[PATCH] hw/display/xenfb: Replace unreachable code by abort()

[PATCH] hw/display/xenfb: Replace unreachable code by abort()

[Markus Armbruster]

xenfb_mouse_event() has a switch statement whose controlling
expression move->axis is an enum InputAxis.  The enum values are
INPUT_AXIS_X and INPUT_AXIS_Y, encoded as 0 and 1.  The switch has a
case for both axes.  In addition, it has an unreachable default label.
This convinces Coverity that move->axis can be greater than 1.  It
duly reports a buffer overrun when it is used to subscript an array
with two elements.

Replace the unreachable code by abort().

Resolves: Coverity CID 1613906
Signed-off-by: Markus Armbruster <armbru@redhat.com>
---
 hw/display/xenfb.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
index 22822fecea..5e6c691779 100644
--- a/hw/display/xenfb.c
+++ b/hw/display/xenfb.c
@@ -283,8 +283,7 @@ static void xenfb_mouse_event(DeviceState *dev, QemuConsole *src,
                 scale = surface_height(surface) - 1;
                 break;
             default:
-                scale = 0x8000;
-                break;
+                abort();
             }
             xenfb->axis[move->axis] = move->value * scale / 0x7fff;
         }
-- 
2.49.0

Re: [PATCH] hw/display/xenfb: Replace unreachable code by abort()

[Philippe Mathieu-Daudé]

On 29/7/25 13:12, Markus Armbruster wrote:
> xenfb_mouse_event() has a switch statement whose controlling
> expression move->axis is an enum InputAxis.  The enum values are
> INPUT_AXIS_X and INPUT_AXIS_Y, encoded as 0 and 1.  The switch has a
> case for both axes.  In addition, it has an unreachable default label.
> This convinces Coverity that move->axis can be greater than 1.  It
> duly reports a buffer overrun when it is used to subscript an array
> with two elements.
> 
> Replace the unreachable code by abort().
> 
> Resolves: Coverity CID 1613906
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---
>   hw/display/xenfb.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> index 22822fecea..5e6c691779 100644
> --- a/hw/display/xenfb.c
> +++ b/hw/display/xenfb.c
> @@ -283,8 +283,7 @@ static void xenfb_mouse_event(DeviceState *dev, QemuConsole *src,
>                   scale = surface_height(surface) - 1;
>                   break;
>               default:
> -                scale = 0x8000;
> -                break;
> +                abort();

We prefer GLib g_assert_not_reached() over abort() because it displays
the file, line number & function before aborting.

>               }
>               xenfb->axis[move->axis] = move->value * scale / 0x7fff;
>           }

Re: [PATCH] hw/display/xenfb: Replace unreachable code by abort()

[Markus Armbruster]

Philippe Mathieu-Daudé <philmd@linaro.org> writes:

> On 29/7/25 13:12, Markus Armbruster wrote:
>> xenfb_mouse_event() has a switch statement whose controlling
>> expression move->axis is an enum InputAxis.  The enum values are
>> INPUT_AXIS_X and INPUT_AXIS_Y, encoded as 0 and 1.  The switch has a
>> case for both axes.  In addition, it has an unreachable default label.
>> This convinces Coverity that move->axis can be greater than 1.  It
>> duly reports a buffer overrun when it is used to subscript an array
>> with two elements.
>> Replace the unreachable code by abort().
>> Resolves: Coverity CID 1613906
>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>> ---
>>   hw/display/xenfb.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
>> index 22822fecea..5e6c691779 100644
>> --- a/hw/display/xenfb.c
>> +++ b/hw/display/xenfb.c
>> @@ -283,8 +283,7 @@ static void xenfb_mouse_event(DeviceState *dev, QemuConsole *src,
>>                   scale = surface_height(surface) - 1;
>>                   break;
>>               default:
>> -                scale = 0x8000;
>> -                break;
>> +                abort();
>
> We prefer GLib g_assert_not_reached() over abort() because it displays
> the file, line number & function before aborting.

The purpose of this line is to tell the compiler we can't get there,
with the least amount of ceremony.

We have ~600 calls of abort().

Whoever merges this: feel free to replace by g_assert_not_reached().

>>               }
>>               xenfb->axis[move->axis] = move->value * scale / 0x7fff;
>>           }

Re: [PATCH] hw/display/xenfb: Replace unreachable code by abort()

[Philippe Mathieu-Daudé]

On 29/7/25 14:16, Markus Armbruster wrote:
> Philippe Mathieu-Daudé <philmd@linaro.org> writes:
> 
>> On 29/7/25 13:12, Markus Armbruster wrote:
>>> xenfb_mouse_event() has a switch statement whose controlling
>>> expression move->axis is an enum InputAxis.  The enum values are
>>> INPUT_AXIS_X and INPUT_AXIS_Y, encoded as 0 and 1.  The switch has a
>>> case for both axes.  In addition, it has an unreachable default label.
>>> This convinces Coverity that move->axis can be greater than 1.  It
>>> duly reports a buffer overrun when it is used to subscript an array
>>> with two elements.
>>> Replace the unreachable code by abort().
>>> Resolves: Coverity CID 1613906
>>> Signed-off-by: Markus Armbruster <armbru@redhat.com>
>>> ---
>>>    hw/display/xenfb.c | 3 +--
>>>    1 file changed, 1 insertion(+), 2 deletions(-)
>>> diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
>>> index 22822fecea..5e6c691779 100644
>>> --- a/hw/display/xenfb.c
>>> +++ b/hw/display/xenfb.c
>>> @@ -283,8 +283,7 @@ static void xenfb_mouse_event(DeviceState *dev, QemuConsole *src,
>>>                    scale = surface_height(surface) - 1;
>>>                    break;
>>>                default:
>>> -                scale = 0x8000;
>>> -                break;
>>> +                abort();
>>
>> We prefer GLib g_assert_not_reached() over abort() because it displays
>> the file, line number & function before aborting.
> 
> The purpose of this line is to tell the compiler we can't get there,
> with the least amount of ceremony.
> 
> We have ~600 calls of abort().

And ~1600 of g_assert_not_reached() =)

$ git grep -w 'abort();' | wc -l
      556
$ git grep -w 'g_assert_not_reached();' | wc -l
     1551

> Whoever merges this: feel free to replace by g_assert_not_reached().
> 
>>>                }
>>>                xenfb->axis[move->axis] = move->value * scale / 0x7fff;
>>>            }
> 

Re: [PATCH] hw/display/xenfb: Replace unreachable code by abort()

[Daniel P. Berrangé]

On Tue, Jul 29, 2025 at 02:59:46PM +0200, Philippe Mathieu-Daudé wrote:
> On 29/7/25 14:16, Markus Armbruster wrote:
> > Philippe Mathieu-Daudé <philmd@linaro.org> writes:
> > 
> > > On 29/7/25 13:12, Markus Armbruster wrote:
> > > > xenfb_mouse_event() has a switch statement whose controlling
> > > > expression move->axis is an enum InputAxis.  The enum values are
> > > > INPUT_AXIS_X and INPUT_AXIS_Y, encoded as 0 and 1.  The switch has a
> > > > case for both axes.  In addition, it has an unreachable default label.
> > > > This convinces Coverity that move->axis can be greater than 1.  It
> > > > duly reports a buffer overrun when it is used to subscript an array
> > > > with two elements.
> > > > Replace the unreachable code by abort().
> > > > Resolves: Coverity CID 1613906
> > > > Signed-off-by: Markus Armbruster <armbru@redhat.com>
> > > > ---
> > > >    hw/display/xenfb.c | 3 +--
> > > >    1 file changed, 1 insertion(+), 2 deletions(-)
> > > > diff --git a/hw/display/xenfb.c b/hw/display/xenfb.c
> > > > index 22822fecea..5e6c691779 100644
> > > > --- a/hw/display/xenfb.c
> > > > +++ b/hw/display/xenfb.c
> > > > @@ -283,8 +283,7 @@ static void xenfb_mouse_event(DeviceState *dev, QemuConsole *src,
> > > >                    scale = surface_height(surface) - 1;
> > > >                    break;
> > > >                default:
> > > > -                scale = 0x8000;
> > > > -                break;
> > > > +                abort();
> > > 
> > > We prefer GLib g_assert_not_reached() over abort() because it displays
> > > the file, line number & function before aborting.
> > 
> > The purpose of this line is to tell the compiler we can't get there,
> > with the least amount of ceremony.
> > 
> > We have ~600 calls of abort().
> 
> And ~1600 of g_assert_not_reached() =)
> 
> $ git grep -w 'abort();' | wc -l
>      556
> $ git grep -w 'g_assert_not_reached();' | wc -l
>     1551

Sounds like we could create a gitlab issue for "replace abort with
g_assert_not_reached" that could be a easy on-ramp for someone to
start contributing.

With regards,
Daniel
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