RE: [PATCH 10/10] Add minimal and functional NetBSD/amd64 gdbserver

["Aktemur, Tankut Baris" <tankut.baris.aktemur@intel.com>]

On September 2, 2020 7:59 PM, Kamil Rytarowski wrote:
> Implement the following functionality: create_inferior,
> post_create_inferior, attach, kill, detach, mourn, join, thread_alive,
> resume, wait, fetch_registers, store_registers, read_memory, write_memory,
> request_interrupt, supports_read_auxv, read_auxv,
> supports_hardware_single_step, sw_breakpoint_from_kind,
> supports_z_point_type, insert_point, remove_point,
> stopped_by_sw_breakpoint, supports_qxfer_siginfo, qxfer_siginfo,
> supports_stopped_by_sw_breakpoint, supports_non_stop,
> supports_multi_process, supports_fork_events, supports_vfork_events,
> supports_exec_events, supports_disable_randomization,
> supports_qxfer_libraries_svr4, qxfer_libraries_svr4,
> supports_pid_to_exec_file, pid_to_exec_file, thread_name,
> supports_catch_syscall.
> 
> The only CPU architecture supported: x86_64.
> 
> Implement only support for hardware assisted single step and
> software breakpoint.
> 
> Implement support only for regular X86 registers, thus no FPU.
> 
> gdb/ChangeLog:

This should be gdbserver/ChangeLog.

> 
>        * netbsd-low.cc: Add.
>        * netbsd-low.h: Likewise.
>        * netbsd-x86_64-low.cc: Likewise.
>        * Makefile.in (SFILES): Register "netbsd-low.cc", "netbsd-low.h",
>        "netbsd-x86_64-low.cc".
>        * configure.srv: Add x86_64-*-netbsd*.
> ---
>  gdbserver/ChangeLog            |    9 +
>  gdbserver/Makefile.in          |    3 +
>  gdbserver/configure.srv        |    7 +
>  gdbserver/netbsd-low.cc        | 1352 ++++++++++++++++++++++++++++++++
>  gdbserver/netbsd-low.h         |  157 ++++
>  gdbserver/netbsd-x86_64-low.cc |  250 ++++++
>  6 files changed, 1778 insertions(+)
>  create mode 100644 gdbserver/netbsd-low.cc
>  create mode 100644 gdbserver/netbsd-low.h
>  create mode 100644 gdbserver/netbsd-x86_64-low.cc
> 
> diff --git a/gdbserver/ChangeLog b/gdbserver/ChangeLog
> index e437493b56d..c0732d1a28a 100644
> --- a/gdbserver/ChangeLog
> +++ b/gdbserver/ChangeLog
> @@ -1,3 +1,12 @@
> +2020-08-13  Kamil Rytarowski  <n54@gmx.com>
> +
> +	* netbsd-low.cc: Add.
> +	* netbsd-low.h: Likewise.
> +	* netbsd-x86_64-low.cc: Likewise.
> +	* Makefile.in (SFILES): Register "netbsd-low.cc", "netbsd-low.h",
> +	"netbsd-x86_64-low.cc".
> +	* configure.srv: Add x86_64-*-netbsd*.
> +
>  2020-08-13  Simon Marchi  <simon.marchi@polymtl.ca>
> 
>  	* server.cc (captured_main): Accept multiple `--selftest=`
> diff --git a/gdbserver/Makefile.in b/gdbserver/Makefile.in
> index 9d7687be534..1d597b14523 100644
> --- a/gdbserver/Makefile.in
> +++ b/gdbserver/Makefile.in
> @@ -193,6 +193,9 @@ SFILES = \
>  	$(srcdir)/linux-x86-low.cc \
>  	$(srcdir)/linux-xtensa-low.cc \
>  	$(srcdir)/mem-break.cc \
> +	$(srcdir)/netbsd-low.cc \
> +	$(srcdir)/netbsd-low.h \
> +	$(srcdir)/netbsd-x86_64-low.cc \
>  	$(srcdir)/proc-service.cc \
>  	$(srcdir)/proc-service.list \
>  	$(srcdir)/regcache.cc \
> diff --git a/gdbserver/configure.srv b/gdbserver/configure.srv
> index 5e33bd9c54d..12f9d410b7c 100644
> --- a/gdbserver/configure.srv
> +++ b/gdbserver/configure.srv
> @@ -349,6 +349,13 @@ case "${gdbserver_host}" in
>  			srv_tgtobj="${srv_tgtobj} nat/windows-nat.o"
>  			srv_tgtobj="${srv_tgtobj} arch/amd64.o arch/i386.o"
>  			;;
> +  x86_64-*-netbsd*)	srv_regobj=""
> +			srv_tgtobj="netbsd-low.o netbsd-x86_64-low.o fork-child.o"
> +			srv_tgtobj="${srv_tgtobj} nat/fork-inferior.o"
> +			srv_tgtobj="${srv_tgtobj} nat/netbsd-nat.o"
> +			srv_tgtobj="${srv_tgtobj} arch/amd64.o"
> +			srv_netbsd=yes

Is `srv_netbsd` used anywhere?

> +			;;
> 
>    xtensa*-*-linux*)	srv_regobj=reg-xtensa.o
>  			srv_tgtobj="$srv_linux_obj linux-xtensa-low.o"
> diff --git a/gdbserver/netbsd-low.cc b/gdbserver/netbsd-low.cc
> new file mode 100644
> index 00000000000..db9531c66d3
> --- /dev/null
> +++ b/gdbserver/netbsd-low.cc
> @@ -0,0 +1,1352 @@
> +/* Copyright (C) 2020 Free Software Foundation, Inc.
> +
> +   This file is part of GDB.
> +
> +   This program is free software; you can redistribute it and/or modify
> +   it under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +#include "server.h"
> +#include "target.h"
> +#include "netbsd-low.h"
> +#include "nat/netbsd-nat.h"
> +
> +#include <sys/param.h>
> +#include <sys/types.h>
> +
> +#include <sys/ptrace.h>
> +#include <sys/sysctl.h>
> +
> +#include <limits.h>
> +#include <unistd.h>
> +#include <signal.h>
> +
> +#include <elf.h>
> +
> +#include <type_traits>
> +
> +#include "gdbsupport/eintr.h"
> +#include "gdbsupport/gdb_wait.h"
> +#include "gdbsupport/filestuff.h"
> +#include "gdbsupport/common-inferior.h"
> +#include "nat/fork-inferior.h"
> +#include "hostio.h"
> +
> +int using_threads = 1;
> +
> +const struct target_desc *netbsd_tdesc;
> +
> +/* Call add_process with the given parameters, and initializes

To be consistent with the imperative "Call", "initializes" could read
"initialize".

> +   the process' private data.  */
> +
> +static struct process_info *
> +netbsd_add_process (int pid, int attached)

There are two uses of this function and in both the return value is ignored.
The function could as well be void.

> +{
> +  struct process_info *proc = add_process (pid, attached);
> +  proc->tdesc = netbsd_tdesc;
> +  proc->priv = nullptr;
> +
> +  return proc;
> +}
> +
> +/* Callback used by fork_inferior to start tracing the inferior.  */
> +
> +static void
> +netbsd_ptrace_fun ()
> +{
> +  /* Switch child to its own process group so that signals won't
> +     directly affect GDBserver. */
> +  if (setpgid (0, 0) < 0)
> +    trace_start_error_with_name (("setpgid"));
> +
> +  if (ptrace (PT_TRACE_ME, 0, nullptr, 0) < 0)
> +    trace_start_error_with_name (("ptrace"));
> +
> +  /* If GDBserver is connected to gdb via stdio, redirect the inferior's
> +     stdout to stderr so that inferior i/o doesn't corrupt the connection.
> +     Also, redirect stdin to /dev/null.  */
> +  if (remote_connection_is_stdio ())
> +    {
> +      if (close (0) < 0)
> +	trace_start_error_with_name (("close"));
> +      if (open ("/dev/null", O_RDONLY) < 0)
> +	trace_start_error_with_name (("open"));
> +      if (dup2 (2, 1) < 0)
> +	trace_start_error_with_name (("dup2"));
> +      if (write (2, "stdin/stdout redirected\n",
> +		 sizeof ("stdin/stdout redirected\n") - 1) < 0)
> +	{
> +	  /* Errors ignored.  */;

Any particular reason for the explicit ';'?

> +	}
> +    }
> +}
> +
> +/* Register threads in the process specified by PID.  */
> +
> +static void
> +netbsd_add_threads (pid_t pid)
> +{
> +  auto fn
> +    = [] (ptid_t ptid)
> +      {
> +	add_thread (ptid, nullptr);
> +      };
> +
> +  netbsd_nat::list_threads (pid, fn);

IMHO, the name "list_threads" gives the impression that it only lists the threads
and is side-effect free.  Maybe use something like "for_each_thread", as in gdbthread.h?

> +}
> +
> +/* Implement the create_inferior method of the target_ops vector.  */
> +
> +int
> +netbsd_process_target::create_inferior (const char *program,
> +					const std::vector<char *> &program_args)
> +{
> +  std::string str_program_args = construct_inferior_arguments (program_args);
> +
> +  pid_t pid = fork_inferior (program, str_program_args.c_str (),
> +			     get_environ ()->envp (), netbsd_ptrace_fun,
> +			     nullptr, nullptr, nullptr, nullptr);
> +
> +  netbsd_add_process (pid, 0);
> +
> +  post_fork_inferior (pid, program);
> +
> +  return pid;
> +}
> +
> +/* Implement the post_create_inferior target_ops method.  */
> +
> +void
> +netbsd_process_target::post_create_inferior ()
> +{
> +  pid_t pid = current_process ()->pid;
> +  netbsd_nat::enable_proc_events (pid);
> +}
> +
> +/* Implement the attach target_ops method.  */
> +
> +int
> +netbsd_process_target::attach (unsigned long pid)
> +{
> +
> +  if (ptrace (PT_ATTACH, pid, nullptr, 0) != 0)
> +    error ("Cannot attach to process %lu: %s (%d)\n", pid,
> +	    safe_strerror (errno), errno);
> +
> +  netbsd_add_process (pid, 1);
> +  netbsd_nat::enable_proc_events (pid);
> +  netbsd_add_threads (pid);
> +
> +  return 0;
> +}
> +
> +/* Returns true if GDB is interested in any child syscalls.  */
> +
> +static bool
> +gdb_catching_syscalls_p (pid_t pid)
> +{
> +  struct process_info *proc = find_process_pid (pid);
> +  return !proc->syscalls_to_catch.empty ();
> +}
> +
> +/* Implement the resume target_ops method.  */
> +
> +void
> +netbsd_process_target::resume (struct thread_resume *resume_info, size_t n)
> +{
> +  ptid_t resume_ptid = resume_info[0].thread;
> +  const int signal = resume_info[0].sig;
> +  const bool step = resume_info[0].kind == resume_step;
> +
> +  if (resume_ptid == minus_one_ptid)
> +    resume_ptid = ptid_of (current_thread);
> +
> +  const pid_t pid = resume_ptid.pid ();
> +  const lwpid_t lwp = resume_ptid.lwp ();
> +  regcache_invalidate_pid (pid);
> +
> +  auto fn
> +    = [&] (ptid_t ptid)
> +      {
> +	if (step)
> +	  {
> +	    if (ptid.lwp () == lwp || n != 1)
> +	      {
> +		if (ptrace (PT_SETSTEP, pid, NULL, ptid.lwp ()) == -1)
> +		  perror_with_name (("ptrace"));
> +		if (ptrace (PT_RESUME, pid, NULL, ptid.lwp ()) == -1)
> +		  perror_with_name (("ptrace"));
> +	      }
> +	    else
> +	      {
> +		if (ptrace (PT_CLEARSTEP, pid, NULL, ptid.lwp ()) == -1)
> +		  perror_with_name (("ptrace"));
> +		if (ptrace (PT_SUSPEND, pid, NULL, ptid.lwp ()) == -1)
> +		  perror_with_name (("ptrace"));
> +	      }
> +	  }
> +	else
> +	  {
> +	    if (ptrace (PT_CLEARSTEP, pid, NULL, ptid.lwp ()) == -1)
> +	      perror_with_name (("ptrace"));
> +	    if (ptrace (PT_RESUME, pid, NULL, ptid.lwp ()) == -1)
> +	      perror_with_name (("ptrace"));
> +	  }
> +      };
> +
> +  netbsd_nat::list_threads (pid, fn);
> +
> +  int request = gdb_catching_syscalls_p (pid) ? PT_CONTINUE : PT_SYSCALL;
> +
> +  errno = 0;
> +  ptrace (request, pid, (void *)1, signal);
> +  if (errno)
> +    perror_with_name (("ptrace"));
> +}
> +
> +/* Returns true if GDB is interested in the reported SYSNO syscall.  */
> +
> +static bool
> +netbsd_catch_this_syscall (int sysno)
> +{
> +  struct process_info *proc = current_process ();
> +
> +  if (proc->syscalls_to_catch.empty ())
> +    return false;
> +
> +  if (proc->syscalls_to_catch[0] == ANY_SYSCALL)
> +    return true;
> +
> +  for (int iter : proc->syscalls_to_catch)
> +    if (iter == sysno)
> +      return true;
> +
> +  return false;
> +}
> +
> +/* Helper function for child_wait and the derivatives of child_wait.
> +   HOSTSTATUS is the waitstatus from wait() or the equivalent; store our
> +   translation of that in OURSTATUS.  */

A blank line here between the comment and the function header.

> +static void
> +netbsd_store_waitstatus (struct target_waitstatus *ourstatus, int hoststatus)
> +{
> +  if (WIFEXITED (hoststatus))
> +    {
> +      ourstatus->kind = TARGET_WAITKIND_EXITED;
> +      ourstatus->value.integer = WEXITSTATUS (hoststatus);
> +    }
> +  else if (!WIFSTOPPED (hoststatus))
> +    {
> +      ourstatus->kind = TARGET_WAITKIND_SIGNALLED;
> +      ourstatus->value.sig = gdb_signal_from_host (WTERMSIG (hoststatus));
> +    }
> +  else
> +    {
> +      ourstatus->kind = TARGET_WAITKIND_STOPPED;
> +      ourstatus->value.sig = gdb_signal_from_host (WSTOPSIG (hoststatus));
> +    }
> +}
> +
> +/* Implement a safe wrapper around waitpid().  */
> +
> +static pid_t
> +netbsd_waitpid (ptid_t ptid, struct target_waitstatus *ourstatus, int options)
> +{
> +  int status;
> +
> +  pid_t pid = gdb::handle_eintr (::waitpid, ptid.pid (), &status, options);
> +
> +  if (pid == -1)
> +    perror_with_name (_("Child process unexpectedly missing"));
> +
> +  netbsd_store_waitstatus (ourstatus, status);
> +  return pid;
> +}
> +
> +
> +/* Implement the wait target_ops method.
> +
> +   Wait for the child specified by PTID to do something.  Return the
> +   process ID of the child, or MINUS_ONE_PTID in case of error; store
> +   the status in *OURSTATUS.  */
> +
> +static ptid_t
> +netbsd_wait (ptid_t ptid, struct target_waitstatus *ourstatus,
> +	     int target_options)
> +{
> +  pid_t pid = netbsd_waitpid (ptid, ourstatus, target_options);
> +  ptid_t wptid = ptid_t (pid);
> +
> +  if (pid == 0)
> +    {
> +      gdb_assert (target_options & TARGET_WNOHANG);
> +      ourstatus->kind = TARGET_WAITKIND_IGNORE;
> +      return null_ptid;
> +    }
> +
> +  gdb_assert (pid != -1);
> +
> +  /* If the child stopped, keep investigating its status.  */
> +  if (ourstatus->kind != TARGET_WAITKIND_STOPPED)
> +    return wptid;
> +
> +  /* Extract the event and thread that received a signal.  */
> +  ptrace_siginfo_t psi;
> +  if (ptrace (PT_GET_SIGINFO, pid, &psi, sizeof (psi)) == -1)
> +    perror_with_name (("ptrace"));
> +
> +  /* Pick child's siginfo_t.  */
> +  siginfo_t *si = &psi.psi_siginfo;
> +
> +  lwpid_t lwp = psi.psi_lwpid;
> +
> +  int signo = si->si_signo;
> +  const int code = si->si_code;
> +
> +  /* Construct PTID with a specified thread that received the event.
> +     If a signal was targeted to the whole process, lwp is 0.  */
> +  wptid = ptid_t (pid, lwp, 0);
> +
> +  /* Bail out on non-debugger oriented signals..  */

Double periods.

> +  if (signo != SIGTRAP)
> +    return wptid;
> +
> +  /* Stop examining non-debugger oriented SIGTRAP codes.  */
> +  if (code <= SI_USER || code == SI_NOINFO)
> +    return wptid;
> +
> +  /* Process state for threading events */

End with dot-space-space?

> +  ptrace_state_t pst = {};
> +  if (code == TRAP_LWP)
> +    {
> +      if (ptrace (PT_GET_PROCESS_STATE, pid, &pst, sizeof (pst)) == -1)
> +	perror_with_name (("ptrace"));
> +    }

I think braces can be removed.

> +
> +  if (code == TRAP_LWP && pst.pe_report_event == PTRACE_LWP_EXIT)
> +    {
> +      /* If GDB attaches to a multi-threaded process, exiting
> +	 threads might be skipped during post_attach that
> +	 have not yet reported their PTRACE_LWP_EXIT event.
> +	 Ignore exited events for an unknown LWP.  */
> +      thread_info *thr = find_thread_ptid (wptid);
> +      if (thr == nullptr)
> +	  ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
> +      else
> +	{
> +	  ourstatus->kind = TARGET_WAITKIND_THREAD_EXITED;
> +	  /* NetBSD does not store an LWP exit status.  */
> +	  ourstatus->value.integer = 0;
> +
> +	  remove_thread (thr);
> +	}
> +      return wptid;
> +    }
> +
> +  if (find_thread_ptid (ptid_t (pid)))
> +    current_thread = find_thread_ptid (wptid);
> +
> +  if (code == TRAP_LWP && pst.pe_report_event == PTRACE_LWP_CREATE)
> +    {
> +      /* If GDB attaches to a multi-threaded process, newborn
> +	 threads might be added by nbsd_add_threads that have
> +	 not yet reported their PTRACE_LWP_CREATE event.  Ignore
> +	 born events for an already-known LWP.  */
> +      if (find_thread_ptid (wptid))
> +	  ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
> +      else
> +	{
> +	  add_thread (wptid, NULL);
> +	  ourstatus->kind = TARGET_WAITKIND_THREAD_CREATED;
> +	}
> +      return wptid;
> +    }
> +
> +  if (code == TRAP_EXEC)
> +    {
> +      ourstatus->kind = TARGET_WAITKIND_EXECD;
> +      ourstatus->value.execd_pathname
> +	= xstrdup (netbsd_nat::pid_to_exec_file (pid));
> +      return wptid;
> +    }
> +
> +  if (code == TRAP_TRACE)
> +    {
> +      return wptid;
> +    }

Braces can be removed.

> +
> +  if (code == TRAP_SCE || code == TRAP_SCX)
> +    {
> +      int sysnum = si->si_sysnum;
> +
> +      if (!netbsd_catch_this_syscall(sysnum))
> +	{
> +	  /* If the core isn't interested in this event, ignore it.  */
> +	  ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
> +	  return wptid;
> +	}
> +
> +      ourstatus->kind =

The assignment operator should be on the next line.

> +	(code == TRAP_SCE) ? TARGET_WAITKIND_SYSCALL_ENTRY :
> +	TARGET_WAITKIND_SYSCALL_RETURN;
> +      ourstatus->value.syscall_number = sysnum;
> +      return wptid;
> +    }
> +
> +  if (code == TRAP_BRKPT)
> +    {
> +#ifdef PTRACE_BREAKPOINT_ADJ
> +      CORE_ADDR pc;
> +      struct reg r;
> +      ptrace (PT_GETREGS, pid, &r, psi.psi_lwpid);
> +      pc = PTRACE_REG_PC (&r);
> +      PTRACE_REG_SET_PC (&r, pc - PTRACE_BREAKPOINT_ADJ);
> +      ptrace (PT_SETREGS, pid, &r, psi.psi_lwpid);
> +#endif
> +      return wptid;
> +    }
> +
> +  /* Unclassified SIGTRAP event.  */
> +  ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
> +  return wptid;
> +}
> +
> +/* Implement the wait target_ops method.  */
> +
> +ptid_t
> +netbsd_process_target::wait (ptid_t ptid, struct target_waitstatus *ourstatus,
> +			     int target_options)
> +{
> +  while (true)
> +    {
> +      ptid_t wptid = netbsd_wait (ptid, ourstatus, target_options);
> +
> +      /* Register thread in the gdbcore if a thread was not reported earlier.
> +	 This is required after ::create_inferior, when the gdbcore does not
> +	 know about the first internal thread.
> +	 This may also happen on attach, when an event is registered on a thread
> +	 that was not fully initialized during the attach stage. */

Dot-space-space.

> +      if (wptid.lwp () != 0 && !find_thread_ptid (wptid)
> +	  && ourstatus->kind != TARGET_WAITKIND_THREAD_EXITED)
> +	add_thread (wptid, nullptr);
> +
> +      switch (ourstatus->kind)
> +	{
> +	case TARGET_WAITKIND_EXITED:
> +	case TARGET_WAITKIND_STOPPED:
> +	case TARGET_WAITKIND_SIGNALLED:
> +	case TARGET_WAITKIND_FORKED:
> +	case TARGET_WAITKIND_VFORKED:
> +	case TARGET_WAITKIND_EXECD:
> +	case TARGET_WAITKIND_VFORK_DONE:
> +	case TARGET_WAITKIND_SYSCALL_ENTRY:
> +	case TARGET_WAITKIND_SYSCALL_RETURN:
> +	  /* Pass the result to the generic code.  */
> +	  return wptid;
> +	case TARGET_WAITKIND_THREAD_CREATED:
> +	case TARGET_WAITKIND_THREAD_EXITED:
> +	  /* The core needlessly stops on these events.  */
> +	  /* FALLTHROUGH */
> +	case TARGET_WAITKIND_SPURIOUS:
> +	  /* Spurious events are unhandled by the gdbserver core.  */
> +	  if (ptrace (PT_CONTINUE, current_process ()->pid, (void *) 1, 0)
> +	      == -1)
> +	    perror_with_name (("ptrace"));
> +	  break;
> +	default:
> +	  error (("Unknwon stopped status"));

Typo "Unknwon".

> +	}
> +    }
> +}
> +
> +/* Implement the kill target_ops method.  */
> +
> +int
> +netbsd_process_target::kill (process_info *process)
> +{
> +  pid_t pid = process->pid;
> +  ptrace (PT_KILL, pid, nullptr, 0);
> +
> +  int status;
> +  gdb::handle_eintr (::waitpid, pid, &status, 0);

The "kill" target op is supposed to return -1 on failure.
So, maybe not ignore the return value of handle_eintr here?

> +  mourn (process);
> +  return 0;
> +}
> +
> +/* Implement the detach target_ops method.  */
> +
> +int
> +netbsd_process_target::detach (process_info *process)
> +{
> +  pid_t pid = process->pid;
> +
> +  ptrace (PT_DETACH, pid, (void *) 1, 0);
> +  mourn (process);
> +  return 0;
> +}
> +
> +/* Implement the mourn target_ops method.  */
> +
> +void
> +netbsd_process_target::mourn (struct process_info *proc)
> +{
> +  for_each_thread (proc->pid, remove_thread);
> +
> +  remove_process (proc);
> +}
> +
> +/* Implement the join target_ops method.  */
> +
> +void
> +netbsd_process_target::join (int pid)
> +{
> +  /* The PT_DETACH is sufficient to detach from the process.
> +     So no need to do anything extra.  */
> +}
> +
> +/* Implement the thread_alive target_ops method.  */
> +
> +bool
> +netbsd_process_target::thread_alive (ptid_t ptid)
> +{
> +  return netbsd_nat::thread_alive (ptid);
> +}
> +
> +/* Implement the fetch_registers target_ops method.  */
> +
> +void
> +netbsd_process_target::fetch_registers (struct regcache *regcache, int regno)
> +{
> +  struct netbsd_regset_info *regset = netbsd_target_regsets;
> +  ptid_t inferior_ptid = ptid_of (current_thread);
> +
> +  while (regset->size >= 0)
> +    {
> +      char *buf = (char *) xmalloc (regset->size);
> +      int res = ptrace (regset->get_request, inferior_ptid.pid (), buf,
> +			inferior_ptid.lwp ());
> +      if (res == -1)
> +	perror_with_name (("ptrace"));
> +      regset->store_function (regcache, buf);
> +      free (buf);
> +      regset++;
> +    }
> +}
> +
> +/* Implement the store_registers target_ops method.  */
> +
> +void
> +netbsd_process_target::store_registers (struct regcache *regcache, int regno)
> +{
> +  struct netbsd_regset_info *regset = netbsd_target_regsets;
> +  ptid_t inferior_ptid = ptid_of (current_thread);
> +
> +  while (regset->size >= 0)
> +    {
> +      char *buf = (char *)xmalloc (regset->size);
> +      int res = ptrace (regset->get_request, inferior_ptid.pid (), buf,
> +			inferior_ptid.lwp ());
> +      if (res == -1)
> +	perror_with_name (("ptrace"));
> +      if (res == 0)
> +	{
> +	  /* Then overlay our cached registers on that.  */
> +	  regset->fill_function (regcache, buf);
> +	  /* Only now do we write the register set.  */
> +	  res = ptrace (regset->set_request, inferior_ptid.pid (), buf,
> +			inferior_ptid.lwp ());
> +	}
> +      free (buf);
> +      regset++;
> +    }
> +}
> +
> +/* Implement the read_memory target_ops method.  */
> +
> +int
> +netbsd_process_target::read_memory (CORE_ADDR memaddr, unsigned char *myaddr,
> +				    int size)
> +{
> +  struct ptrace_io_desc io;
> +  io.piod_op = PIOD_READ_D;
> +  io.piod_len = size;
> +
> +  pid_t pid = current_process ()->pid;
> +
> +  int bytes_read = 0;
> +
> +  if (size == 0)
> +    {
> +      /* Zero length write always succeeds.  */
> +      return 0;
> +    }
> +  do
> +    {
> +      io.piod_offs = (void *)(memaddr + bytes_read);
> +      io.piod_addr = myaddr + bytes_read;
> +
> +      int rv = ptrace (PT_IO, pid, &io, 0);
> +      if (rv == -1)
> +	return errno;
> +      if (io.piod_len == 0)
> +	return 0;
> +
> +      bytes_read += io.piod_len;
> +      io.piod_len = size - bytes_read;
> +    }
> +  while (bytes_read < size);
> +
> +  return 0;
> +}
> +
> +/* Implement the write_memory target_ops method.  */
> +
> +int
> +netbsd_process_target::write_memory (CORE_ADDR memaddr,
> +				     const unsigned char *myaddr, int size)
> +{
> +  struct ptrace_io_desc io;
> +  io.piod_op = PIOD_WRITE_D;
> +  io.piod_len = size;
> +
> +  pid_t pid = current_process ()->pid;
> +
> +  int bytes_written = 0;
> +
> +  if (size == 0)
> +    {
> +      /* Zero length write always succeeds.  */
> +      return 0;
> +    }
> +
> +  do
> +    {
> +      io.piod_addr = (void *)(myaddr + bytes_written);
> +      io.piod_offs = (void *)(memaddr + bytes_written);
> +
> +      int rv = ptrace (PT_IO, pid, &io, 0);
> +      if (rv == -1)
> +	return errno;
> +      if (io.piod_len == 0)
> +	return 0;
> +
> +      bytes_written += io.piod_len;
> +      io.piod_len = size - bytes_written;
> +    }
> +  while (bytes_written < size);
> +
> +  return 0;
> +}
> +
> +/* Implement the kill_request target_ops method.  */

"kill_request" -> "request_interrupt".

> +
> +void
> +netbsd_process_target::request_interrupt ()
> +{
> +  ptid_t inferior_ptid = ptid_of (get_first_thread ());
> +
> +  ::kill (inferior_ptid.pid(), SIGINT);
> +}
> +

I think a function comment is needed here.

> +static size_t
> +netbsd_read_auxv(pid_t pid, void *offs, void *addr, size_t len)
> +{
> +  struct ptrace_io_desc pio;
> +
> +  pio.piod_op = PIOD_READ_AUXV;
> +  pio.piod_offs = offs;
> +  pio.piod_addr = addr;
> +  pio.piod_len = len;
> +
> +  if (ptrace (PT_IO, pid, &pio, 0) == -1)
> +    perror_with_name (("ptrace"));
> +
> +  return pio.piod_len;
> +}
> +
> +/* Copy LEN bytes from inferior's auxiliary vector starting at OFFSET
> +   to debugger memory starting at MYADDR.  */
> +
> +int
> +netbsd_process_target::read_auxv (CORE_ADDR offset,
> +				  unsigned char *myaddr, unsigned int len)
> +{
> +  pid_t pid = pid_of (current_thread);
> +
> +  return netbsd_read_auxv (pid, (void *) (intptr_t) offset, myaddr, len);
> +}
> +
> +bool
> +netbsd_process_target::supports_z_point_type (char z_type)
> +{
> +  switch (z_type)
> +    {
> +    case Z_PACKET_SW_BP:
> +      return true;
> +    case Z_PACKET_HW_BP:
> +    case Z_PACKET_WRITE_WP:
> +    case Z_PACKET_READ_WP:
> +    case Z_PACKET_ACCESS_WP:
> +    default:
> +      return false; /* Not supported.  */
> +    }
> +}
> +
> +/* Insert {break/watch}point at address ADDR.  SIZE is not used.  */
> +
> +int
> +netbsd_process_target::insert_point (enum raw_bkpt_type type, CORE_ADDR addr,
> +		     int size, struct raw_breakpoint *bp)
> +{
> +  switch (type)
> +    {
> +    case raw_bkpt_type_sw:
> +      return insert_memory_breakpoint (bp);
> +    case raw_bkpt_type_hw:
> +    case raw_bkpt_type_write_wp:
> +    case raw_bkpt_type_read_wp:
> +    case raw_bkpt_type_access_wp:
> +    default:
> +      return 1; /* Not supported.  */
> +    }
> +}
> +
> +/* Remove {break/watch}point at address ADDR.  SIZE is not used.  */
> +
> +int
> +netbsd_process_target::remove_point (enum raw_bkpt_type type, CORE_ADDR addr,
> +				     int size, struct raw_breakpoint *bp)
> +{
> +  switch (type)
> +    {
> +    case raw_bkpt_type_sw:
> +      return remove_memory_breakpoint (bp);
> +    case raw_bkpt_type_hw:
> +    case raw_bkpt_type_write_wp:
> +    case raw_bkpt_type_read_wp:
> +    case raw_bkpt_type_access_wp:
> +    default:
> +      return 1; /* Not supported.  */
> +    }
> +}
> +
> +/* Implement the to_stopped_by_sw_breakpoint target_ops
> +   method.  */

"to_stopped_by_sw_breakpoint" -> "stopped_by_sw_breakpoint".

> +
> +bool
> +netbsd_process_target::stopped_by_sw_breakpoint ()
> +{
> +  ptrace_siginfo_t psi;
> +  pid_t pid = current_process ()->pid;
> +
> +  if (ptrace (PT_GET_SIGINFO, pid, &psi, sizeof (psi)) == -1)
> +    perror_with_name (("ptrace"));
> +
> +  return psi.psi_siginfo.si_signo == SIGTRAP &&
> +	 psi.psi_siginfo.si_code == TRAP_BRKPT;
> +}
> +
> +/* Implement the to_supports_stopped_by_sw_breakpoint target_ops
> +   method.  */

The "to_" prefix shall be removed here, too.

> +
> +bool
> +netbsd_process_target::supports_stopped_by_sw_breakpoint ()
> +{
> +  return true;
> +}
> +
> +/* Implement the supports_qxfer_siginfo target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_qxfer_siginfo ()
> +{
> +  return true;
> +}
> +
> +/* Implement the qxfer_siginfo target_ops method.  */
> +
> +int
> +netbsd_process_target::qxfer_siginfo (const char *annex, unsigned char *readbuf,
> +				      unsigned const char *writebuf,
> +				      CORE_ADDR offset, int len)
> +{
> +  if (current_thread == nullptr)
> +    return -1;
> +
> +  pid_t pid = current_process ()->pid;
> +
> +  return netbsd_nat::qxfer_siginfo(pid, annex, readbuf, writebuf, offset, len);
> +}
> +
> +/* Implement the supports_non_stop target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_non_stop ()
> +{
> +  return false;
> +}
> +
> +/* Implement the supports_multi_process target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_multi_process ()
> +{
> +  return true;
> +}
> +
> +/* Check if fork events are supported.  */
> +
> +bool
> +netbsd_process_target::supports_fork_events ()
> +{
> +  return false;
> +}
> +
> +/* Check if vfork events are supported.  */
> +
> +bool
> +netbsd_process_target::supports_vfork_events ()
> +{
> +  return false;
> +}
> +
> +/* Check if exec events are supported.  */
> +
> +bool
> +netbsd_process_target::supports_exec_events ()
> +{
> +  return true;
> +}
> +
> +/* Implement the supports_disable_randomization target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_disable_randomization ()
> +{
> +  return false;
> +}

Some of the target ops above do not change the base class behavior.  The override
can be omitted for code simplicity.  I think it's also OK to retain them if you want
to make the definitions explicit.

> +
> +/* Extract &phdr and num_phdr in the inferior.  Return 0 on success.  */
> +
> +template <typename T>
> +int get_phdr_phnum_from_proc_auxv (const pid_t pid,
> +				   CORE_ADDR *phdr_memaddr, int *num_phdr)
> +{
> +  typedef typename std::conditional<sizeof(T) == sizeof(int64_t),
> +				    Aux64Info, Aux32Info>::type auxv_type;
> +  const size_t auxv_size = sizeof (auxv_type);
> +  const size_t auxv_buf_size = 128 * sizeof (auxv_type);
> +
> +  char *auxv_buf = (char *) xmalloc (auxv_buf_size);
> +
> +  netbsd_read_auxv (pid, nullptr, auxv_buf, auxv_buf_size);
> +
> +  *phdr_memaddr = 0;
> +  *num_phdr = 0;
> +
> +  for (char *buf = auxv_buf; buf < (auxv_buf + auxv_buf_size); buf += auxv_size)
> +    {
> +      auxv_type *const aux = (auxv_type *) buf;
> +
> +      switch (aux->a_type)
> +	{
> +	case AT_PHDR:
> +	  *phdr_memaddr = aux->a_v;
> +	  break;
> +	case AT_PHNUM:
> +	  *num_phdr = aux->a_v;
> +	  break;
> +	}
> +
> +      if (*phdr_memaddr != 0 && *num_phdr != 0)
> +	break;
> +    }
> +
> +  xfree (auxv_buf);
> +
> +  if (*phdr_memaddr == 0 || *num_phdr == 0)
> +    {
> +      warning ("Unexpected missing AT_PHDR and/or AT_PHNUM: "
> +	       "phdr_memaddr = %ld, phdr_num = %d",
> +	       (long) *phdr_memaddr, *num_phdr);

It might be better to use `core_addr_to_string` or `core_addr_to_string_nz`
to print phdr_memaddr.

> +      return 2;
> +    }
> +
> +  return 0;
> +}
> +
> +/* Return &_DYNAMIC (via PT_DYNAMIC) in the inferior, or 0 if not present.  */
> +
> +template <typename T>
> +static CORE_ADDR
> +get_dynamic (netbsd_process_target *target, const pid_t pid)
> +{
> +  typedef typename std::conditional<sizeof(T) == sizeof(int64_t),
> +				    Elf64_Phdr, Elf32_Phdr>::type phdr_type;
> +  const int phdr_size = sizeof (phdr_type);
> +
> +  CORE_ADDR phdr_memaddr;
> +  int num_phdr;
> +  if (get_phdr_phnum_from_proc_auxv<T> (pid, &phdr_memaddr, &num_phdr))
> +    return 0;
> +
> +  std::vector<unsigned char> phdr_buf;
> +  phdr_buf.resize (num_phdr * phdr_size);
> +
> +  if ((*target).read_memory (phdr_memaddr, phdr_buf.data (), phdr_buf.size ()))

Why not simply `target->read_memory`?
Also, it might be better to explicitly check != 0.

> +    return 0;
> +
> +  /* Compute relocation: it is expected to be 0 for "regular" executables,
> +     non-zero for PIE ones.  */
> +  CORE_ADDR relocation = -1;
> +  for (int i = 0; relocation == -1 && i < num_phdr; i++)
> +    {
> +      phdr_type *const p = (phdr_type *) (phdr_buf.data() + i * phdr_size);
> +
> +      if (p->p_type == PT_PHDR)
> +	relocation = phdr_memaddr - p->p_vaddr;
> +    }
> +
> +  if (relocation == -1)
> +    {
> +      /* PT_PHDR is optional, but necessary for PIE in general.  Fortunately
> +	 any real world executables, including PIE executables, have always
> +	 PT_PHDR present.  PT_PHDR is not present in some shared libraries or
> +	 in fpc (Free Pascal 2.4) binaries but neither of those have a need for
> +	 or present DT_DEBUG anyway (fpc binaries are statically linked).
> +
> +	 Therefore if there exists DT_DEBUG there is always also PT_PHDR.
> +
> +	 GDB could find RELOCATION also from AT_ENTRY - e_entry.  */
> +
> +      return 0;
> +    }
> +
> +  for (int i = 0; i < num_phdr; i++)
> +    {
> +      phdr_type *const p = (phdr_type *) (phdr_buf.data () + i * phdr_size);
> +
> +      if (p->p_type == PT_DYNAMIC)
> +	return p->p_vaddr + relocation;
> +    }
> +
> +  return 0;
> +}
> +
> +/* Return &_r_debug in the inferior, or -1 if not present.  Return value
> +   can be 0 if the inferior does not yet have the library list initialized.
> +   We look for DT_MIPS_RLD_MAP first.  MIPS executables use this instead of
> +   DT_DEBUG, although they sometimes contain an unused DT_DEBUG entry too.  */
> +
> +template <typename T>
> +static CORE_ADDR
> +get_r_debug (netbsd_process_target *target, const int pid)
> +{
> +  typedef typename std::conditional<sizeof(T) == sizeof(int64_t),
> +				    Elf64_Dyn, Elf32_Dyn>::type dyn_type;
> +  const int dyn_size = sizeof (dyn_type);
> +  unsigned char buf[sizeof (dyn_type)];  /* The larger of the two.  */
> +  CORE_ADDR map = -1;
> +
> +  CORE_ADDR dynamic_memaddr = get_dynamic<T> (target, pid);
> +  if (dynamic_memaddr == 0)
> +    return map;
> +
> +  while ((*target).read_memory (dynamic_memaddr, buf, dyn_size) == 0)

The `target->read_memory` comment here, too.

> +    {
> +      dyn_type *const dyn = (dyn_type *) buf;
> +#if defined DT_MIPS_RLD_MAP
> +      union
> +      {
> +	T map;
> +	unsigned char buf[sizeof (T)];
> +      }
> +      rld_map;
> +
> +      if (dyn->d_tag == DT_MIPS_RLD_MAP)
> +	{
> +	  if (read_memory (dyn->d_un.d_val,
> +			   rld_map.buf, sizeof (rld_map.buf)) == 0)
> +	    return rld_map.map;
> +	  else
> +	    break;
> +	}
> +#endif  /* DT_MIPS_RLD_MAP */
> +
> +      if (dyn->d_tag == DT_DEBUG && map == -1)
> +	map = dyn->d_un.d_val;
> +
> +      if (dyn->d_tag == DT_NULL)
> +	break;
> +
> +      dynamic_memaddr += dyn_size;
> +    }
> +
> +  return map;
> +}
> +
> +/* Read one pointer from MEMADDR in the inferior.  */
> +
> +static int
> +read_one_ptr (netbsd_process_target *target, CORE_ADDR memaddr, CORE_ADDR *ptr,
> +	      int ptr_size)
> +{
> +  /* Go through a union so this works on either big or little endian
> +     hosts, when the inferior's pointer size is smaller than the size
> +     of CORE_ADDR.  It is assumed the inferior's endianness is the
> +     same of the superior's.  */
> +
> +  union
> +  {
> +    CORE_ADDR core_addr;
> +    unsigned int ui;
> +    unsigned char uc;
> +  } addr;
> +
> +  int ret = (*target).read_memory (memaddr, &addr.uc, ptr_size);

Here, too.

> +  if (ret == 0)
> +    {
> +      if (ptr_size == sizeof (CORE_ADDR))
> +	*ptr = addr.core_addr;
> +      else if (ptr_size == sizeof (unsigned int))
> +	*ptr = addr.ui;
> +      else
> +	gdb_assert_not_reached ("unhandled pointer size");
> +    }
> +  return ret;
> +}
> +
> +/* Construct qXfer:libraries-svr4:read reply.  */
> +
> +template <typename T>
> +int
> +netbsd_qxfer_libraries_svr4 (netbsd_process_target *target,
> +			     const pid_t pid, const char *annex,
> +			     unsigned char *readbuf,
> +			     unsigned const char *writebuf,
> +			     CORE_ADDR offset, int len)
> +{
> +  struct link_map_offsets
> +  {
> +    /* Offset and size of r_debug.r_version.  */
> +    int r_version_offset;
> +
> +    /* Offset and size of r_debug.r_map.  */
> +    int r_map_offset;
> +
> +    /* Offset to l_addr field in struct link_map.  */
> +    int l_addr_offset;
> +
> +    /* Offset to l_name field in struct link_map.  */
> +    int l_name_offset;
> +
> +    /* Offset to l_ld field in struct link_map.  */
> +    int l_ld_offset;
> +
> +    /* Offset to l_next field in struct link_map.  */
> +    int l_next_offset;
> +
> +    /* Offset to l_prev field in struct link_map.  */
> +    int l_prev_offset;
> +  };
> +
> +  static const struct link_map_offsets lmo_32bit_offsets =
> +    {
> +      0,     /* r_version offset. */
> +      4,     /* r_debug.r_map offset.  */
> +      0,     /* l_addr offset in link_map.  */
> +      4,     /* l_name offset in link_map.  */
> +      8,     /* l_ld offset in link_map.  */
> +      12,    /* l_next offset in link_map.  */
> +      16     /* l_prev offset in link_map.  */
> +    };
> +
> +  static const struct link_map_offsets lmo_64bit_offsets =
> +    {
> +      0,     /* r_version offset. */
> +      8,     /* r_debug.r_map offset.  */
> +      0,     /* l_addr offset in link_map.  */
> +      8,     /* l_name offset in link_map.  */
> +      16,    /* l_ld offset in link_map.  */
> +      24,    /* l_next offset in link_map.  */
> +      32     /* l_prev offset in link_map.  */
> +    };
> +
> +  CORE_ADDR lm_addr = 0, lm_prev = 0;
> +  CORE_ADDR l_name, l_addr, l_ld, l_next, l_prev;
> +  int header_done = 0;
> +
> +  const struct link_map_offsets *lmo
> +    = (sizeof (T) == sizeof (int64_t))
> +    ? &lmo_64bit_offsets : &lmo_32bit_offsets;

An extra pair of parens can be used to better align '?'.  Like this:

  const struct link_map_offsets *lmo
    = ((sizeof (T) == sizeof (int64_t))
       ? &lmo_64bit_offsets : &lmo_32bit_offsets);

> +  int ptr_size = sizeof (T);
> +
> +  while (annex[0] != '\0')
> +    {
> +      const char *sep = strchr (annex, '=');
> +      if (sep == nullptr)
> +	break;
> +
> +      int name_len = sep - annex;
> +      CORE_ADDR *addrp;
> +      if (name_len == 5 && startswith (annex, "start"))
> +	addrp = &lm_addr;
> +      else if (name_len == 4 && startswith (annex, "prev"))
> +	addrp = &lm_prev;
> +      else
> +	{
> +	  annex = strchr (sep, ';');
> +	  if (annex == nullptr)
> +	    break;
> +	  annex++;
> +	  continue;
> +	}
> +
> +      annex = decode_address_to_semicolon (addrp, sep + 1);
> +    }
> +
> +  if (lm_addr == 0)
> +    {
> +      CORE_ADDR r_debug = get_r_debug<T> (target, pid);
> +
> +      /* We failed to find DT_DEBUG.  Such situation will not change
> +	 for this inferior - do not retry it.  Report it to GDB as
> +	 E01, see for the reasons at the GDB solib-svr4.c side.  */
> +      if (r_debug == (CORE_ADDR) -1)
> +	return -1;
> +
> +      if (r_debug != 0)
> +	{
> +	  if (read_one_ptr (target, r_debug + lmo->r_map_offset,
> +			    &lm_addr, ptr_size) != 0)
> +	    {
> +	      warning ("unable to read r_map from 0x%lx",
> +		       (long) r_debug + lmo->r_map_offset);

Again, `core_addr_to_string` could be used.

> +	    }
> +	}

Braces can be removed in both conditional statements above.  And, if you prefer,
the conditions can be merged via &&.

> +    }
> +
> +  std::string document = "<library-list-svr4 version=\"1.0\"";
> +
> +  while (lm_addr
> +	 && read_one_ptr (target, lm_addr + lmo->l_name_offset,
> +			  &l_name, ptr_size) == 0
> +	 && read_one_ptr (target, lm_addr + lmo->l_addr_offset,
> +			  &l_addr, ptr_size) == 0
> +	 && read_one_ptr (target, lm_addr + lmo->l_ld_offset,
> +			  &l_ld, ptr_size) == 0
> +	 && read_one_ptr (target, lm_addr + lmo->l_prev_offset,
> +			  &l_prev, ptr_size) == 0
> +	 && read_one_ptr (target, lm_addr + lmo->l_next_offset,
> +			  &l_next, ptr_size) == 0)
> +    {
> +      if (lm_prev != l_prev)
> +	{
> +	  warning ("Corrupted shared library list: 0x%lx != 0x%lx",
> +		   (long) lm_prev, (long) l_prev);
> +	  break;
> +	}
> +
> +      /* Ignore the first entry even if it has valid name as the first entry
> +	 corresponds to the main executable.  The first entry should not be
> +	 skipped if the dynamic loader was loaded late by a static executable
> +	 (see solib-svr4.c parameter ignore_first).  But in such case the main
> +	 executable does not have PT_DYNAMIC present and this function already
> +	 exited above due to failed get_r_debug.  */
> +      if (lm_prev == 0)
> +	string_appendf (document, " main-lm=\"0x%lx\"",
> +			(unsigned long) lm_addr);
> +      else
> +	{
> +	  unsigned char libname[PATH_MAX];
> +
> +	  /* Not checking for error because reading may stop before
> +	     we've got PATH_MAX worth of characters.  */
> +	  libname[0] = '\0';
> +	  (*target).read_memory (l_name, libname, sizeof (libname) - 1);
> +	  libname[sizeof (libname) - 1] = '\0';
> +	  if (libname[0] != '\0')
> +	    {
> +	      if (!header_done)
> +		{
> +		  /* Terminate `<library-list-svr4'.  */
> +		  document += '>';
> +		  header_done = 1;
> +		}
> +
> +	      string_appendf (document, "<library name=\"");
> +	      xml_escape_text_append (&document, (char *) libname);
> +	      string_appendf (document, "\" lm=\"0x%lx\" "
> +			      "l_addr=\"0x%lx\" l_ld=\"0x%lx\"/>",
> +			      (unsigned long) lm_addr, (unsigned long) l_addr,
> +			      (unsigned long) l_ld);
> +	    }
> +	}
> +
> +      lm_prev = lm_addr;
> +      lm_addr = l_next;
> +    }
> +
> +  if (!header_done)
> +    {
> +      /* Empty list; terminate `<library-list-svr4'.  */
> +      document += "/>";
> +    }
> +  else
> +    document += "</library-list-svr4>";
> +
> +  int document_len = document.length ();
> +  if (offset < document_len)
> +    document_len -= offset;
> +  else
> +    document_len = 0;
> +  if (len > document_len)
> +    len = document_len;
> +
> +  memcpy (readbuf, document.data () + offset, len);
> +
> +  return len;
> +}
> +
> +/* Return true if FILE is a 64-bit ELF file,
> +   false if the file is not a 64-bit ELF file,
> +   and error if the file is not accessible or doesn't exist.  */
> +
> +static bool
> +elf_64_file_p (const char *file)
> +{
> +  int fd = gdb::handle_eintr (::open, file, O_RDONLY);
> +  if (fd < 0)
> +    perror_with_name (("open"));
> +
> +  Elf64_Ehdr header;
> +  ssize_t ret = gdb::handle_eintr (::read, fd, &header, sizeof (header));
> +  if (ret == -1)
> +    perror_with_name (("read"));
> +  gdb::handle_eintr (::close, fd);
> +  if (ret != sizeof (header))
> +    error ("Cannot read ELF file header: %s", file);
> +
> +  if (header.e_ident[EI_MAG0] != ELFMAG0
> +      || header.e_ident[EI_MAG1] != ELFMAG1
> +      || header.e_ident[EI_MAG2] != ELFMAG2
> +      || header.e_ident[EI_MAG3] != ELFMAG3)
> +    error ("Unrecognized ELF file header: %s", file);
> +
> +  return header.e_ident[EI_CLASS] == ELFCLASS64;
> +}
> +
> +/* Construct qXfer:libraries-svr4:read reply.  */
> +
> +int
> +netbsd_process_target::qxfer_libraries_svr4 (const char *annex,
> +					     unsigned char *readbuf,
> +					     unsigned const char *writebuf,
> +					     CORE_ADDR offset, int len)
> +{
> +  if (writebuf != nullptr)
> +    return -2;
> +  if (readbuf == nullptr)
> +    return -1;
> +
> +  struct process_info *proc = current_process ();
> +  pid_t pid = proc->pid;
> +  bool is_elf64 = elf_64_file_p (netbsd_nat::pid_to_exec_file (pid));
> +
> +  if (is_elf64)
> +    return netbsd_qxfer_libraries_svr4<int64_t> (this, pid, annex, readbuf,
> +						 writebuf, offset, len);
> +  else
> +    return netbsd_qxfer_libraries_svr4<int32_t> (this, pid, annex, readbuf,
> +						 writebuf, offset, len);
> +}
> +
> +/* Implement the supports_qxfer_libraries_svr4 target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_qxfer_libraries_svr4 ()
> +{
> +  return true;
> +}
> +
> +/* Return the name of a file that can be opened to get the symbols for
> +   the child process identified by PID.  */
> +
> +char *
> +netbsd_process_target::pid_to_exec_file (pid_t pid)
> +{
> +  return netbsd_nat::pid_to_exec_file (pid);
> +}
> +
> +/* Implementation of the target_ops method "supports_pid_to_exec_file".  */
> +
> +bool
> +netbsd_process_target::supports_pid_to_exec_file ()
> +{
> +  return true;
> +}
> +
> +/* Implementation of the target_ops method "supports_hardware_single_step".  */
> +bool
> +netbsd_process_target::supports_hardware_single_step ()
> +{
> +  return true;
> +}
> +
> +/* Implementation of the target_ops method "sw_breakpoint_from_kind".  */
> +
> +const gdb_byte *
> +netbsd_process_target::sw_breakpoint_from_kind (int kind, int *size)
> +{
> +  static gdb_byte brkpt[PTRACE_BREAKPOINT_SIZE];
> +
> +  *size = PTRACE_BREAKPOINT_SIZE;
> +
> +  memcpy (brkpt, PTRACE_BREAKPOINT, PTRACE_BREAKPOINT_SIZE);

Is there a need to memcpy each time the function is called?

> +
> +  return brkpt;
> +}
> +
> +/* Implement the thread_name target_ops method.  */
> +
> +const char *
> +netbsd_process_target::thread_name (ptid_t ptid)
> +{
> +  return netbsd_nat::thread_name (ptid);
> +}
> +
> +/* Implement the supports_catch_syscall target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_catch_syscall ()
> +{
> +  return true;
> +}
> +
> +/* Implement the supports_read_auxv target_ops method.  */
> +
> +bool
> +netbsd_process_target::supports_read_auxv ()
> +{
> +  return true;
> +}
> +
> +/* The NetBSD target ops object.  */
> +
> +static netbsd_process_target the_netbsd_target;
> +
> +void
> +initialize_low ()
> +{
> +  set_target_ops (&the_netbsd_target);
> +  the_low_target.arch_setup ();
> +}
> diff --git a/gdbserver/netbsd-low.h b/gdbserver/netbsd-low.h
> new file mode 100644
> index 00000000000..2bd57c51726
> --- /dev/null
> +++ b/gdbserver/netbsd-low.h
> @@ -0,0 +1,157 @@
> +/* Copyright (C) 2020 Free Software Foundation, Inc.
> +
> +   This file is part of GDB.
> +
> +   This program is free software; you can redistribute it and/or modify
> +   it under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +#ifndef GDBSERVER_NETBSD_LOW_H
> +#define GDBSERVER_NETBSD_LOW_H
> +
> +struct regcache;
> +struct target_desc;
> +
> +/*  Some information relative to a given register set.   */
> +
> +struct netbsd_regset_info
> +{
> +  /* The ptrace request needed to get/set registers of this set.  */
> +  int get_request, set_request;
> +  /* The size of the register set.  */
> +  int size;
> +  /* Fill the buffer BUF from the contents of the given REGCACHE.  */
> +  void (*fill_function) (struct regcache *regcache, char *buf);
> +  /* Store the register value in BUF in the given REGCACHE.  */
> +  void (*store_function) (struct regcache *regcache, const char *buf);

Leaving one blank line between each field may improve readability.

> +};
> +
> +/* A list of regsets for the target being debugged, terminated by an entry
> +   where the size is negative.
> +
> +   This list should be created by the target-specific code.  */
> +
> +extern struct netbsd_regset_info netbsd_target_regsets[];
> +
> +/* The target-specific operations for NetBSD support.  */
> +
> +struct netbsd_target_ops
> +{
> +  /* Architecture-specific setup.  */
> +  void (*arch_setup) ();
> +
> +  /* Hook to support target specific qSupported.  */
> +  void (*process_qsupported) (char **, int count);

Is the `process_qsupported` function called anywhere?

> +};

No strong opinion, but the low target could be a derived class of
netbsd_process_target, like the linux low targets.

> +
> +/* Target ops definitions for a NetBSD target.  */
> +
> +class netbsd_process_target : public process_stratum_target
> +{
> +public:
> +
> +  int create_inferior (const char *program,
> +		       const std::vector<char *> &program_args) override;
> +
> +  void post_create_inferior () override;
> +
> +  int attach (unsigned long pid) override;
> +
> +  int kill (process_info *proc) override;
> +
> +  int detach (process_info *proc) override;
> +
> +  void mourn (process_info *proc) override;
> +
> +  void join (int pid) override;
> +
> +  bool thread_alive (ptid_t pid) override;
> +
> +  void resume (thread_resume *resume_info, size_t n) override;
> +
> +  ptid_t wait (ptid_t ptid, target_waitstatus *status,
> +	       int options) override;
> +
> +  void fetch_registers (regcache *regcache, int regno) override;
> +
> +  void store_registers (regcache *regcache, int regno) override;
> +
> +  int read_memory (CORE_ADDR memaddr, unsigned char *myaddr,
> +		   int len) override;
> +
> +  int write_memory (CORE_ADDR memaddr, const unsigned char *myaddr,
> +		    int len) override;
> +
> +  void request_interrupt () override;
> +
> +  bool supports_read_auxv () override;
> +
> +  int read_auxv (CORE_ADDR offset, unsigned char *myaddr,
> +		 unsigned int len) override;
> +
> +  bool supports_hardware_single_step () override;
> +
> +  const gdb_byte *sw_breakpoint_from_kind (int kind, int *size) override;
> +
> +  bool supports_z_point_type (char z_type) override;
> +
> +  int insert_point (enum raw_bkpt_type type, CORE_ADDR addr,
> +		    int size, struct raw_breakpoint *bp) override;
> +
> +  int remove_point (enum raw_bkpt_type type, CORE_ADDR addr,
> +		    int size, struct raw_breakpoint *bp) override;
> +
> +  bool stopped_by_sw_breakpoint () override;
> +
> +  bool supports_qxfer_siginfo () override;
> +
> +  int qxfer_siginfo (const char *annex, unsigned char *readbuf,
> +		     unsigned const char *writebuf, CORE_ADDR offset,
> +		     int len) override;
> +
> +  bool supports_stopped_by_sw_breakpoint () override;
> +
> +  bool supports_non_stop () override;
> +
> +  bool supports_multi_process () override;
> +
> +  bool supports_fork_events () override;
> +
> +  bool supports_vfork_events () override;
> +
> +  bool supports_exec_events () override;
> +
> +  bool supports_disable_randomization () override;
> +
> +  bool supports_qxfer_libraries_svr4 () override;
> +
> +  int qxfer_libraries_svr4 (const char*, unsigned char*, const unsigned char*,
> +			    CORE_ADDR, int) override;
> +
> +  bool supports_pid_to_exec_file () override;
> +
> +  char *pid_to_exec_file (int pid) override;
> +
> +  const char *thread_name (ptid_t thread) override;
> +
> +  bool supports_catch_syscall () override;
> +};
> +
> +/* The inferior's target description.  This is a global because the
> +   NetBSD ports support neither bi-arch nor multi-process.  */

The "nor multi-process" part of this comment is confusing.  What does the multi-process
feature have to do with the_low_target?  And netbsd_process_target was defined to
support multi-process above in the .cc file.

> +
> +extern struct netbsd_target_ops the_low_target;
> +
> +/* XXX: multilib */
> +extern const struct target_desc *netbsd_tdesc;
> +
> +#endif /* GDBSERVER_NETBSD_LOW_H */
> diff --git a/gdbserver/netbsd-x86_64-low.cc b/gdbserver/netbsd-x86_64-low.cc
> new file mode 100644
> index 00000000000..c8679d9f153
> --- /dev/null
> +++ b/gdbserver/netbsd-x86_64-low.cc
> @@ -0,0 +1,250 @@
> +/* Copyright (C) 2020 Free Software Foundation, Inc.
> +
> +   This file is part of GDB.
> +
> +   This program is free software; you can redistribute it and/or modify
> +   it under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +#include <sys/types.h>
> +#include <sys/ptrace.h>
> +#include <limits.h>
> +
> +#include "server.h"
> +#include "netbsd-low.h"
> +#include "gdbsupport/x86-xstate.h"
> +#include "arch/amd64.h"
> +#include "x86-tdesc.h"
> +#include "tdesc.h"
> +
> +static int use_xml;
> +
> +/* The index of various registers inside the regcache.  */
> +
> +enum netbsd_x86_64_gdb_regnum
> +{
> +  AMD64_RAX_REGNUM,	     /* %rax */
> +  AMD64_RBX_REGNUM,	     /* %rbx */
> +  AMD64_RCX_REGNUM,	     /* %rcx */
> +  AMD64_RDX_REGNUM,	     /* %rdx */
> +  AMD64_RSI_REGNUM,	     /* %rsi */
> +  AMD64_RDI_REGNUM,	     /* %rdi */
> +  AMD64_RBP_REGNUM,	     /* %rbp */
> +  AMD64_RSP_REGNUM,	     /* %rsp */
> +  AMD64_R8_REGNUM,	      /* %r8 */
> +  AMD64_R9_REGNUM,	      /* %r9 */
> +  AMD64_R10_REGNUM,	     /* %r10 */
> +  AMD64_R11_REGNUM,	     /* %r11 */
> +  AMD64_R12_REGNUM,	     /* %r12 */
> +  AMD64_R13_REGNUM,	     /* %r13 */
> +  AMD64_R14_REGNUM,	     /* %r14 */
> +  AMD64_R15_REGNUM,	     /* %r15 */
> +  AMD64_RIP_REGNUM,	     /* %rip */
> +  AMD64_EFLAGS_REGNUM,	  /* %eflags */
> +  AMD64_CS_REGNUM,	      /* %cs */
> +  AMD64_SS_REGNUM,	      /* %ss */
> +  AMD64_DS_REGNUM,	      /* %ds */
> +  AMD64_ES_REGNUM,	      /* %es */
> +  AMD64_FS_REGNUM,	      /* %fs */
> +  AMD64_GS_REGNUM,	      /* %gs */
> +  AMD64_ST0_REGNUM = 24,     /* %st0 */
> +  AMD64_ST1_REGNUM,	     /* %st1 */
> +  AMD64_FCTRL_REGNUM = AMD64_ST0_REGNUM + 8,
> +  AMD64_FSTAT_REGNUM = AMD64_ST0_REGNUM + 9,
> +  AMD64_FTAG_REGNUM = AMD64_ST0_REGNUM + 10,
> +  AMD64_XMM0_REGNUM = 40,   /* %xmm0 */
> +  AMD64_XMM1_REGNUM,	    /* %xmm1 */
> +  AMD64_MXCSR_REGNUM = AMD64_XMM0_REGNUM + 16,
> +  AMD64_YMM0H_REGNUM,	   /* %ymm0h */
> +  AMD64_YMM15H_REGNUM = AMD64_YMM0H_REGNUM + 15,
> +  AMD64_BND0R_REGNUM = AMD64_YMM15H_REGNUM + 1,
> +  AMD64_BND3R_REGNUM = AMD64_BND0R_REGNUM + 3,
> +  AMD64_BNDCFGU_REGNUM,
> +  AMD64_BNDSTATUS_REGNUM,
> +  AMD64_XMM16_REGNUM,
> +  AMD64_XMM31_REGNUM = AMD64_XMM16_REGNUM + 15,
> +  AMD64_YMM16H_REGNUM,
> +  AMD64_YMM31H_REGNUM = AMD64_YMM16H_REGNUM + 15,
> +  AMD64_K0_REGNUM,
> +  AMD64_K7_REGNUM = AMD64_K0_REGNUM + 7,
> +  AMD64_ZMM0H_REGNUM,
> +  AMD64_ZMM31H_REGNUM = AMD64_ZMM0H_REGNUM + 31,
> +  AMD64_PKRU_REGNUM,
> +  AMD64_FSBASE_REGNUM,
> +  AMD64_GSBASE_REGNUM
> +};
> +
> +/* The fill_function for the general-purpose register set.  */
> +
> +static void
> +netbsd_x86_64_fill_gregset (struct regcache *regcache, char *buf)
> +{
> +  struct reg *r = (struct reg *)buf;

Space after ')'.

> +
> +#define netbsd_x86_64_collect_gp(regnum, fld) do {		\
> +    collect_register (regcache, regnum, &r->regs[_REG_##fld]);	\
> +  } while (0)
> +
> +  netbsd_x86_64_collect_gp (AMD64_RAX_REGNUM, RAX);
> +  netbsd_x86_64_collect_gp (AMD64_RBX_REGNUM, RBX);
> +  netbsd_x86_64_collect_gp (AMD64_RCX_REGNUM, RCX);
> +  netbsd_x86_64_collect_gp (AMD64_RDX_REGNUM, RDX);
> +  netbsd_x86_64_collect_gp (AMD64_RSI_REGNUM, RSI);
> +  netbsd_x86_64_collect_gp (AMD64_RDI_REGNUM, RDI);
> +  netbsd_x86_64_collect_gp (AMD64_RBP_REGNUM, RBP);
> +  netbsd_x86_64_collect_gp (AMD64_RSP_REGNUM, RSP);
> +  netbsd_x86_64_collect_gp (AMD64_R8_REGNUM, R8);
> +  netbsd_x86_64_collect_gp (AMD64_R9_REGNUM, R9);
> +  netbsd_x86_64_collect_gp (AMD64_R10_REGNUM, R10);
> +  netbsd_x86_64_collect_gp (AMD64_R11_REGNUM, R11);
> +  netbsd_x86_64_collect_gp (AMD64_R12_REGNUM, R12);
> +  netbsd_x86_64_collect_gp (AMD64_R13_REGNUM, R13);
> +  netbsd_x86_64_collect_gp (AMD64_R14_REGNUM, R14);
> +  netbsd_x86_64_collect_gp (AMD64_R15_REGNUM, R15);
> +  netbsd_x86_64_collect_gp (AMD64_RIP_REGNUM, RIP);
> +  netbsd_x86_64_collect_gp (AMD64_EFLAGS_REGNUM, RFLAGS);
> +  netbsd_x86_64_collect_gp (AMD64_CS_REGNUM, CS);
> +  netbsd_x86_64_collect_gp (AMD64_SS_REGNUM, SS);
> +  netbsd_x86_64_collect_gp (AMD64_DS_REGNUM, DS);
> +  netbsd_x86_64_collect_gp (AMD64_ES_REGNUM, ES);
> +  netbsd_x86_64_collect_gp (AMD64_FS_REGNUM, FS);
> +  netbsd_x86_64_collect_gp (AMD64_GS_REGNUM, GS);
> +}
> +
> +/* The store_function for the general-purpose register set.  */
> +
> +static void
> +netbsd_x86_64_store_gregset (struct regcache *regcache, const char *buf)
> +{
> +  struct reg *r = (struct reg *)buf;

Space after ')'.

> +
> +#define netbsd_x86_64_supply_gp(regnum, fld) do {		\
> +    supply_register (regcache, regnum, &r->regs[_REG_##fld]);	\
> +  } while(0)
> +
> +  netbsd_x86_64_supply_gp (AMD64_RAX_REGNUM, RAX);
> +  netbsd_x86_64_supply_gp (AMD64_RBX_REGNUM, RBX);
> +  netbsd_x86_64_supply_gp (AMD64_RCX_REGNUM, RCX);
> +  netbsd_x86_64_supply_gp (AMD64_RDX_REGNUM, RDX);
> +  netbsd_x86_64_supply_gp (AMD64_RSI_REGNUM, RSI);
> +  netbsd_x86_64_supply_gp (AMD64_RDI_REGNUM, RDI);
> +  netbsd_x86_64_supply_gp (AMD64_RBP_REGNUM, RBP);
> +  netbsd_x86_64_supply_gp (AMD64_RSP_REGNUM, RSP);
> +  netbsd_x86_64_supply_gp (AMD64_R8_REGNUM, R8);
> +  netbsd_x86_64_supply_gp (AMD64_R9_REGNUM, R9);
> +  netbsd_x86_64_supply_gp (AMD64_R10_REGNUM, R10);
> +  netbsd_x86_64_supply_gp (AMD64_R11_REGNUM, R11);
> +  netbsd_x86_64_supply_gp (AMD64_R12_REGNUM, R12);
> +  netbsd_x86_64_supply_gp (AMD64_R13_REGNUM, R13);
> +  netbsd_x86_64_supply_gp (AMD64_R14_REGNUM, R14);
> +  netbsd_x86_64_supply_gp (AMD64_R15_REGNUM, R15);
> +  netbsd_x86_64_supply_gp (AMD64_RIP_REGNUM, RIP);
> +  netbsd_x86_64_supply_gp (AMD64_EFLAGS_REGNUM, RFLAGS);
> +  netbsd_x86_64_supply_gp (AMD64_CS_REGNUM, CS);
> +  netbsd_x86_64_supply_gp (AMD64_SS_REGNUM, SS);
> +  netbsd_x86_64_supply_gp (AMD64_DS_REGNUM, DS);
> +  netbsd_x86_64_supply_gp (AMD64_ES_REGNUM, ES);
> +  netbsd_x86_64_supply_gp (AMD64_FS_REGNUM, FS);
> +  netbsd_x86_64_supply_gp (AMD64_GS_REGNUM, GS);
> +}
> +
> +/* Implements the netbsd_target_ops.arch_setup routine.  */
> +
> +static void
> +netbsd_x86_64_arch_setup (void)
> +{
> +  struct target_desc *tdesc
> +    = amd64_create_target_description (X86_XSTATE_SSE_MASK, false, false, false);
> +
> +  init_target_desc (tdesc, amd64_expedite_regs);
> +
> +  netbsd_tdesc = tdesc;
> +}
> +
> +/* Update all the target description of all processes; a new GDB
> +   connected, and it may or not support xml target descriptions.  */
> +
> +static void
> +x86_64_netbsd_update_xmltarget (void)
> +{
> +  struct thread_info *saved_thread = current_thread;
> +
> +  /* Before changing the register cache's internal layout, flush the
> +     contents of the current valid caches back to the threads, and
> +     release the current regcache objects.  */
> +  regcache_release ();
> +
> +  for_each_process ([] (process_info *proc) {
> +		      int pid = proc->pid;
> +
> +		      /* Look up any thread of this process.  */
> +		      current_thread = find_any_thread_of_pid (pid);
> +
> +		      the_low_target.arch_setup ();

I find this confusing because the target object is supposed to be a singleton.
Why is its arch_setup called for each process?

> +		    });
> +
> +  current_thread = saved_thread;
> +}
> +
> +/* Process qSupported query, "xmlRegisters=". */
> +
> +static void
> +netbsd_x86_64_process_qsupported (char **features, int count)
> +{
> +  /* Return if gdb doesn't support XML.  If gdb sends "xmlRegisters="
> +     with "i386" in qSupported query, it supports x86 XML target
> +     descriptions.  */
> +  use_xml = 0;
> +  for (int i = 0; i < count; i++)
> +    {
> +      const char *feature = features[i];
> +
> +      if (startswith (feature, "xmlRegisters="))
> +	{
> +	  char *copy = xstrdup (feature + 13);
> +	  char *last;
> +	  char *p;
> +
> +	  for (p = strtok_r (copy, ",", &last); p != NULL;
> +	       p = strtok_r (NULL, ",", &last))
> +	    {
> +	      if (strcmp (p, "i386") == 0)
> +		{
> +		  use_xml = 1;
> +		  break;
> +		}
> +	    }
> +
> +	  free (copy);
> +	}
> +    }
> +  x86_64_netbsd_update_xmltarget ();
> +}
> +
> +/* Description of all the x86-netbsd register sets.  */
> +
> +struct netbsd_regset_info netbsd_target_regsets[] =
> +{
> + /* General Purpose Registers.  */
> + {PT_GETREGS, PT_SETREGS, sizeof (struct reg),
> +  netbsd_x86_64_fill_gregset, netbsd_x86_64_store_gregset},
> + /* End of list marker.  */
> + {0, 0, -1, NULL, NULL }
> +};
> +
> +/* The netbsd_target_ops vector for x86-netbsd.  */
> +
> +struct netbsd_target_ops the_low_target =
> +{
> + netbsd_x86_64_arch_setup,
> + netbsd_x86_64_process_qsupported,
> +};
> --
> 2.28.0

Regards
-Baris


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