ISR runs with global interrupts initially enabled. The interrupt enable flag is activated by the compiler as early as possible within the ISR to ensure minimal processing delay for nested interrupts.
This may be used to create nested ISRs, however care should be taken to avoid stack overflows, or to avoid infinitely entering the ISR for those cases where the AVR hardware does not clear the respective interrupt flag before entering the ISR.
Use this attribute in the attributes parameter of the ISR macro.