x86-fbsd-nat: Copy debug register state on fork.

Use the FreeBSD native target low_new_fork hook to copy the
per-process debug state from the parent to the child on fork.

gdb/configure.nat

@@ -165,7 +165,7 @@ case ${gdb_host} in
 	    i386)
 		# Host: FreeBSD/i386
 		NATDEPFILES="${NATDEPFILES} x86-nat.o nat/x86-dregs.o \
-		x86-bsd-nat.o i386-fbsd-nat.o bsd-kvm.o"
+		x86-bsd-nat.o x86-fbsd-nat.o i386-fbsd-nat.o bsd-kvm.o"
 		;;
 	    mips)
 		# Host: FreeBSD/mips
@@ -194,7 +194,7 @@ case ${gdb_host} in
 		# Host: FreeBSD/amd64
 		NATDEPFILES="${NATDEPFILES} amd64-nat.o \
 		amd64-fbsd-nat.o bsd-kvm.o x86-nat.o nat/x86-dregs.o \
-		x86-bsd-nat.o"
+		x86-bsd-nat.o x86-fbsd-nat.o"
 		;;
 	esac
 	;;

gdb/x86-fbsd-nat.c

@@ -0,0 +1,45 @@
+/* Native-dependent code for FreeBSD x86.
+
+   Copyright (C) 2022 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 "defs.h"
+#include "x86-fbsd-nat.h"
+
+/* Implement the virtual fbsd_nat_target::low_new_fork method.  */
+
+void
+x86_fbsd_nat_target::low_new_fork (ptid_t parent, pid_t child)
+{
+  struct x86_debug_reg_state *parent_state, *child_state;
+
+  /* If there is no parent state, no watchpoints nor breakpoints have
+     been set, so there is nothing to do.  */
+  parent_state = x86_lookup_debug_reg_state (parent.pid ());
+  if (parent_state == nullptr)
+    return;
+
+  /* The kernel clears debug registers in the new child process after
+     fork, but GDB core assumes the child inherits the watchpoints/hw
+     breakpoints of the parent, and will remove them all from the
+     forked off process.  Copy the debug registers mirrors into the
+     new process so that all breakpoints and watchpoints can be
+     removed together.  */
+
+  child_state = x86_debug_reg_state (child);
+  *child_state = *parent_state;
+}

gdb/x86-fbsd-nat.h

@@ -29,6 +29,8 @@ class x86_fbsd_nat_target : public x86bsd_nat_target<fbsd_nat_target>
 {
   bool supports_stopped_by_hw_breakpoint () override
   { return true; }
+
+  void low_new_fork (ptid_t parent, pid_t child) override;
 };
 
 #endif /* x86-bsd-nat.h */