most backup programs run with root or at the targets own user. There is the possibility to circumvent the permission checks: capabilities. the provide parts of

most backup programs run with root or at the targets own user. There is the possibility to circumvent the permission checks: capabilities. the provide parts of the root user to unprivileged processes, in your case CAP_DAC_READ_SEARCH should be enough for the backup process itself. for restore it would need a bumch of caps set, like CAP_DAC_OVERRIDE, CAP_CHOWN and maybe more