Bug#842513: vlc: immediate crash on launch on powerpc

[Robert Ou]

On Sun, Oct 30, 2016 at 4:40 AM, James Cowgill <jcowgill at debian.org> wrote:
> Control: tags -1 - help
> Control: severity -1 important
> Control: retitle -1 vlc: should prevent installation on powerpc G3
>
> Hi,
>
> On 30/10/16 00:16, Robert Ou wrote:
>> On Sat, Oct 29, 2016 at 3:43 PM, James Cowgill <jcowgill at debian.org> wrote:
>>> Control: tags -1 help
>>> Control: severity -1 grave
>>>
>>> Hi,
>>>
>>> On 29/10/16 23:00, Robert Ou wrote:
>>>> Package: src:vlc
>>>> Version: 2.2.4-7
>>>> Severity: normal
>>>>
>>>> Dear Maintainer,
>>>>
>>>> I decided I wanted to test the performance of Debian PowerPC on my
>>>> ancient iMac, and I discovered that vlc will immediately crash with an
>>>> illegal instruction right after showing the "Privacy and Network Access
>>>> Policy" window and before showing the main window. The crashes look like
>>>> the following:
>>>>
>>>> [ 1560.952016] vlc[997]: unhandled signal 4 at 0ea48f58 nip 0ea48f58 lr 0ea48f4c code 30001
>>>
>>> As powerpc is a release architecture, this bug is RC.
>>>
>>> I tried running vlc on partch. It managed to get further, but then
>>> segfaulted inside QT so it's probably a separate issue. I also had to
>>> run it in xvfb so it probably gets different results.
>>>
>>> Specifically what powerpc hardware do you have? Could you run vlc within
>>> gdb to determine which instruction it SIGILLs on (try 'layout asm')?
>>
>> I was testing on a first-generation iMac with a 333 MHz PowerPC 750
>> (G3). Running vlc under gdb shows that the crash occurs in
>> libqt4_plugin.so in QRect::adjusted. The crash occurs on a "lvx
>> v0,r10,r5" opcode, which is an Altivec opcode. The G3 however does not
>> support Altivec. Here is a backtrace and some more debug information:
>
> This explains it. From the PowerPC FAQ:
> https://wiki.debian.org/PowerPC/FAQ#VLC_crashes_on_startup._What.27s_up_with_that.3F
>
> "
> If VLC immediately crashes, it's probably because you're on a G3 and
> VLC was compiled with Altivec instructions. To use VLC on a G3, you
> must compile it with the configure option --disable-altivec.
> "
>
> Having said that, I think adding something to vlc's preinst to prevent
> installation on systems without altivec would be a good idea here.
>
> I'm not sure going through all the trouble of compiling vlc twice so it
> works on the G3 is worth it here - especially since powerpc may not be
> a release architecture in 24 hours...
>

Sorry, I completely missed that FAQ entry amongst the other random
bugs/quirks listed there. I am totally fine with not having vlc on
this system (I didn't expect vlc to actually be able to play anything
at a reasonable speed on this ancient a system). However, will
blacklisting it in the preinst step break packages like
task-xfce-desktop or break people who already had vlc installed and
tried to upgrade?