The GNU GDB Debugger and NetBSD (Part 3)

August 04, 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 base-system version of GDB (GPLv3) still relies on a set of local patches. I set a goal to reduce the local patches to bare minimum, ideally reaching no local modifications at all.

GDB changes

I've written an integration of GDB with fork(2) and vfork(2) events. Unfortunately, this support (present in a local copy of GDB in the base-system) had not been merged so far, because there is a generic kernel regression with the pg_jobc variable. This variable can be called a reference counter of the number of processes within a process group that has a parent with control over a terminal. The semantics of this variable are not very well defined and in the result the number can become negative. This unexpected state of pg_jobc resulted in spurious crashes during kernel fuzzing. As a result new kernel assertions checking for non-negative pg_jobc values were introduced in order to catch the anomalies quickly. GDB as a ptrace(2)-based application happened to reproduce negative pg_jobc values quickly and reliably and this stopped the further adoption of the fork(2) and vfork(2) patch in GDB, until the pg_jobc behavior is enhanced. I was planning to include support for posix_spawn(3) events as well, as they are implemented as a first-class operation through a syscall, however this is also blocked by the pg_jobc blocker.

A local patch for GDB is stored here for the time being.

I've enable multi-process mode in the NetBSD native target. This enabled proper support for multiple inferiors and ptrace(2) assisted management of the inferior processes and their threads.

    (gdb) info inferior
      Num  Description       Connection           Executable
    * 1    process 14952     1 (native)           /usr/bin/dig
      2    <null>            1 (native)
      3    process 25684     1 (native)           /bin/ls
      4    <null>            1 (native)           /bin/ls

Without this change, additional inferiors could be already added, but not properly controlled.

I've implemented the xfer_partial TARGET_OBJECT_SIGNAL_INFO support for NetBSD. NetBSD implements reading and overwriting siginfo_t received by the tracee. With TARGET_OBJECT_SIGNAL_INFO signal information can be examined and modified through the special variable $_siginfo. Currently NetBSD uses an identical siginfo type on all architectures, so there is no support for architecture-specific fields.

(gdb) b main
Breakpoint 1 at 0x71a0
(gdb) r
Starting program: /bin/ps 

Breakpoint 1, 0x00000000002071a0 in main ()
(gdb) p $_siginfo
$1 = {si_pad = {5, 0, 0, 0, 1, 0 , 1, 0 }, _info = {_signo = 5, 
    _code = 1, _errno = 0, _pad = 0, _reason = {_rt = {_pid = 0, _uid = 0, _value = {sival_int = 1, 
          sival_ptr = 0x1}}, _child = {_pid = 0, _uid = 0, _status = 1, _utime = 0, _stime = 0}, 
      _fault = {_addr = 0x0, _trap = 1, _trap2 = 0, _trap3 = 0}, _poll = {_band = 0, _fd = 1}, 
      _syscall = {_sysnum = 0, _retval = {0, 1}, _error = 0, _args = {0, 0, 0, 0, 0, 0, 0, 0}}, 
      _ptrace_state = {_pe_report_event = 0, _option = {_pe_other_pid = 0, _pe_lwp = 0}}}}}

NetBSD, contrary to Linux and other BSDs, supports a ptrace(2) operation to generate a core(5) file from a running process. This operation is used in the base-system gcore(1) program. The gcore functionality is also delivered by GDB, and I have prepared new code for GDB to wire PT_DUMPCORE into the GDB code for NetBSD, and thus support GDB's gcore functionality. This patch is still waiting in upstream review. A local copy of the patch is here.

(gdb) r
Starting program: /bin/ps 

Breakpoint 1, 0x00000000002071a0 in main ()
(gdb) gcore
Saved corefile core.4378
(gdb) !file core.4378
core.4378: ELF 64-bit LSB core file, x86-64, version 1 (SYSV), NetBSD-style, from 'ps', pid=4378, uid=1000, gid=100, nlwps=1, lwp=4378 (signal 5/code 1)

Plan for the next milestone

Rewrite the gdbserver support and submit upstream. [0 comments]


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