diff mbox

RFC: gdbserver crash on win XP during watchpoint removal

Message ID 543DC1FF.8030109@redhat.com
State New
Headers show

Commit Message

Pedro Alves Oct. 15, 2014, 12:38 a.m. UTC
On 10/14/2014 10:37 PM, Joel Brobecker wrote:
>> dr_control_mirror is never supposed to be updated from the target.  It's a
>> one way street - the state we _want_ dr7 to have on next resume.  Where is this
>> 0x400 coming from then?
>>
>> Also, what's different in native GDB?  This debug registers code is
>> shared between GDB and GDBserver now.
> 
> It is read back in win32-i386-low.c::i386_get_thread_context:
> 
>   if (th->tid == current_event->dwThreadId)
>     {
>       /* Copy dr values from the current thread.  */
>       struct x86_debug_reg_state *dr = &debug_reg_state;
>       dr->dr_mirror[0] = th->context.Dr0;
>       dr->dr_mirror[1] = th->context.Dr1;
>       dr->dr_mirror[2] = th->context.Dr2;
>       dr->dr_mirror[3] = th->context.Dr3;
>       dr->dr_status_mirror = th->context.Dr6;
>       dr->dr_control_mirror = th->context.Dr7;
>     }
> 
> debug_reg_state is a global, which is passed to x86_dr_insert_watchpoint
> in win32-i386-low.c::i386_insert_point:
> 
>         return x86_dr_insert_watchpoint (&debug_reg_state,
>                                          hw_type, addr, size);
> 
> i386_get_thread_context gets called after we receive the watchpoint
> signal (TARGET_WAITKIND_STOPPED) in win32_wait:
> 
>         case TARGET_WAITKIND_STOPPED:
>         case TARGET_WAITKIND_LOADED:
>           OUTMSG2 (("Child Stopped with signal = %d \n",
>                     ourstatus->value.sig));
> 
>           regcache = get_thread_regcache (current_thread, 1);
>           child_fetch_inferior_registers (regcache, -1);
>           return debug_event_ptid (&current_event);
> 
> It looks like just removing the statement that sets dr_control_mirror
> from the thread's DR7 is sufficient to allow hardware watchpoints
> to work again.

Yes, that sounds good enough for 7.8.1.  This is OK.

> I admit I could have spent a little more time trying to understand
> the bigger picture, but I'm a bit under time pressure, so I'm hoping
> this is enough. Otherwise, I'll look at a better fix.

Hmm, I've wanted to do this for a while.  So...  Below's is what I
think is the better fix.

------ 8< ---------
From 64451d0e90e363c86539236aa3b1761ca2eff0a7 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Tue, 14 Oct 2014 22:52:41 +0100
Subject: [PATCH] gdbserver/win32: Rewrite debug registers handling

Don't use debug_reg_state for both:

 * "intent" - what we want the debug registers to look like

 * "reality" - what/which were the contents of the DR registers when
   the event triggered

Reserve it for the former only, like in the GNU/Linux port.

Otherwise the core x86 debug registers code can get confused if the
inferior itself changes the debug registers since GDB last set them.

This is also a requirement for being able to set watchpoints while the
target is running, if/when we get to it on Windows.  See the big
comment in x86_dr_stopped_data_address.

Seems to me this may also fixes propagating watchpoints to all threads
-- continue_one_thread only calls win32_set_thread_context (what
copies the DR registers to the thread), if something already fetched
the thread's context before.  Something else may be masking this
issue, I haven't checked.

Smoke tested by running gdbserver under Wine, connecting to it from
GNU/Linux, and checking that I could trigger a watchpoint as expected.

gdb/
2014-10-14  Pedro Alves  <palves@redhat.com>

	* win32-arm-low.c (arm_set_thread_context): Remove current_event
	parameter.
	(arm_set_thread_context): Delete.
	(the_low_target): Adjust.
	* win32-i386-low.c (debug_registers_changed)
	(debug_registers_used): Delete.
	(update_debug_registers_callback): New function.
	(x86_dr_low_set_addr, x86_dr_low_set_control): Mark all threads as
	needing to update their debug registers.
	(win32_get_current_dr): New function.
	(x86_dr_low_get_addr, x86_dr_low_get_control)
	(x86_dr_low_get_status): Fetch the debug register from the thread
	record's context.
	(i386_initial_stuff): Adjust.
	(i386_get_thread_context): Remove current_event parameter.  Don't
	clear debug_registers_changed nor copy DR values to
	debug_reg_state.
	(i386_set_thread_context): Delete.
	(i386_prepare_to_resume): New function.
	(i386_thread_added): Mark the thread as needing to update irs
	debug registers.
	(the_low_target): Remove i386_set_thread_context and install
	i386_prepare_to_resume.
	* win32-low.c (win32_get_thread_context): Adjust.
	(win32_set_thread_context): Use SetThreadContext
	directly.
	(win32_prepare_to_resume): New function.
	(win32_require_context): New function, factored out from ...
	(thread_rec): ... this.
	(continue_one_thread): Call win32_prepare_to_resume on each thread
	we're about to continue.
	(win32_resume): Call win32_prepare_to_resume on the event thread.
	* win32-low.h (struct win32_thread_info)
	<debug_registers_changed>: New field.
	(struct win32_target_ops): Change prototype of set_thread_context,
	delete set_thread_context and add prepare_to_resume.
	(win32_require_context): New declaration.
---
 gdb/gdbserver/win32-arm-low.c  |  10 +--
 gdb/gdbserver/win32-i386-low.c | 137 ++++++++++++++++++++++-------------------
 gdb/gdbserver/win32-low.c      |  73 ++++++++++++++--------
 gdb/gdbserver/win32-low.h      |  15 +++--
 4 files changed, 134 insertions(+), 101 deletions(-)

Comments

Joel Brobecker Oct. 15, 2014, 2:35 p.m. UTC | #1
> > It looks like just removing the statement that sets dr_control_mirror
> > from the thread's DR7 is sufficient to allow hardware watchpoints
> > to work again.
> 
> Yes, that sounds good enough for 7.8.1.  This is OK.

Thanks, Pedro. I created PR server/17487 for this, and pushed
the patch on gdb-7.8-branch using that PR.

> > I admit I could have spent a little more time trying to understand
> > the bigger picture, but I'm a bit under time pressure, so I'm hoping
> > this is enough. Otherwise, I'll look at a better fix.
> 
> Hmm, I've wanted to do this for a while.  So...  Below's is what I
> think is the better fix.

Thanks! I tested your patch on x86-windows using AdaCore's testsuite,
and it also fixes the problem, with no regression. Do you want to push
it? I left the PR open pending your patch going in...
Pedro Alves Oct. 15, 2014, 7:02 p.m. UTC | #2
On 10/15/2014 03:35 PM, Joel Brobecker wrote:
>>> It looks like just removing the statement that sets dr_control_mirror
>>> from the thread's DR7 is sufficient to allow hardware watchpoints
>>> to work again.
>>
>> Yes, that sounds good enough for 7.8.1.  This is OK.
> 
> Thanks, Pedro. I created PR server/17487 for this, and pushed
> the patch on gdb-7.8-branch using that PR.
> 
>>> I admit I could have spent a little more time trying to understand
>>> the bigger picture, but I'm a bit under time pressure, so I'm hoping
>>> this is enough. Otherwise, I'll look at a better fix.
>>
>> Hmm, I've wanted to do this for a while.  So...  Below's is what I
>> think is the better fix.
> 
> Thanks! I tested your patch on x86-windows using AdaCore's testsuite,
> and it also fixes the problem, with no regression. Do you want to push
> it? 

Awesome, I pushed it in then.

> I left the PR open pending your patch going in...

Thanks,
Pedro Alves
diff mbox

Patch

diff --git a/gdb/gdbserver/win32-arm-low.c b/gdb/gdbserver/win32-arm-low.c
index cf64514..f4667ff 100644
--- a/gdb/gdbserver/win32-arm-low.c
+++ b/gdb/gdbserver/win32-arm-low.c
@@ -27,7 +27,7 @@  void init_registers_arm (void);
 extern const struct target_desc *tdesc_arm;
 
 static void
-arm_get_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
+arm_get_thread_context (win32_thread_info *th)
 {
   th->context.ContextFlags = \
     CONTEXT_FULL | \
@@ -36,12 +36,6 @@  arm_get_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
   GetThreadContext (th->h, &th->context);
 }
 
-static void
-arm_set_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
-{
-  SetThreadContext (th->h, &th->context);
-}
-
 #define context_offset(x) ((int)&(((CONTEXT *)NULL)->x))
 static const int mappings[] = {
   context_offset (R0),
@@ -124,7 +118,7 @@  struct win32_target_ops the_low_target = {
   sizeof (mappings) / sizeof (mappings[0]),
   NULL, /* initial_stuff */
   arm_get_thread_context,
-  arm_set_thread_context,
+  NULL, /* prepare_to_resume */
   NULL, /* thread_added */
   arm_fetch_inferior_register,
   arm_store_inferior_register,
diff --git a/gdb/gdbserver/win32-i386-low.c b/gdb/gdbserver/win32-i386-low.c
index b4b99e8..6d6b310 100644
--- a/gdb/gdbserver/win32-i386-low.c
+++ b/gdb/gdbserver/win32-i386-low.c
@@ -40,29 +40,36 @@  extern const struct target_desc *tdesc_i386;
 
 static struct x86_debug_reg_state debug_reg_state;
 
-static int debug_registers_changed = 0;
-static int debug_registers_used = 0;
+static int
+update_debug_registers_callback (struct inferior_list_entry *entry,
+				 void *pid_p)
+{
+  struct thread_info *thr = (struct thread_info *) entry;
+  win32_thread_info *th = inferior_target_data (thr);
+  int pid = *(int *) pid_p;
+
+  /* Only update the threads of this process.  */
+  if (pid_of (thr) == pid)
+    {
+      /* The actual update is done later just before resuming the lwp,
+	 we just mark that the registers need updating.  */
+      th->debug_registers_changed = 1;
+    }
+
+  return 0;
+}
 
 /* Update the inferior's debug register REGNUM from STATE.  */
 
 static void
 x86_dr_low_set_addr (int regnum, CORE_ADDR addr)
 {
-  gdb_assert (DR_FIRSTADDR <= regnum && regnum <= DR_LASTADDR);
-
-  /* debug_reg_state.dr_mirror is already set.
-     Just notify i386_set_thread_context, i386_thread_added
-     that the registers need to be updated.  */
-  debug_registers_changed = 1;
-  debug_registers_used = 1;
-}
+  /* Only update the threads of this process.  */
+  int pid = pid_of (current_thread);
 
-static CORE_ADDR
-x86_dr_low_get_addr (int regnum)
-{
   gdb_assert (DR_FIRSTADDR <= regnum && regnum <= DR_LASTADDR);
 
-  return debug_reg_state.dr_mirror[regnum];
+  find_inferior (&all_threads, update_debug_registers_callback, &pid);
 }
 
 /* Update the inferior's DR7 debug control register from STATE.  */
@@ -70,17 +77,53 @@  x86_dr_low_get_addr (int regnum)
 static void
 x86_dr_low_set_control (unsigned long control)
 {
-  /* debug_reg_state.dr_control_mirror is already set.
-     Just notify i386_set_thread_context, i386_thread_added
-     that the registers need to be updated.  */
-  debug_registers_changed = 1;
-  debug_registers_used = 1;
+  /* Only update the threads of this process.  */
+  int pid = pid_of (current_thread);
+
+  find_inferior (&all_threads, update_debug_registers_callback, &pid);
+}
+
+/* Return the current value of a DR register of the current thread's
+   context.  */
+
+static DWORD64
+win32_get_current_dr (int dr)
+{
+  win32_thread_info *th = inferior_target_data (current_thread);
+
+  win32_require_context (th);
+
+#define RET_DR(DR)				\
+  case DR:					\
+    return th->context.Dr ## DR
+
+  switch (dr)
+    {
+      RET_DR (0);
+      RET_DR (1);
+      RET_DR (2);
+      RET_DR (3);
+      RET_DR (6);
+      RET_DR (7);
+    }
+
+#undef RET_DR
+
+  gdb_assert_not_reached ("unhandled dr");
+}
+
+static CORE_ADDR
+x86_dr_low_get_addr (int regnum)
+{
+  gdb_assert (DR_FIRSTADDR <= regnum && regnum <= DR_LASTADDR);
+
+  return win32_get_current_dr (regnum - DR_FIRSTADDR);
 }
 
 static unsigned long
 x86_dr_low_get_control (void)
 {
-  return debug_reg_state.dr_control_mirror;
+  return win32_get_current_dr (7);
 }
 
 /* Get the value of the DR6 debug status register from the inferior
@@ -89,9 +132,7 @@  x86_dr_low_get_control (void)
 static unsigned long
 x86_dr_low_get_status (void)
 {
-  /* We don't need to do anything here, the last call to thread_rec for
-     current_event.dwThreadId id has already set it.  */
-  return debug_reg_state.dr_status_mirror;
+  return win32_get_current_dr (6);
 }
 
 /* Low-level function vector.  */
@@ -181,12 +222,10 @@  static void
 i386_initial_stuff (void)
 {
   x86_low_init_dregs (&debug_reg_state);
-  debug_registers_changed = 0;
-  debug_registers_used = 0;
 }
 
 static void
-i386_get_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
+i386_get_thread_context (win32_thread_info *th)
 {
   /* Requesting the CONTEXT_EXTENDED_REGISTERS register set fails if
      the system doesn't support extended registers.  */
@@ -210,28 +249,17 @@  i386_get_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
 
       error ("GetThreadContext failure %ld\n", (long) e);
     }
-
-  debug_registers_changed = 0;
-
-  if (th->tid == current_event->dwThreadId)
-    {
-      /* Copy dr values from the current thread.  */
-      struct x86_debug_reg_state *dr = &debug_reg_state;
-      dr->dr_mirror[0] = th->context.Dr0;
-      dr->dr_mirror[1] = th->context.Dr1;
-      dr->dr_mirror[2] = th->context.Dr2;
-      dr->dr_mirror[3] = th->context.Dr3;
-      dr->dr_status_mirror = th->context.Dr6;
-      dr->dr_control_mirror = th->context.Dr7;
-    }
 }
 
 static void
-i386_set_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
+i386_prepare_to_resume (win32_thread_info *th)
 {
-  if (debug_registers_changed)
+  if (th->debug_registers_changed)
     {
       struct x86_debug_reg_state *dr = &debug_reg_state;
+
+      win32_require_context (th);
+
       th->context.Dr0 = dr->dr_mirror[0];
       th->context.Dr1 = dr->dr_mirror[1];
       th->context.Dr2 = dr->dr_mirror[2];
@@ -239,32 +267,15 @@  i386_set_thread_context (win32_thread_info *th, DEBUG_EVENT* current_event)
       /* th->context.Dr6 = dr->dr_status_mirror;
 	 FIXME: should we set dr6 also ?? */
       th->context.Dr7 = dr->dr_control_mirror;
-    }
 
-  SetThreadContext (th->h, &th->context);
+      th->debug_registers_changed = 0;
+    }
 }
 
 static void
 i386_thread_added (win32_thread_info *th)
 {
-  /* Set the debug registers for the new thread if they are used.  */
-  if (debug_registers_used)
-    {
-      struct x86_debug_reg_state *dr = &debug_reg_state;
-      th->context.ContextFlags = CONTEXT_DEBUG_REGISTERS;
-      GetThreadContext (th->h, &th->context);
-
-      th->context.Dr0 = dr->dr_mirror[0];
-      th->context.Dr1 = dr->dr_mirror[1];
-      th->context.Dr2 = dr->dr_mirror[2];
-      th->context.Dr3 = dr->dr_mirror[3];
-      /* th->context.Dr6 = dr->dr_status_mirror;
-	 FIXME: should we set dr6 also ?? */
-      th->context.Dr7 = dr->dr_control_mirror;
-
-      SetThreadContext (th->h, &th->context);
-      th->context.ContextFlags = 0;
-    }
+  th->debug_registers_changed = 1;
 }
 
 static void
@@ -450,7 +461,7 @@  struct win32_target_ops the_low_target = {
   sizeof (mappings) / sizeof (mappings[0]),
   i386_initial_stuff,
   i386_get_thread_context,
-  i386_set_thread_context,
+  i386_prepare_to_resume,
   i386_thread_added,
   i386_fetch_inferior_register,
   i386_store_inferior_register,
diff --git a/gdb/gdbserver/win32-low.c b/gdb/gdbserver/win32-low.c
index ee99fe4..e714933 100644
--- a/gdb/gdbserver/win32-low.c
+++ b/gdb/gdbserver/win32-low.c
@@ -129,7 +129,7 @@  static void
 win32_get_thread_context (win32_thread_info *th)
 {
   memset (&th->context, 0, sizeof (CONTEXT));
-  (*the_low_target.get_thread_context) (th, &current_event);
+  (*the_low_target.get_thread_context) (th);
 #ifdef _WIN32_WCE
   memcpy (&th->base_context, &th->context, sizeof (CONTEXT));
 #endif
@@ -153,23 +153,24 @@  win32_set_thread_context (win32_thread_info *th)
      it between stopping and resuming.  */
   if (memcmp (&th->context, &th->base_context, sizeof (CONTEXT)) != 0)
 #endif
-    (*the_low_target.set_thread_context) (th, &current_event);
+    SetThreadContext (th->h, &th->context);
 }
 
-/* Find a thread record given a thread id.  If GET_CONTEXT is set then
-   also retrieve the context for this thread.  */
-static win32_thread_info *
-thread_rec (ptid_t ptid, int get_context)
+/* Set the thread context of the thread associated with TH.  */
+
+static void
+win32_prepare_to_resume (win32_thread_info *th)
 {
-  struct thread_info *thread;
-  win32_thread_info *th;
+  if (the_low_target.prepare_to_resume != NULL)
+    (*the_low_target.prepare_to_resume) (th);
+}
 
-  thread = (struct thread_info *) find_inferior_id (&all_threads, ptid);
-  if (thread == NULL)
-    return NULL;
+/* See win32-low.h.  */
 
-  th = inferior_target_data (thread);
-  if (get_context && th->context.ContextFlags == 0)
+void
+win32_require_context (win32_thread_info *th)
+{
+  if (th->context.ContextFlags == 0)
     {
       if (!th->suspended)
 	{
@@ -185,7 +186,23 @@  thread_rec (ptid_t ptid, int get_context)
 
       win32_get_thread_context (th);
     }
+}
 
+/* Find a thread record given a thread id.  If GET_CONTEXT is set then
+   also retrieve the context for this thread.  */
+static win32_thread_info *
+thread_rec (ptid_t ptid, int get_context)
+{
+  struct thread_info *thread;
+  win32_thread_info *th;
+
+  thread = (struct thread_info *) find_inferior_id (&all_threads, ptid);
+  if (thread == NULL)
+    return NULL;
+
+  th = inferior_target_data (thread);
+  if (get_context)
+    win32_require_context (th);
   return th;
 }
 
@@ -419,22 +436,26 @@  continue_one_thread (struct inferior_list_entry *this_thread, void *id_ptr)
   int thread_id = * (int *) id_ptr;
   win32_thread_info *th = inferior_target_data (thread);
 
-  if ((thread_id == -1 || thread_id == th->tid)
-      && th->suspended)
+  if (thread_id == -1 || thread_id == th->tid)
     {
-      if (th->context.ContextFlags)
-	{
-	  win32_set_thread_context (th);
-	  th->context.ContextFlags = 0;
-	}
+      win32_prepare_to_resume (th);
 
-      if (ResumeThread (th->h) == (DWORD) -1)
+      if (th->suspended)
 	{
-	  DWORD err = GetLastError ();
-	  OUTMSG (("warning: ResumeThread failed in continue_one_thread, "
-		   "(error %d): %s\n", (int) err, strwinerror (err)));
+	  if (th->context.ContextFlags)
+	    {
+	      win32_set_thread_context (th);
+	      th->context.ContextFlags = 0;
+	    }
+
+	  if (ResumeThread (th->h) == (DWORD) -1)
+	    {
+	      DWORD err = GetLastError ();
+	      OUTMSG (("warning: ResumeThread failed in continue_one_thread, "
+		       "(error %d): %s\n", (int) err, strwinerror (err)));
+	    }
+	  th->suspended = 0;
 	}
-      th->suspended = 0;
     }
 
   return 0;
@@ -937,6 +958,8 @@  win32_resume (struct thread_resume *resume_info, size_t n)
   th = thread_rec (ptid, FALSE);
   if (th)
     {
+      win32_prepare_to_resume (th);
+
       if (th->context.ContextFlags)
 	{
 	  /* Move register values from the inferior into the thread
diff --git a/gdb/gdbserver/win32-low.h b/gdb/gdbserver/win32-low.h
index 4e84957..f4422ed 100644
--- a/gdb/gdbserver/win32-low.h
+++ b/gdb/gdbserver/win32-low.h
@@ -47,6 +47,10 @@  typedef struct win32_thread_info
 
   /* The context of the thread, including any manipulations.  */
   CONTEXT context;
+
+  /* Whether debug registers change since we last set CONTEXT back to
+     the thread.  */
+  int debug_registers_changed;
 } win32_thread_info;
 
 struct win32_target_ops
@@ -61,12 +65,10 @@  struct win32_target_ops
   void (*initial_stuff) (void);
 
   /* Fetch the context from the inferior.  */
-  void (*get_thread_context) (win32_thread_info *th,
-			      DEBUG_EVENT *current_event);
+  void (*get_thread_context) (win32_thread_info *th);
 
-  /* Flush the context back to the inferior.  */
-  void (*set_thread_context) (win32_thread_info *th,
-			      DEBUG_EVENT *current_event);
+  /* Called just before resuming the thread.  */
+  void (*prepare_to_resume) (win32_thread_info *th);
 
   /* Called when a thread was added.  */
   void (*thread_added) (win32_thread_info *th);
@@ -96,6 +98,9 @@  struct win32_target_ops
 
 extern struct win32_target_ops the_low_target;
 
+/* Retrieve the context for this thread, if not already retrieved.  */
+extern void win32_require_context (win32_thread_info *th);
+
 /* Map the Windows error number in ERROR to a locale-dependent error
    message string and return a pointer to it.  Typically, the values
    for ERROR come from GetLastError.