core: skip ReadOnlyPaths= and other permission-related mounts on PermissionsStartOnly= (#5309)

ReadOnlyPaths=, ProtectHome=, InaccessiblePaths= and ProtectSystem= are
about restricting access and little more, hence they should be disabled
if PermissionsStartOnly= is used or ExecStart= lines are prefixed with a
"+". Do that.

(Note that we will still create namespaces and stuff, since that's about
a lot more than just permissions. We'll simply disable the effect of
the four options mentioned above, but nothing else mount related.)

This also adds a test for this, to ensure this works as intended.

No documentation updates, as the documentation are already vague enough
to support the new behaviour ("If true, the permission-related execution
options…"). We could clarify this further, but I think we might want to
extend the switches' behaviour a bit more in future, hence leave it at
this for now.

Fixes: #5308

1 files changed, 8 insertions, 0 deletions

diff --git a/test/test-execute/exec-read-only-path-succeed.service b/test/test-execute/exec-read-only-path-succeed.service
new file mode 100644
index 0000000000..b54d48f281
--- /dev/null
+++ b/test/test-execute/exec-read-only-path-succeed.service
@@ -0,0 +1,8 @@
+[Service]
+Type=oneshot
+# This should work, as we explicitly disable the effect of ReadOnlyPaths=
+ExecStart=+/bin/touch /tmp/thisisasimpletest
+# This should also work, as we do not disable the effect of ReadOnlyPaths= but invert the exit code
+ExecStart=/bin/sh -x -c '! /bin/touch /tmp/thisisasimpletest'
+ExecStart=+/bin/rm /tmp/thisisasimpletest
+ReadOnlyPaths=/tmp