Debugging userspace threads


The seL4 microkernel leverages the hardware debugging capabilities of modern processors, and exports hardware breakpoint, watchpoint, and single-stepping to user threads via conditionally compiled-in APIs. These kernel-level APIs can be used as the backend for implementing a full-featured debugger, or porting an existing one such as the GNU Debugger.

Enabling this feature

You can enable and disable the hardware debugging API by going through the kernel’s configuration system: make menuconfig -> seL4 Kernel -> Build Options -> Enable hardware breakpoint and single-stepping API.

Not all platforms support this feature for two main reasons:

  • The feature is gated behind certain hardware signals, such as #DBGEN, being active. If the hardware isn’t asserting these signals, the kernel will be unable to use them.
  • Your processor supports only the “Baseline” set of Coprocessor 14 registers, and doesn’t reliably expose the debug features through the debug coprocessor.

Caveat lector: If you compile the kernel with support for the debug API, and your ARM platform doesn’t support it, your kernel will abort at boot, with a message (or without a message if you are compiling the kernel in release mode (CONFIG_DEBUG_BUILD=n)).

Summary of the invocations

The invocations are documented in detail in the seL4 manual. This article will cover how to practically call them and use them in a prospective debugger.

To view a current version of the seL4 manuals, please download the kernel source code and then cd manual and execute make.

Additionally, there is available source code that demonstrates how the invocations can be used practically: the seL4-Test repository holds code tests the debug APIs, and shows how to use them to set breakpoints, watchpoints and single-stepping, on both x86 and ARM:

The invocations take capabilities to TCBs, and perform operations on the TCB register context to virtualize the hardware debug feature for each thread.

  • seL4_TCB_SetBreakpoint: Takes a capability to a TCB, and a hardware breakpoint register ID, and sets a breakpoint on a specified virtual address, for a range of addresses, for a certain access type (Read, Write, or both).
  • seL4_TCB_UnsetBreakpoint: Takes a capability to a TCB, and both disables and clears the specific hardware breakpoint for that thread.
  • seL4_TCB_GetBreakpoint: Takes a capability to a TCB, and returns information on whether or not the hardware breakpoint is enabled, and if enabled, what virtual address it will trigger on, and what types of accesses will trigger it.
  • seL4_TCB_ConfigureSingleStepping: Takes a capability to a TCB, and configures a hardware debugging register to break on every instruction (or every Nth instruction), and send a message on the thread’s fault endpoint everytime it faults. You may reply to the fault endpoint to resume the thread’s execution.