The GNU GDB Debugger and NetBSD (Part 1)

April 02, 2020 posted by Kamil Rytarowski

The NetBSD team of developers maintains two copies of GDB:
  • One in the base-system with a stack of local patches.
  • One in pkgsrc with mostly build fix patches.

The process of maintaining a modern version (GPLv3) of GDB in basesystem is tainted with a constant extra cost. The NetBSD developers need to rebase the stack of local patches for the newer releases of the debugger and resurrect the support. The GDB project is under an active development and in active refactoring of the code, that was originally written in C, to C++.

Unfortunately we cannot abandon the local basesystem patches and rely on a pristine version as there is lack of feature parity in the pkgsrc version of GDB: no threading support, not operational support for most targets, no fork/vfork/etc events support, no auxv reading support on 64-bit kernels, no proper support of signals, single step etc.

Additionally there are extra GDB patches stored in pkgsrc-wip (created by me last year), that implement the gdbserver support for NetBSD/amd64. gdbserver is a GDB version that makes it possible to remotely debug other programs even across different Operating Systems and CPUs. This code has still not been merged into the mainline base-system version. This month, I have discovered that support needs to be reworked, as the preexisting source code directory hierarchy was rearranged.

Unless otherwise specified all the following changes were upstreamed to the mainstream GDB repository. According to the GDB schedule, the GDB10 branch point is planned on 2020-05-15 with release on 2020-06-05. It's a challenge to see how much the GDB support can be improved by then for NetBSD!


The GDB debugger contains PSIM (Model of the PowerPC Architecture) originally developed by Andrew Cagney between 1994 and 1996. This is a simulator that contains, among other things, NetBSD support in the UEA mode. This means that GDB can run static programs prebuilt for NetBSD without execution on a real PowerPC hardware. In order to make it work, there is need to wrap the kernel interfaces such as syscalls, errno values and signals and handle them in the simulator.

I have updated the list of errno names and signal names with NetBSD 9.99.49.

It would be nice to still update the list of syscalls to reflect the current kernels, but I have deferred this into future.

bfd changes

The AArch64 (NetBSD/evbarm) target uses PT_GETREGS and PT_GETFPREGS operation names with the same Machine Dependent values as NetBSD/alpha and NetBSD/sparc. This knowledge is required as these values are used in core(5) files, as emitted by a crashing program. I've added a patch that recognizes these ELF notes in arm64 coredumps appropriately.

I've also added a new define constant NT_NETBSDCORE_AUXV. This allows properly identifying AUXV ELF notes in core files. Meanwhile I have implemented and added detection of LWPSTATUS notes. This note ships with meta information (name, signal context, TLS base, etc) about threads in a process in a core.

The number of ARM and MIPS boards supported by NetBSD is huge and there are multiple variations of them. I have fixed the detection macro in bfd to recognize more arm and mips NetBSD installations.

GDB/NetBSD fixes in CPU specific files

I have reached the state of GDB being more operational for more NetBSD ports out of the box. There were missing features and build issues that has been addressed. I have committed the following changes:

Now support for NetBSD in various CPU-specific files improved significantly, however there are still missing features, especially KGDB debugging and unwinding the stack over the signal trampoline. There are still smaller or larger changes that might be needed on per-port basis and I will keep working on them. There is need to develop at least proper aarch64 support as it is missing upstream. We might evaluate what to do with at least Itanium and RISCV.

CPU Generic improvements in the GDB codebase

I've switched the nbsd_nat_target::pid_to_exec_file() function from a logic of reading the /proc entries to a sysctl(3) based solution.

As the gdbserver support is around the corner, I have improved small parts of the code base to be compatibile with NetBSD. I've fixed the unconditional inclusion of alloca.h in gdbsupport. Another fix namespaced a local class reg, because it conflicted with the struct reg from the NetBSD headers.

The current logic of get_ptrace_pid function matches the semantics of other kernels suchs as Linux and FreeBSD. With the guidance of upstream developers, I have disabled this function completely for NetBSD instead of patching it for the NetBSD specific behavior of maintaining pairs PID+LWP for each internal ptid_t entry (that reflects the relation of PID, LWP and TID).

Plan for the next milestone

Finish reimplementing operational support of debugging of multi-threaded programs and upstream more patches, especially CPU-independent ones. [0 comments]


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