Document the fact that read-only properties may not have the flag SD_BUS_VTABLE_UNPRIVILEGED as that is not obvious especially given the flag is accepted for writable properties. Based on the check in `add_object_vtable_internal` called by `sd_bus_add_object_vtable` (as of the current tip of the main branch f7f5ba019206cacd486b0892fec76f70f525e04d): case _SD_BUS_VTABLE_PROPERTY: { [...] if ([...] || [...] (v->flags & SD_BUS_VTABLE_UNPRIVILEGED && v->type == _SD_BUS_VTABLE_PROPERTY)) { r = -EINVAL; goto fail; } (where `_SD_BUS_VTABLE_PROPERTY` means read-only property whereas `_SD_BUS_VTABLE_WRITABLE_PROPERTY` maps to writable property). This was implemented in the commit adacb9575a09981fcf11279f2f661e3fc21e58ff ("bus: introduce "trusted" bus concept and encode access control in object vtables") where `SD_BUS_VTABLE_UNPRIVILEGED` was introduced: Writable properties are also subject to SD_BUS_VTABLE_UNPRIVILEGED and SD_BUS_VTABLE_CAPABILITY() for controlling write access to them. Note however that read access is unrestricted, as PropertiesChanged messages might send out the values anyway as an unrestricted broadcast. (cherry picked from commit 3ca09aa4dd57327989eceb1298754601046ac041) (cherry picked from commit cd727031a4daafe19f491df360c512433562f469) (cherry picked from commit f694a84faf082ce4a18cc2478d7843bb2b7e7fc4) (cherry picked from commit 0aea62ba3364192e70f426aaf1e8596f67fdc2c3) (cherry picked from commit 413d82c4620b5e2bcc7001f98a9f90bc109c1998)
System and Service Manager
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