[PATCH v2 25/47] Windows gdbserver: Eliminate soft-interrupt mechanism

Pedro Alves
2025-05-19T13:32:19Z

I noticed that faked_breakpoint is write only. And then I hacked
win32_process_target::request_interrupt to force it to stop threads
using the soft_interrupt_requested mechanism (which suspends threads,
and then fakes a breakpoint event in the main thread), and saw that it
no longer works -- gdbserver crashes accessing a NULL current_thread,
because fake_breakpoint_event does not switch to a thread.

This code was originally added for Windows CE, as neither
GenerateConsoleCtrlEvent nor DebugBreakProcess worked there. Windows
CE support has since been removed.

We nowadays require Windows XP or later, and XP has DebugBreakProcess.

The soft_interrupt_requested mechanism has other problems, like for
example faking the event in the main thread, even if that thread was
previously stopped, due to scheduler-locking.

A following patch will add a similar mechanism stopping all threads
with SuspendThread to native GDB, for non-stop mode, which doesn't
have these problems. It's different enough from this old code that I
think we should just rip the old code out, and reimplement it from
scratch (based on gdb's version) when we need it.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I89e98233a9c40c6dcba7c8e1dacee08603843fb1
---
gdbserver/win32-low.cc | 32 +-------------------------------
gdbserver/win32-low.h | 8 --------
2 files changed, 1 insertion(+), 39 deletions(-)

diff --git a/gdbserver/win32-low.cc b/gdbserver/win32-low.cc
index 2e3e4442012..ebce67cd72d 100644
--- a/gdbserver/win32-low.cc
+++ b/gdbserver/win32-low.cc
@@ -294,8 +294,6 @@ do_initial_child_stuff (HANDLE proch, DWORD pid, int attached)
windows_process.process_id = pid;
windows_process.main_thread_id = 0;

- windows_process.soft_interrupt_requested = 0;
- windows_process.faked_breakpoint = 0;
windows_process.open_process_used = true;

memset (&windows_process.current_event, 0,
@@ -418,7 +416,6 @@ child_continue_for_kill (DWORD continue_status, int thread_id)
{
continue_one_thread (thread, thread_id);
});
- windows_process.faked_breakpoint = 0;

return continue_last_debug_event (continue_status, debug_threads);
}
@@ -918,23 +915,6 @@ suspend_one_thread (thread_info *thread)
th->suspend ();
}

-static void
-fake_breakpoint_event (void)
-{
- OUTMSG2(("fake_breakpoint_event\n"));
-
- windows_process.faked_breakpoint = 1;
-
- memset (&windows_process.current_event, 0,
- sizeof (windows_process.current_event));
- windows_process.current_event.dwThreadId = windows_process.main_thread_id;
- windows_process.current_event.dwDebugEventCode = EXCEPTION_DEBUG_EVENT;
- windows_process.current_event.u.Exception.ExceptionRecord.ExceptionCode
- = EXCEPTION_BREAKPOINT;
-
- for_each_thread (suspend_one_thread);
-}
-
/* See nat/windows-nat.h. */

bool
@@ -989,13 +969,6 @@ get_child_debug_event (DWORD *continue_status,

DEBUG_EVENT *current_event = &windows_process.current_event;

- if (windows_process.soft_interrupt_requested)
- {
- windows_process.soft_interrupt_requested = 0;
- fake_breakpoint_event ();
- goto gotevent;
- }
-
windows_process.attaching = 0;
{
process_info *proc = find_process_pid (windows_process.process_id);
@@ -1036,8 +1009,6 @@ get_child_debug_event (DWORD *continue_status,
}
}

- gotevent:
-
switch (current_event->dwDebugEventCode)
{
case CREATE_THREAD_DEBUG_EVENT:
@@ -1291,8 +1262,7 @@ win32_process_target::request_interrupt ()
if (DebugBreakProcess (windows_process.handle))
return;

- /* Last resort, suspend all threads manually. */
- windows_process.soft_interrupt_requested = 1;
+ OUTMSG2 (("Could not interrupt.\n"));
}

bool
diff --git a/gdbserver/win32-low.h b/gdbserver/win32-low.h
index 3c49ca7de9b..b170eb898f6 100644
--- a/gdbserver/win32-low.h
+++ b/gdbserver/win32-low.h
@@ -194,14 +194,6 @@ struct gdbserver_windows_process : public windows_nat::windows_process_info
debug event off the win32 API. */
struct target_waitstatus cached_status;

- /* Non zero if an interrupt request is to be satisfied by suspending
- all threads. */
- int soft_interrupt_requested = 0;
-
- /* Non zero if the inferior is stopped in a simulated breakpoint done
- by suspending all the threads. */
- int faked_breakpoint = 0;
-
/* True if current_process_handle needs to be closed. */
bool open_process_used = false;

--
2.49.0