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Backport fix for a race condition in GCancellable (rhbz#1825230)

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Tomas Popela 2020-04-28 08:20:28 +02:00
parent ba78ee1988
commit ec78e22b1b
2 changed files with 228 additions and 1 deletions
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220
gcancellable-race-fix.patch Normal file
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@ -0,0 +1,220 @@
From e4a690f5dd959e74b2d6054826f61509892c8aa7 Mon Sep 17 00:00:00 2001
From: Philip Withnall <withnall@endlessm.com>
Date: Fri, 21 Feb 2020 14:44:44 +0000
Subject: [PATCH] gcancellable: Fix minor race between GCancellable and
GCancellableSource
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
There’s a minor race condition between cancellation of a `GCancellable`,
and disposal/finalisation of a `GCancellableSource` in another thread.
Thread A Thread B
g_cancellable_cancel(C)
→cancellable_source_cancelled(C, S)
g_source_unref(S)
cancellable_source_dispose(S)
→→g_source_ref(S)
→→# S is invalid at this point; crash
Thankfully, the `GCancellable` sets `cancelled_running` while it’s
emitting the `cancelled` signal, so if `cancellable_source_dispose()` is
called while that’s high, we know that the thread which is doing the
cancellation has already started (or is committed to starting) calling
`cancellable_source_cancelled()`.
Fix the race by resurrecting the `GCancellableSource` in
`cancellable_source_dispose()`, and signalling this using
`GCancellableSource.resurrected_during_cancellation`. Check for that
flag in `cancellable_source_cancelled()` and ignore cancellation if it’s
set.
The modifications to `resurrected_during_cancellation` and the
cancellable source’s refcount have to be done with `cancellable_mutex`
held so that they are seen atomically by each thread. This should not
affect performance too much, as it only happens during cancellation or
disposal of a `GCancellableSource`.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Fixes: #1841
---
gio/gcancellable.c | 43 +++++++++++++++++++++++
gio/tests/cancellable.c | 77 +++++++++++++++++++++++++++++++++++++++++
2 files changed, 120 insertions(+)
diff --git a/gio/gcancellable.c b/gio/gcancellable.c
index d9e58b8e8..e687cca23 100644
--- a/gio/gcancellable.c
+++ b/gio/gcancellable.c
@@ -643,6 +643,8 @@ typedef struct {
GCancellable *cancellable;
gulong cancelled_handler;
+ /* Protected by cancellable_mutex: */
+ gboolean resurrected_during_cancellation;
} GCancellableSource;
/*
@@ -661,8 +663,24 @@ cancellable_source_cancelled (GCancellable *cancellable,
gpointer user_data)
{
GSource *source = user_data;
+ GCancellableSource *cancellable_source = (GCancellableSource *) source;
+
+ g_mutex_lock (&cancellable_mutex);
+
+ /* Drop the reference added in cancellable_source_dispose(); see the comment there.
+ * The reference must be dropped after unlocking @cancellable_mutex since
+ * it could be the final reference, and the dispose function takes
+ * @cancellable_mutex. */
+ if (cancellable_source->resurrected_during_cancellation)
+ {
+ cancellable_source->resurrected_during_cancellation = FALSE;
+ g_mutex_unlock (&cancellable_mutex);
+ g_source_unref (source);
+ return;
+ }
g_source_ref (source);
+ g_mutex_unlock (&cancellable_mutex);
g_source_set_ready_time (source, 0);
g_source_unref (source);
}
@@ -684,12 +702,37 @@ cancellable_source_dispose (GSource *source)
{
GCancellableSource *cancellable_source = (GCancellableSource *)source;
+ g_mutex_lock (&cancellable_mutex);
+
if (cancellable_source->cancellable)
{
+ if (cancellable_source->cancellable->priv->cancelled_running)
+ {
+ /* There can be a race here: if thread A has called
+ * g_cancellable_cancel() and has got as far as committing to call
+ * cancellable_source_cancelled(), then thread B drops the final
+ * ref on the GCancellableSource before g_source_ref() is called in
+ * cancellable_source_cancelled(), then cancellable_source_dispose()
+ * will run through and the GCancellableSource will be finalised
+ * before cancellable_source_cancelled() gets to g_source_ref(). It
+ * will then be left in a state where it’s committed to using a
+ * dangling GCancellableSource pointer.
+ *
+ * Eliminate that race by resurrecting the #GSource temporarily, and
+ * then dropping that reference in cancellable_source_cancelled(),
+ * which should be guaranteed to fire because we’re inside a
+ * @cancelled_running block.
+ */
+ g_source_ref (source);
+ cancellable_source->resurrected_during_cancellation = TRUE;
+ }
+
g_clear_signal_handler (&cancellable_source->cancelled_handler,
cancellable_source->cancellable);
g_clear_object (&cancellable_source->cancellable);
}
+
+ g_mutex_unlock (&cancellable_mutex);
}
static gboolean
diff --git a/gio/tests/cancellable.c b/gio/tests/cancellable.c
index 4ba9f6326..002bdccca 100644
--- a/gio/tests/cancellable.c
+++ b/gio/tests/cancellable.c
@@ -228,6 +228,82 @@ test_cancel_null (void)
g_cancellable_cancel (NULL);
}
+typedef struct
+{
+ GCond cond;
+ GMutex mutex;
+ GSource *cancellable_source; /* (owned) */
+} ThreadedDisposeData;
+
+static gboolean
+cancelled_cb (GCancellable *cancellable,
+ gpointer user_data)
+{
+ /* Nothing needs to be done here. */
+ return G_SOURCE_CONTINUE;
+}
+
+static gpointer
+threaded_dispose_thread_cb (gpointer user_data)
+{
+ ThreadedDisposeData *data = user_data;
+
+ /* Synchronise with the main thread before trying to reproduce the race. */
+ g_mutex_lock (&data->mutex);
+ g_cond_broadcast (&data->cond);
+ g_mutex_unlock (&data->mutex);
+
+ /* Race with cancellation of the cancellable. */
+ g_source_unref (data->cancellable_source);
+
+ return NULL;
+}
+
+static void
+test_cancellable_source_threaded_dispose (void)
+{
+ guint i;
+
+ g_test_summary ("Test a thread race between disposing of a GCancellableSource "
+ "(in one thread) and cancelling the GCancellable it refers "
+ "to (in another thread)");
+ g_test_bug ("https://gitlab.gnome.org/GNOME/glib/issues/1841");
+
+ for (i = 0; i < 100000; i++)
+ {
+ GCancellable *cancellable = NULL;
+ GSource *cancellable_source = NULL;
+ ThreadedDisposeData data;
+ GThread *thread = NULL;
+
+ /* Create a cancellable and a cancellable source for it. For this test,
+ * there’s no need to attach the source to a #GMainContext. */
+ cancellable = g_cancellable_new ();
+ cancellable_source = g_cancellable_source_new (cancellable);
+ g_source_set_callback (cancellable_source, G_SOURCE_FUNC (cancelled_cb), NULL, NULL);
+
+ /* Create a new thread and wait until it’s ready to execute before
+ * cancelling our cancellable. */
+ g_cond_init (&data.cond);
+ g_mutex_init (&data.mutex);
+ data.cancellable_source = g_steal_pointer (&cancellable_source);
+
+ g_mutex_lock (&data.mutex);
+ thread = g_thread_new ("/cancellable-source/threaded-dispose",
+ threaded_dispose_thread_cb, &data);
+ g_cond_wait (&data.cond, &data.mutex);
+ g_mutex_unlock (&data.mutex);
+
+ /* Race with disposal of the cancellable source. */
+ g_cancellable_cancel (cancellable);
+
+ g_thread_join (g_steal_pointer (&thread));
+ g_mutex_clear (&data.mutex);
+ g_cond_clear (&data.cond);
+ g_object_unref (cancellable);
+ }
+}
+
int
main (int argc, char *argv[])
{
@@ -235,6 +311,7 @@ main (int argc, char *argv[])
g_test_add_func ("/cancellable/multiple-concurrent", test_cancel_multiple_concurrent);
g_test_add_func ("/cancellable/null", test_cancel_null);
+ g_test_add_func ("/cancellable-source/threaded-dispose", test_cancellable_source_threaded_dispose);
return g_test_run ();
}
--
2.24.1

9
glib2.spec
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@ -2,13 +2,17 @@
Name: glib2
Version: 2.64.2
Release: 1%{?dist}
Release: 2%{?dist}
Summary: A library of handy utility functions
License: LGPLv2+
URL: http://www.gtk.org
Source0: http://download.gnome.org/sources/glib/2.64/glib-%{version}.tar.xz
# https://bugzilla.redhat.com/show_bug.cgi?id=1825230
# https://gitlab.gnome.org/GNOME/glib/-/commit/e4a690f5dd959e74b2d6054826f61509892c8aa7
Patch0: gcancellable-race-fix.patch
BuildRequires: chrpath
BuildRequires: gcc
BuildRequires: gcc-c++
@ -215,6 +219,9 @@ glib-compile-schemas %{_datadir}/glib-2.0/schemas &> /dev/null || :
%{_datadir}/installed-tests
%changelog
* Tue Apr 28 2020 Tomas Popela <tpopela@redhat.com> - 2.64.2-2
- Backport fix for a race condition in GCancellable (rhbz#1825230)
* Fri Apr 10 2020 Kalev Lember <klember@redhat.com> - 2.64.2-1
- Update to 2.64.2