The asker is building a custom operating system for the RP2040 (ARM Cortex-M0+) and compiling with position-independent code flags. After returning to the project following a multi-year hiatus, the toolchain now emits a linker error: dangerous relocation: unsupported relocation (R_ARM_SBREL32). The code compiled fine before; nothing in the source changed. This is a toolchain regression or behavioral change surfacing a latent issue.

This question is interesting for several reasons:

• It sits at the intersection of compiler internals and embedded constraints. The Cortex-M0+ is a Thumb-only, ARMv6-M core with no ARM mode. PIC on this target is already fraught because the M0+ lacks certain addressing modes that make position-independent code natural on larger ARM cores.
• R_ARM_SBREL32 is a static-base-relative relocation. It computes an offset from a "static base" register (typically r9). GCC emits this when compiling with -msingle-pic-base or when the RWPI (Read-Write Position Independence) model is active. Newer GCC/Binutils versions may have changed how they emit or resolve these relocations for Thumb-only targets, which explains why the same flags broke after a toolchain update.
• The minimal reproducer likely involves a global or static variable access. SBREL relocations appear when the compiler needs to reference writable data relative to a base register rather than using a GOT. If the linker for this target doesn't support SBREL32 in the output format being used, it rejects it outright.

The path toward a solution involves a few steps:

• Inspect the compiler flags carefully. Flags like -fpic, -fpie, -msingle-pic-base, -mpic-register=r9, and -frwpi all influence which relocation types GCC emits. The asker should determine exactly which PIC model they need for their OS and use only that.
• Check the toolchain version delta. Running arm-none-eabi-objdump -r stat.o will reveal exactly which relocations the compiler is emitting. Comparing this output between the old and new toolchain versions pinpoints what changed.
• Consider switching PIC strategy. For Cortex-M0+ OS work, using GOT-based PIC (-fpic with a proper linker script) or manually managing position independence through a custom loader may be more robust than RWPI, which has always had patchy support on Thumb-only targets.

A key gotcha: even if you suppress this specific error, mixing RWPI and GOT-based relocations in the same binary can produce silently broken code. Every object file in the link must agree on the PIC model. Another edge case is that -msingle-pic-base assumes r9 is reserved and never clobbered, but some C library implementations or RTOS context-switch code may not preserve it.