apply: do not read from beyond a symbolic link

We should reject a patch, whether it renames/copies dir/file to
elsewhere with or without modificiation, or updates dir/file in
place, if "dir/" part is actually a symbolic link to elsewhere,
by making sure that the code to read the preimage does not read
from a path that is beyond a symbolic link.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/apply.c

@@ -3145,6 +3145,8 @@ static int load_patch_target(struct strbuf *buf,
 				return read_file_or_gitlink(ce, buf);
 			else
 				return SUBMODULE_PATCH_WITHOUT_INDEX;
+		} else if (has_symlink_leading_path(name, strlen(name))) {
+			return error(_("reading from '%s' beyond a symbolic link"), name);
 		} else {
 			if (read_old_data(st, name, buf))
 				return error(_("read of %s failed"), name);

t/t4122-apply-symlink-inside.sh

@@ -52,4 +52,23 @@ test_expect_success 'check result' '
 
 '
 
+test_expect_success SYMLINKS 'do not read from beyond symbolic link' '
+	git reset --hard &&
+	mkdir -p arch/x86_64/dir &&
+	>arch/x86_64/dir/file &&
+	git add arch/x86_64/dir/file &&
+	echo line >arch/x86_64/dir/file &&
+	git diff >patch &&
+	git reset --hard &&
+
+	mkdir arch/i386/dir &&
+	>arch/i386/dir/file &&
+	ln -s ../i386/dir arch/x86_64/dir &&
+
+	test_must_fail git apply patch &&
+	test_must_fail git apply --cached patch &&
+	test_must_fail git apply --index patch
+
+'
+
 test_done