The linker reports "region FLASH overflowed by N bytes." What does that mean and what do you do?

It means the total size of everything you placed in the FLASH region (code, constants, the vector table, and the stored initial values of .data) exceeds the LENGTH you declared for FLASH in the linker script, your program doesn't fit the chip's flash. This is the linker doing you a favor by catching at build time what would otherwise be a corrupt or non-functional image. The responses, roughly in order: first verify the region size in the script actually matches the part (a wrong LENGTH is an easy mistake); then reduce the footprint, enable -ffunction-sections -fdata-sections with --gc-sections so unreferenced code/data is dropped, build with size optimization (-Os), switch to a smaller C library (newlib-nano, and avoid pulling in full floating-point printf), and remove dead code or large constant tables (or move them if there's a better region); inspect what's big with size and the linker map file (-Wl,-Map), which shows per-symbol and per-section sizes so you can find the offender; and if it genuinely needs more, choose a part with more flash. The analogous RAM overflow means your data/bss/stack don't fit RAM. The key mindset is that this is a real resource constraint surfaced early, not a tooling glitch, you analyze the map file to see where the space went and trim accordingly.