Fix ctrl-c when debugging WOW64 processes

DebugBreakProcess starts a new thread in the target process with the
entry point DbgUiRemoteBreakin, where an int3 triggers a breakpoint
exception for gdb.

But this uses DbgUiRemoteBreakin of the 64bit ntdll.dll even for
WOW64 processes.
It stops in 64bit code, Wow64GetThreadContext reports a wrong pc without
the int3, and gdb lets the target process continue.

So this uses DbgUiRemoteBreakin of the 32bit ntdll.dll as the thread
entry point for WOW64 processes instead.

gdb/ChangeLog:

2020-09-17  Hannes Domani  <ssbssa@yahoo.de>

	* windows-nat.c (ctrl_c_handler): Use 32bit DbgUiRemoteBreakin
	for WOW64 processes.

2 files changed, 37 insertions, 3 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 3c8e4a1..4b54f4e 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,8 @@
+2020-09-17  Hannes Domani  <ssbssa@yahoo.de>
+
+	* windows-nat.c (ctrl_c_handler): Use 32bit DbgUiRemoteBreakin
+	for WOW64 processes.
+
 2020-09-17  Tom Tromey  <tom@tromey.com>
 
 	* dwarf2/read.c (compute_compunit_symtab_includes): Use htab_up.
diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index 188a920..ec5e428 100644
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -71,6 +71,7 @@
 #include "gdbsupport/pathstuff.h"
 #include "gdbsupport/gdb_wait.h"
 #include "nat/windows-nat.h"
+#include "gdbsupport/symbol.h"
 
 using namespace windows_nat;
 
@@ -235,6 +236,7 @@ static int saw_create;
 static int open_process_used = 0;
 #ifdef __x86_64__
 static bool wow64_process = false;
+static void *wow64_dbgbreak;
 #endif
 
 /* User options.  */
@@ -1522,9 +1524,36 @@ ctrl_c_handler (DWORD event_type)
   if (!new_console && !attach_flag)
     return TRUE;
 
-  if (!DebugBreakProcess (current_process_handle))
-    warning (_("Could not interrupt program.  "
-	       "Press Ctrl-c in the program console."));
+#ifdef __x86_64__
+  if (wow64_process)
+    {
+      /* Call DbgUiRemoteBreakin of the 32bit ntdll.dll in the target process.
+	 DebugBreakProcess would call the one of the 64bit ntdll.dll, which
+	 can't be correctly handled by gdb.  */
+      if (wow64_dbgbreak == nullptr)
+	{
+	  CORE_ADDR addr;
+	  if (!find_minimal_symbol_address ("ntdll!DbgUiRemoteBreakin",
+					    &addr, 0))
+	    wow64_dbgbreak = (void *) addr;
+	}
+
+      if (wow64_dbgbreak != nullptr)
+	{
+	  HANDLE thread = CreateRemoteThread (current_process_handle, NULL,
+					      0, (LPTHREAD_START_ROUTINE)
+					      wow64_dbgbreak, NULL, 0, NULL);
+	  if (thread)
+	    CloseHandle (thread);
+	}
+    }
+  else
+#endif
+    {
+      if (!DebugBreakProcess (current_process_handle))
+	warning (_("Could not interrupt program.  "
+		   "Press Ctrl-c in the program console."));
+    }
 
   /* Return true to tell that Ctrl-C has been handled.  */
   return TRUE;