data.is_safe;
 }

t/t0033-safe-directory.sh

@@ -16,24 +16,20 @@ test_expect_success 'safe.directory is not set' '
 	expect_rejected_dir
 '
 
-test_expect_success 'ignoring safe.directory on the command line' '
-	test_must_fail git -c safe.directory="$(pwd)" status 2>err &&
-	grep "dubious ownership" err
+test_expect_success 'safe.directory on the command line' '
+	git -c safe.directory="$(pwd)" status
 '
 
-test_expect_success 'ignoring safe.directory in the environment' '
-	test_must_fail env GIT_CONFIG_COUNT=1 \
-		GIT_CONFIG_KEY_0="safe.directory" \
-		GIT_CONFIG_VALUE_0="$(pwd)" \
-		git status 2>err &&
-	grep "dubious ownership" err
+test_expect_success 'safe.directory in the environment' '
+	env GIT_CONFIG_COUNT=1 \
+	    GIT_CONFIG_KEY_0="safe.directory" \
+	    GIT_CONFIG_VALUE_0="$(pwd)" \
+	    git status
 '
 
-test_expect_success 'ignoring safe.directory in GIT_CONFIG_PARAMETERS' '
-	test_must_fail env \
-		GIT_CONFIG_PARAMETERS="${SQ}safe.directory${SQ}=${SQ}$(pwd)${SQ}" \
-		git status 2>err &&
-	grep "dubious ownership" err
+test_expect_success 'safe.directory in GIT_CONFIG_PARAMETERS' '
+	env GIT_CONFIG_PARAMETERS="${SQ}safe.directory${SQ}=${SQ}$(pwd)${SQ}" \
+	    git status
 '
 
 test_expect_success 'ignoring safe.directory in repo config' '