Re: [PATCH 2/3] Fix crash if connection drops in scoped_restore_current_thread's ctor, part 2

[Pedro Alves <pedro@palves.net>]

On 7/9/20 4:31 AM, Simon Marchi wrote:
> On 2020-07-08 7:31 p.m., Pedro Alves wrote:
>> Running the testsuite against an Asan-enabled build of GDB makes
>> gdb.base/multi-target.exp expose this bug.
>>
>> scoped_restore_current_thread's ctor calls get_frame_id to record the
>> selected frame's ID to restore later.  If the frame ID hasn't been
>> computed yet, it will be computed on the spot, and that will usually
>> require accessing the target's memory and registers.  If the remote
>> connection closes, while we're computing the frame ID, the remote
>> target exits its inferiors, unpushes itself, and throws a
>> TARGET_CLOSE_ERROR error.  Exiting the inferiors deletes the
>> inferior's threads.
>>
>> scoped_restore_current_thread increments the current thread's refcount
>> to prevent the thread from being deleted from under its feet.
>> However, the code that does that isn't considering the case of the
>> thread being deleted from within get_frame_id.  It only increments the
>> refcount _after_ get_frame_id returns.  So if the current thread is
>> indeed deleted, the
>>
>>      tp->incref ();
>>
>> statement references a stale TP pointer.
>>
>> Incrementing the refcounts earlier fixes it.
>>
>> We should probably also let the TARGET_CLOSE_ERROR error propagate in
>> this case.  That alone would fix it, though it seems better to tweak
>> the refcount handling too.
> 
> So, when the target closes while we (scoped_restore_current_thread) own
> a reference on the inferior and thread, the inferior and thread are still
> destroyed, and so we shouldn't decref them?

Aw, no, I got confused and misremembered how exceptions in ctors work.
The dtor for scoped_restore_current_thread isn't called, and I assumed
it was called.  We need to decref the inferior and thread before letting
the exception propagate, otherwise we leak them.  I'm testing the updated
version of the patch below, which does that:

	  /* Better let this propagate.  */
	  if (ex.error == TARGET_CLOSE_ERROR)
	    {
	      m_thread->decref ();
	      m_inf->decref ();
	      throw;
	    }

It passes multi-target.exp with Asan-enabled GDB.  Running the full
testsuite now.

From 1ad36a4b892fc4425d6f24c298713eeafece7b04 Mon Sep 17 00:00:00 2001
From: Pedro Alves <pedro@palves.net>
Date: Tue, 7 Jul 2020 01:50:10 +0100
Subject: [PATCH] Fix crash if connection drops in
 scoped_restore_current_thread's ctor, part 2

Running the testsuite against an Asan-enabled build of GDB makes
gdb.base/multi-target.exp expose this bug.

scoped_restore_current_thread's ctor calls get_frame_id to record the
selected frame's ID to restore later.  If the frame ID hasn't been
computed yet, it will be computed on the spot, and that will usually
require accessing the target's memory and registers.  If the remote
connection closes, while we're computing the frame ID, the remote
target exits its inferiors, unpushes itself, and throws a
TARGET_CLOSE_ERROR error.  Exiting the inferiors deletes the
inferior's threads.

scoped_restore_current_thread increments the current thread's refcount
to prevent the thread from being deleted from under its feet.
However, the code that does that isn't considering the case of the
thread being deleted from within get_frame_id.  It only increments the
refcount _after_ get_frame_id returns.  So if the current thread is
indeed deleted, the

     tp->incref ();

statement references a stale TP pointer.

Incrementing the refcounts earlier fixes it.

We should probably also let the TARGET_CLOSE_ERROR error propagate in
this case.  That alone would fix it, though it seems better to tweak
the refcount handling too.

gdb/ChangeLog:

	* thread.c
	(scoped_restore_current_thread::scoped_restore_current_thread):
	Incref the thread before calling get_frame_id instead of after.
	Let TARGET_CLOSE_ERROR propagate.
---
 gdb/thread.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/gdb/thread.c b/gdb/thread.c
index f0722d3588..a3c2be7dd0 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -1433,15 +1433,17 @@ scoped_restore_current_thread::~scoped_restore_current_thread ()
 
 scoped_restore_current_thread::scoped_restore_current_thread ()
 {
-  m_thread = NULL;
   m_inf = current_inferior ();
+  m_inf->incref ();
 
   if (inferior_ptid != null_ptid)
     {
-      thread_info *tp = inferior_thread ();
+      m_thread = inferior_thread ();
+      m_thread->incref ();
+
       struct frame_info *frame;
 
-      m_was_stopped = tp->state == THREAD_STOPPED;
+      m_was_stopped = m_thread->state == THREAD_STOPPED;
       if (m_was_stopped
 	  && target_has_registers
 	  && target_has_stack
@@ -1466,13 +1468,18 @@ scoped_restore_current_thread::scoped_restore_current_thread ()
 	{
 	  m_selected_frame_id = null_frame_id;
 	  m_selected_frame_level = -1;
-	}
 
-      tp->incref ();
-      m_thread = tp;
+	  /* Better let this propagate.  */
+	  if (ex.error == TARGET_CLOSE_ERROR)
+	    {
+	      m_thread->decref ();
+	      m_inf->decref ();
+	      throw;
+	    }
+	}
     }
-
-  m_inf->incref ();
+  else
+    m_thread = NULL;
 }
 
 /* See gdbthread.h.  */

base-commit: ad8464f799a4c96c7ab8bdfec3f95846cf54f9b0
prerequisite-patch-id: 32ffdda7d7d774bc4df88bf848bcb796559b53ce
prerequisite-patch-id: 02021b74355b70debd344a6e445285c67dfef7d6
prerequisite-patch-id: c87fcf5a54f6805967cbf8ab107606c57d9ecf52
prerequisite-patch-id: ac7dee583d0ffa519c9d1cd89d27664bca68d8c1
prerequisite-patch-id: eac59ae2ea85d2d51e5be1b03e88a5641cc12c22
prerequisite-patch-id: 13da42ad04dc8e2e3bd6a556a0be0e17cf23669b
prerequisite-patch-id: fd3f09fdb58ddc1c595ea014716851f4c8fca48c
prerequisite-patch-id: 8230262cb24020a5b37e915843180e940807ba1f
-- 
2.14.5