dangerous relocation: unsupported relocation (R_ARM_SBREL32)

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Summary

The postmortem analyzes a compilation failure on an ARM Cortex-M0+ project, where the linker flags a relocation type R_ARM_SBREL32 as “dangerous” and “unsupported.” The issue occurs because the developer used the -frwpi flag, which stands for Read-Only Write-Once Pointer Initialization. This flag is designed for a specific code model that requires the linker to support Static Base (SB) relative addressing. However, standard linkers (like GNU ld or LLVM ld.lld) configured for generic embedded targets often lack built-in support for generating the necessary SB initialization sequences or resolving these specific relocation types for executables. The root cause is a mismatch between the compiler flag selected and the capabilities of the default linker script and loader environment.

Root Cause

The immediate cause of the build failure is the invocation of the linker on object files containing R_ARM_SBREL32 relocations without a linker script or runtime environment that supports the RWPI (Read-Write Position Independent) code model.

  • Flag Mismatch: The -frwpi flag instructs the compiler to generate position-independent code where data accesses are relative to a register (usually r9 or sb). This creates R_ARM_SBREL32 relocations.
  • Linker Limitation: The linker (invoked as arm-none-eabi-ld) does not provide the necessary default support for this relocation in the output executable. It cannot calculate the offset relative to a Static Base that will be valid at runtime because the standard embedded ABI does not reserve r9 for this purpose by default.
  • Missing Runtime Support: For RWPI to work, the r9 register must be initialized at runtime to point to the Read-Only Write-Once data segment. The linker command provided does not include a linker script that handles this initialization, nor does the code provide a mechanism to set r9.

Why This Happens in Real Systems

In real-world embedded systems, developers often attempt to achieve Position Independent Code (PIC) on architectures without an MMU.

  • Text vs. Data Independence: Standard PIC (often via -fPIC) usually relies on the Processors Program Counter (PC) for code, but for data, if no MMU exists, you need a separate mechanism.
  • RWPI Implementation: ARM compilers (like ARMClang or GCC with specific specs) support RWPI. This model assumes a Static Base register (r9) points to the writable data segment.
  • Ecosystem Support: This feature requires tight integration between the compiler, linker, and startup code. If a developer enables the compiler flag but uses a generic “bare-metal” linker command without a tailored script, the toolchain chain breaks down.
  • Deprecation of R_ARM_SBREL32: In newer ARM specifications, R_ARM_SBREL32 has been superseded or is handled differently in strict ABI modes, leading modern linkers to reject it unless explicitly configured to handle legacy or specific RWPI scenarios.

Real-World Impact

If this issue is not resolved, it results in a complete inability to build the software.

  • Build Failure: The project cannot produce a binary, halting development entirely.
  • Code Model Confusion: The developer might waste time trying to fix linker flags or compiler versions, missing the architectural mismatch.
  • Runtime Instability: If one manages to force the linker to accept the relocation (e.g., by ignoring errors or using a different flag), the resulting binary will likely crash immediately upon boot because the r9 register will not be initialized. The CPU will attempt to access global variables relative to an undefined base address, leading to HardFaults or memory corruption.
  • Debuggability: The error message “dangerous relocation” is a warning that the relocation type might overflow or cannot be resolved. It is a critical stop sign from the linker indicating that the memory layout required by the code cannot be represented by the output format.

Example or Code

The user provided the minimal reproduction case effectively. The core logic is:

int bar;
void baz() {
    bar = 0;
}

Compiled with -frwpi. This generates a relocation for bar.

The specific error originates because the bar variable address is calculated relative to the Static Base (SB). The linker expects to perform a specific fixup that it does not know how to perform for the target executable format.

The relocation in the object file looks like this (as shown in the user’s output):
Offset Info Type Sym.Value Sym.Name
000000c 00000709 R_ARM_SBREL32 00000000 bar

The 0x09 type is R_ARM_SBREL32. The linker fails when it sees this in a non-shared object context without specific SB support enabled in the linker script.

How Senior Engineers Fix It

There are three distinct paths to fixing this, depending on the actual goal.

  1. The “Standard Bare-Metal” Fix (Recommended for RP2040):

    • Remove -frwpi: Unless you have a specific requirement for static base register usage, revert to standard absolute addressing or standard Position Independent Code (-fPIC or -fpie).
    • Use -fPIE: Generate position-independent code for the executable. The linker will then resolve relocations relative to the Program Counter (PC) or use GOT (Global Offset Table) mechanisms, which are supported by the standard linker.
    • Linker Script: Ensure you are using the standard pico linker script (provided by the Raspberry Pi Pico SDK), which handles memory layout correctly.

  2. The “Strict RWPI” Fix:

    • Custom Linker Script: You must write a linker script that defines a region for the Static Base.
    • Startup Code Modification: You must modify the startup assembly (e.g., crt0.s) to calculate the address of the data segment (or the SB region) and load it into the r9 register before C code execution (main or _start).
    • Linker Support: You may need to verify that your specific version of arm-none-eabi-ld supports --symdiff or specific RWPI emulation flags, though these are often handled by armclang rather than generic GNU ld.

  3. The “Quick Hack” (Not Recommended):

    • Force the linker to treat the object as position-independent using -r (partial link) first, then link again, or use --no-warn-rwx-segments (though this won’t fix the relocation logic).

For the RP2040 (Cortex-M0+): The best fix is Option 1. Remove -frwpi. Cortex-M0+ is usually simple absolute code or standard PC-relative code. RWPI is rare on M0+ unless writing a custom OS kernel that requires shared libraries without an MMU.

Why Juniors Miss It

  • Cargo Culting: Developers often copy compiler flags from tutorials or older projects without understanding what they do. -frwpi looks like “RWPI” which sounds like “Position Independent,” which sounds good for OS development.
  • Reading the Docs: The error dangerous relocation is a linker error. Juniors often focus on the C code or the compiler flags, not realizing the linker is the actor enforcing the architecture constraints.
  • Assuming Toolchain Magic: There is a belief that if you pass a flag like -frwpi, the compiler and linker will automatically “make it work” (like -fPIC often does on Linux). It fails to realize that -frwpi implies a very specific System ABI (Static Base usage) that the standard arm-none-eabi toolchain does not implement by default.
  • Confusion between -fPIC and -frwpi: These are two different ways to achieve independence. -fPIC uses PC-relative addressing (mostly safe). -frwpi uses a register-relative addressing (requires runtime setup). Juniors often don’t know the difference.