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| From: | Tony Krowiak |
| Subject: | Re: [Qemu-devel] [PATCH v3] qdev/core: fix qbus_is_full() |
| Date: | Wed, 9 Jan 2019 10:36:11 -0500 |
| User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 |
On 1/9/19 5:14 AM, Cornelia Huck wrote:
On Tue, 8 Jan 2019 15:34:37 -0500 Tony Krowiak <address@hidden> wrote:On 1/8/19 12:06 PM, Cornelia Huck wrote:On Tue, 8 Jan 2019 17:50:21 +0100 Halil Pasic <address@hidden> wrote:On Tue, 8 Jan 2019 17:31:13 +0100 Cornelia Huck <address@hidden> wrote:On Tue, 8 Jan 2019 11:08:56 -0500 Tony Krowiak <address@hidden> wrote:On 12/17/18 10:57 AM, Tony Krowiak wrote:The qbus_is_full(BusState *bus) function (qdev_monitor.c) compares the max_index value of the BusState structure with the max_dev value of the BusClass structure to determine whether the maximum number of children has been reached for the bus. The problem is, the max_index field of the BusState structure does not necessarily reflect the number of devices that have been plugged into the bus. Whenever a child device is plugged into the bus, the bus's max_index value is assigned to the child device and then incremented. If the child is subsequently unplugged, the value of the max_index does not change and no longer reflects the number of children. When the bus's max_index value reaches the maximum number of devices allowed for the bus (i.e., the max_dev field in the BusClass structure), attempts to plug another device will be rejected claiming that the bus is full -- even if the bus is actually empty. To resolve the problem, a new 'num_children' field is being added to the BusState structure to keep track of the number of children plugged into the bus. It will be incremented when a child is plugged, and decremented when a child is unplugged. Signed-off-by: Tony Krowiak <address@hidden> Reviewed-by: Pierre Morel<address@hidden> Reviewed-by: Halil Pasic <address@hidden> --- hw/core/qdev.c | 3 +++ include/hw/qdev-core.h | 1 + qdev-monitor.c | 2 +- 3 files changed, 5 insertions(+), 1 deletion(-)Ping ... I could not determine who the maintainer is for the three files listed above. I checked the MAINTAINERS file and the prologue of each individual file. Can someone please tell me who is responsible for merging these changes? Any additional review comments?diff --git a/hw/core/qdev.c b/hw/core/qdev.c index 6b3cc55b27c2..956923f33520 100644 --- a/hw/core/qdev.c +++ b/hw/core/qdev.c @@ -59,6 +59,8 @@ static void bus_remove_child(BusState *bus, DeviceState *child) snprintf(name, sizeof(name), "child[%d]", kid->index); QTAILQ_REMOVE(&bus->children, kid, sibling);+ bus->num_children--;+ /* This gives back ownership of kid->child back to us. */ object_property_del(OBJECT(bus), name, NULL); object_unref(OBJECT(kid->child)); @@ -73,6 +75,7 @@ static void bus_add_child(BusState *bus, DeviceState *child) char name[32]; BusChild *kid = g_malloc0(sizeof(*kid));+ bus->num_children++;kid->index = bus->max_index++;Hm... I'm wondering what happens for insane numbers of hotplugging operations here? (Preexisting problem for busses without limit, but busses with a limit could now run into that as well.)How does this patch change things? I mean bus->num_children gets decremented on unplug.We don't stop anymore if max_index >= max_dev, which means that we can now trigger that even if max_dev != 0.I guess I am missing your point. If max_dev == 0, then there is nothing stopping an insane number of hot plug operations; either before this patch, or with this patch. With the patch, once the number of children hot plugged reaches max_dev, the qbus_is_full function will return false and no more children can be plugged. If a child device is unplugged, the num_children - which counts the number of children attached to the bus - will be decremented, so it always reflects the number of children added to the bus. Besides, checking max_index against max_dev is erroneous, because max_index is incremented every time a child device is plugged and is never decremented. It really operates as more of a uinique identifier than a counter and is also used to create a unique property name when the child device is linked to the bus as a property (see bus_add_child function in hw/core/qdev.c).Checking num_children against max_dev is the right thing to do, no argument here. However, max_index continues to be incremented even if devices have been unplugged again. That means it can overflow, as it is never bound by the max_dev constraint. This has been a problem before for busses with an unrestricted number of devices before, but with your patch, the problem is now triggerable for all busses. Not a problem with your patch, but we might want to look into making max_index overflow/wraparound save.
I see your point. It does beg the question, what are the chances that max_index reaches INT_MAX? In the interest of making this code more bullet proof, I suppose it is something that should be dealt with. A search reveals that max_index is used in only two places: It is used to set the index for a child of the bus (i.e., bus_add_child function in hw/core/qdev.c); and prior to this patch, to determine if max_dev has been exceeded (i.e., qbus_is_full function in qdev_monitor.c). From what I can see, the bus child index is used only to generate a property name of the format "child[%d]" so the child can be linked as a property of the bus (see bus_add_child and bus_remove_child functions in hw/core/qdev.c). Wraparound of the max_index would most likely result in generating a duplicate property name for the child. I propose two possible solutions: 1. When max_index reaches INT_MAX, do not allow any additional children to be added to the bus. 2. Set a max_dev value of INT_MAX for the BusClass instance if the value is not set (in the bus_class_init function in hw/core/bus.c). 3. Instead of generating the property name from the BusChild index value, generate a UUID string. Since the index field of the BusChild appears to be used only to generate the child's name, maybe change the index field to a UUID field or a name field.
Regards, Halilkid->child = child; object_ref(OBJECT(kid->child)); diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h index a24d0dd566e3..521f0a947ead 100644 --- a/include/hw/qdev-core.h +++ b/include/hw/qdev-core.h @@ -206,6 +206,7 @@ struct BusState { HotplugHandler *hotplug_handler; int max_index; bool realized; + int num_children; QTAILQ_HEAD(ChildrenHead, BusChild) children; QLIST_ENTRY(BusState) sibling; }; diff --git a/qdev-monitor.c b/qdev-monitor.c index 07147c63bf8b..45a8ba49644c 100644 --- a/qdev-monitor.c +++ b/qdev-monitor.c @@ -414,7 +414,7 @@ static DeviceState *qbus_find_dev(BusState *bus, char *elem) static inline bool qbus_is_full(BusState *bus) { BusClass *bus_class = BUS_GET_CLASS(bus); - return bus_class->max_dev && bus->max_index >= bus_class->max_dev; + return bus_class->max_dev && bus->num_children >= bus_class->max_dev; }/*The approach the patch takes looks sane to me.
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