What does a linker script do, and why do you need a custom one on bare metal?

A linker script tells the linker the target's memory layout and how to place the program's output sections into it, essentially "here are the physical memory regions (flash, RAM) with their addresses and sizes, and here's which sections go where." Its MEMORY command declares regions like FLASH at 0x08000000 and RAM at 0x20000000 with their lengths, and its SECTIONS command assigns .text and .rodata to flash, .data and .bss to RAM, places the vector table at the start of flash, and defines symbols and the entry point. You need a custom one on bare metal because there's no OS to provide a default memory layout; the addresses are dictated by your specific chip's memory map, and the linker must produce an image whose vector table sits where the CPU looks at reset, whose code runs from the right flash address, and whose RAM sections are set up for the startup code to initialize. The default host linker script targets a hosted OS environment and is wrong for an MCU, using it (or a mismatched one) yields an image that doesn't boot, faults, or has uninitialized globals. So the linker script is the bridge between your compiled objects and the physical reality of the chip.