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stm32wrapper4dbg

A tool that adds a debug wrapper to a stm32 fsbl image.

Tool description

To handle secure boot, STM32 MPU prevents the debugger to halt the system at reset vector. This cause the debugger to halt each core at a random address, as soon as it get access to the debug port. Such random halt address is not optimal to debug the early boot steps of either the First-Stage Boot Loader (fsbl) (e.g. TF-A) or the Second-Stage Boot Loader (e.g. U-Boot).

A common workaround consists by adding an infinite loop at the entry of fsbl, so the debugger can attach, halt and skip the loop. This has the drawback of requiring modification in the source code of fsbl.

stm32wrapper4dbg adds a binary wrapper to an existing fsbl image in stm32 binary format and generates a new wrapped stm32 image. The wrapped image is intended to replace the existing fsbl image flashed on STM32 MPU board during the debug activity.

The wrapper code is executed before the fsbl code and:

  • enables the debug port (JTAG and SWD);
  • waits at maximum two seconds for a debugger to attach and to toggle debug claim bit zero, then jumps to fsbl;
  • if the debugger has requested to halt, then the boot will halt at the very first instruction of fsbl. If no debugger is detected or the debugger has not requested any halt, the fsbl is executed.

ATTENTION: on devices closed with OTP, the debug port is not accessible, for obvious security reasons. A wrapped image created with stm32wrapper4dbg will open the debug port. A wrapped image must be signed with the user key to be used in the secured boot. Do not distribute the signed wrapped image to prevent its use to compromise the secure boot.

Getting Started

Prerequisite

The fsbl in stm32 binary image format must be available.

Installing

Simply run:

make

then copy the file stm32wrapper4dbg and the man page file stm32wrapper4dbg.1.

Running stm32wrapper4dbg

Check the man page stm32wrapper4dbg (1) for syntax and examples.

Contributing

See contributing.md for contribution guidelines to stm32wrapper4dbg.

Maintainers

License

The tool stm32wrapper4dbg is dual-licensed under GPL-2.0-or-later OR BSD-3-Clause.

You may use this work according to either of these licenses as is most appropriate for your project on a case-by-case basis.