Windows gdb: Avoid writing debug registers if watchpoint hit pending

Several watchpoint-related testcases, such as
gdb.threads/watchthreads.exp for example, when tested with the backend
in non-stop mode, exposed an interesting detail of the Windows debug
API that wasn't considered before.  The symptom observed is spurious
SIGTRAPs, like:

  Thread 1 "watchthreads" received signal SIGTRAP, Trace/breakpoint trap.
  0x00000001004010b1 in main () at .../src/gdb/testsuite/gdb.threads/watchthreads.c:48
  48              args[i] = 1; usleep (1); /* Init value.  */

After a good amount of staring at logs and headscratching, I realized
the problem:

 #0 - It all starts with the fact that multiple threads can hit an
      event at the same time.  Say, a watchpoint for thread A, and a
      breakpoint for thread B.

 #1 - Say, WaitForDebugEvent reports the breakpoint hit for thread B
      first, then GDB for some reason decides to update debug
      registers, and continue.  Updating debug registers means writing
      the debug registers to _all_ threads, with SetThreadContext.

 #2 - WaitForDebugEvent reports the watchpoint hit for thread A.
      Watchpoint hits are reported as EXCEPTION_SINGLE_STEP.

 #3 - windows-nat checks the Dr6 debug register to check if the step
      was a watchpoint or hardware breakpoint stop, and finds that Dr6
      is completely cleared.  So windows-nat reports a plain SIGTRAP
      (given EXCEPTION_SINGLE_STEP) to the core.

 #4 - Thread A was not supposed to be stepping, so infrun reports the
      SIGTRAP to the user as a random signal.

The strange part is #3 above.  Why was Dr6 cleared?

Turns out that (at least in Windows 10 & 11), writing to _any_ debug
register has the side effect of clearing Dr6, even if you write the
same values the registers already had, back to the registers.

I confirmed it clearly by adding this hack to GDB:

  if (th->context.ContextFlags == 0)
    {
      th->context.ContextFlags = CONTEXT_DEBUGGER_DR;

      /* Get current values of debug registers.  */
      CHECK (GetThreadContext (th->h, &th->context));

      DEBUG_EVENTS ("For 0x%x (once),  Dr6=0x%llx", th->tid, th->context.Dr6);

      /* Write debug registers back to thread, same values,
	 and re-read them.  */
      CHECK (SetThreadContext (th->h, &th->context));
      CHECK (GetThreadContext (th->h, &th->context));

      DEBUG_EVENTS ("For 0x%x (twice), Dr6=0x%llx", th->tid, th->context.Dr6);
    }

Which showed Dr6=0 after the write + re-read:

  [windows events] fill_thread_context: For 0x6a0 (once),  Dr6=0xffff0ff1
  [windows events] fill_thread_context: For 0x6a0 (twice), Dr6=0x0

This commit fixes the issue by detecting that a thread has a pending
watchpoint hit to report (Dr6 has interesting bits set), and if so,
avoid modifying any debug register.  Instead, let the pending
watchpoint hit be reported by WaitForDebugEvent.  If infrun did want
to modify watchpoints, it will still be done when the thread is
eventually re-resumed after the pending watchpoint hit is reported.
(infrun knows how to gracefully handle the case of a watchpoint hit
for a watchpoint that has since been deleted.)

Move the fill_thread_context method from windows_nat_target to
windows_per_inferior so it can be used by gdbserver too.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I21a3daa9e34eecfa054f0fea706e5ab40aabe70a
commit-id:a28f8d4e

diff --git a/gdb/aarch64-windows-nat.c b/gdb/aarch64-windows-nat.c
index 871531bb93a4d8f2c2460cd4d5a8f25baf49b051..2b0828057080675ec8d503a07dda1b15546a695b 100644 (file)
--- a/gdb/aarch64-windows-nat.c
+++ b/gdb/aarch64-windows-nat.c
@@ -30,6 +30,7 @@ struct aarch64_windows_per_inferior : public windows_per_inferior
 {
   aarch64_debug_reg_state dr_state;
 
+  void fill_thread_context (windows_thread_info *th) override;
   void invalidate_thread_context (windows_thread_info *th) override;
 };
 
@@ -42,8 +43,6 @@ struct aarch64_windows_nat_target final
   void initialize_windows_arch (bool attaching) override;
   void cleanup_windows_arch () override;
 
-  void fill_thread_context (windows_thread_info *th) override;
-
   void thread_context_continue (windows_thread_info *th, int killed) override;
   void thread_context_step (windows_thread_info *th) override;
 
@@ -176,10 +175,10 @@ aarch64_windows_nat_target::cleanup_windows_arch ()
   aarch64_remove_debug_reg_state (inferior_ptid.pid ());
 }
 
-/* See windows-nat.h.  */
+/* See nat/windows-nat.h.  */
 
 void
-aarch64_windows_nat_target::fill_thread_context (windows_thread_info *th)
+aarch64_windows_per_inferior::fill_thread_context (windows_thread_info *th)
 {
   CONTEXT *context = &th->context;
 

diff --git a/gdb/nat/windows-nat.h b/gdb/nat/windows-nat.h
index 5258ce52a0f87b85512d58a9584e8049df8e9e3d..45d186f771ab55583e11615647b0abdc656037f7 100644 (file)
--- a/gdb/nat/windows-nat.h
+++ b/gdb/nat/windows-nat.h
@@ -174,6 +174,12 @@ struct windows_process_info
      This function must be supplied by the embedding application.  */
   virtual windows_thread_info *find_thread (ptid_t ptid) = 0;
 
+  /* Fill in the thread's CONTEXT/WOW64_CONTEXT, if it wasn't filled
+     in yet.
+
+     This function must be supplied by the embedding application.  */
+  virtual void fill_thread_context (windows_thread_info *th) = 0;
+
   /* Handle OUTPUT_DEBUG_STRING_EVENT from child process.  Updates
      OURSTATUS and returns the thread id if this represents a thread
      change (this is specific to Cygwin), otherwise 0.

diff --git a/gdb/windows-nat.c b/gdb/windows-nat.c
index fba1ae6c40b0ff81f7d89bfe4bb89c0a5961e5b8..17166fdcfad1c787d62aac81a0a81e9e9f8768db 100644 (file)
--- a/gdb/windows-nat.c
+++ b/gdb/windows-nat.c
@@ -386,7 +386,7 @@ windows_nat_target::fetch_registers (struct regcache *regcache, int r)
   if (th == NULL)
     return;
 
-  fill_thread_context (th);
+  windows_process->fill_thread_context (th);
 
   if (r < 0)
     for (r = 0; r < gdbarch_num_regs (regcache->arch()); r++)

diff --git a/gdb/windows-nat.h b/gdb/windows-nat.h
index e5084631e5d8ac98f341738b9e97a087081b329a..235844a7c957b3434c3e1fa89d0da99e91517589 100644 (file)
--- a/gdb/windows-nat.h
+++ b/gdb/windows-nat.h
@@ -226,9 +226,6 @@ protected:
   /* Cleanup arch-specific data after inferior exit.  */
   virtual void cleanup_windows_arch () = 0;
 
-  /* Reload the thread context.  */
-  virtual void fill_thread_context (windows_thread_info *th) = 0;
-
   /* Prepare the thread context for continuing.  */
   virtual void thread_context_continue (windows_thread_info *th,
                                        int killed) = 0;

diff --git a/gdb/x86-windows-nat.c b/gdb/x86-windows-nat.c
index 5c09efce32fc97f515f0bd130147398a7e25d4ca..5a4876b7fbe3e07c666f168709c3444ef4e8aff9 100644 (file)
--- a/gdb/x86-windows-nat.c
+++ b/gdb/x86-windows-nat.c
@@ -48,6 +48,7 @@ struct x86_windows_per_inferior : public windows_per_inferior
      a segment register or not.  */
   segment_register_p_ftype *segment_register_p = nullptr;
 
+  void fill_thread_context (windows_thread_info *th) override;
   void invalidate_thread_context (windows_thread_info *th) override;
 };
 
@@ -56,8 +57,6 @@ struct x86_windows_nat_target final : public x86_nat_target<windows_nat_target>
   void initialize_windows_arch (bool attaching) override;
   void cleanup_windows_arch () override;
 
-  void fill_thread_context (windows_thread_info *th) override;
-
   void thread_context_continue (windows_thread_info *th, int killed) override;
   void thread_context_step (windows_thread_info *th) override;
 
@@ -102,10 +101,10 @@ x86_windows_nat_target::cleanup_windows_arch ()
   x86_cleanup_dregs ();
 }
 
-/* See windows-nat.h.  */
+/* See nat/windows-nat.h.  */
 
 void
-x86_windows_nat_target::fill_thread_context (windows_thread_info *th)
+x86_windows_per_inferior::fill_thread_context (windows_thread_info *th)
 {
   x86_windows_process.with_context (th, [&] (auto *context)
     {
@@ -141,13 +140,56 @@ x86_windows_nat_target::thread_context_continue (windows_thread_info *th,
 
       if (th->debug_registers_changed)
        {
-         context->ContextFlags |= WindowsContext<decltype(context)>::debug;
-         context->Dr0 = state->dr_mirror[0];
-         context->Dr1 = state->dr_mirror[1];
-         context->Dr2 = state->dr_mirror[2];
-         context->Dr3 = state->dr_mirror[3];
-         context->Dr6 = DR6_CLEAR_VALUE;
-         context->Dr7 = state->dr_control_mirror;
+         windows_process->fill_thread_context (th);
+
+         gdb_assert ((context->ContextFlags & CONTEXT_DEBUG_REGISTERS) != 0);
+
+         /* Check whether the thread has Dr6 set indicating a
+            watchpoint hit, and we haven't seen the watchpoint event
+            yet (reported as
+            EXCEPTION_SINGLE_STEP/STATUS_WX86_SINGLE_STEP).  In that
+            case, don't change the debug registers.  Changing debug
+            registers, even if to the same values, makes the kernel
+            clear Dr6.  The result would be we would lose the
+            unreported watchpoint hit.  */
+         if ((context->Dr6 & ~DR6_CLEAR_VALUE) != 0)
+           {
+             if (th->last_event.dwDebugEventCode == EXCEPTION_DEBUG_EVENT
+                 && (th->last_event.u.Exception.ExceptionRecord.ExceptionCode
+                     == EXCEPTION_SINGLE_STEP
+                     || (th->last_event.u.Exception.ExceptionRecord.ExceptionCode
+                         == STATUS_WX86_SINGLE_STEP)))
+               {
+                 DEBUG_EVENTS ("0x%x already reported watchpoint", th->tid);
+               }
+             else
+               {
+                 DEBUG_EVENTS ("0x%x last reported something else (0x%x)",
+                               th->tid,
+                               th->last_event.dwDebugEventCode);
+
+                 /* Don't touch debug registers.  Let the pending
+                    watchpoint event be reported instead.  We will
+                    update the debug registers later when the thread
+                    is re-resumed by the core after the watchpoint
+                    event.  */
+                 context->ContextFlags &= ~CONTEXT_DEBUG_REGISTERS;
+               }
+           }
+         else
+           DEBUG_EVENTS ("0x%x has no dr6 set", th->tid);
+
+         if ((context->ContextFlags & CONTEXT_DEBUG_REGISTERS) != 0)
+           {
+             DEBUG_EVENTS ("0x%x changing dregs", th->tid);
+             context->Dr0 = state->dr_mirror[0];
+             context->Dr1 = state->dr_mirror[1];
+             context->Dr2 = state->dr_mirror[2];
+             context->Dr3 = state->dr_mirror[3];
+             context->Dr6 = DR6_CLEAR_VALUE;
+             context->Dr7 = state->dr_control_mirror;
+           }
+
          th->debug_registers_changed = false;
        }
 

diff --git a/gdbserver/win32-low.cc b/gdbserver/win32-low.cc
index e712033898c317050275cb16b1727fa18fe60caa..826d21867ae85b27caab0c769dafb545a2ca2c8a 100644 (file)
--- a/gdbserver/win32-low.cc
+++ b/gdbserver/win32-low.cc
@@ -118,6 +118,14 @@ win32_require_context (windows_thread_info *th)
 
 /* See nat/windows-nat.h.  */
 
+void
+gdbserver_windows_process::fill_thread_context (windows_thread_info *th)
+{
+  win32_require_context (th);
+}
+
+/* See nat/windows-nat.h.  */
+
 windows_thread_info *
 gdbserver_windows_process::find_thread (ptid_t ptid)
 {

diff --git a/gdbserver/win32-low.h b/gdbserver/win32-low.h
index 01a904043ebf8931e36aabc17429bb693256ae4f..ff680492756111ae00227910d2e6972cb1a61f34 100644 (file)
--- a/gdbserver/win32-low.h
+++ b/gdbserver/win32-low.h
@@ -185,6 +185,8 @@ struct gdbserver_windows_process : public windows_nat::windows_process_info
   void handle_unload_dll (const DEBUG_EVENT &current_event) override;
   bool handle_access_violation (const EXCEPTION_RECORD *rec) override;
 
+  void fill_thread_context (windows_nat::windows_thread_info *th) override;
+
   int attaching = 0;
 
   /* A status that hasn't been reported to the core yet, and so