Re: [Qemu-devel] [PATCH] tci: don't write zero for reloc in tci_out_label

[Stefan Weil <sw@weilnetz.de>]

Am 19.06.2012 20:02, schrieb Blue Swirl:
> On Tue, Jun 19, 2012 at 5:53 AM, Stefan Weil <sw@weilnetz.de> wrote:
>> Am 19.06.2012 04:31, schrieb Scott Wood:
>>
>>> If tci_out_label is called in the context of tcg_gen_code_search_pc, we
>>> could be overwriting an already patched relocation with zero -- and not
>>> repatch it because the set_label is past search_pc, causing a QEMU crash
>>> when it tries to branch to a zero label.
>>>
>>> Not writing anything to the relocation area seems to be in line with 
>>> what
>>> other backends do from the couple I looked at (x86, ppc).
>>
>>
>> Thanks, this might fix a crash which I have seen from time to time.
>> I'll run tests as soon as possible.

Tested-by: Stefan Weil <sw@weilnetz.de>

The patch fixes my test scenario with a guest booting a Debian ARM kernel
on a x86_64 host (Linux or W64).

The following command crashes while the ARM Linux guest is booting
(shortly after "Freeing init memory") with SIGSEGV caused by tb_ptr == 2:

$ qemu-system-arm -M versatilepb -m 192 \
     -kernel vmlinuz-3.2.0-2-versatile -initrd initrd.img-3.2.0-2-versatile

With the patch applied, the ARM Linux boots correctly.

Blue, maybe that test also works with a SPARC host.
I have copied the ARM kernel and initrd to http://qemu.weilnetz.de/arm/.

Regards,
Stefan W.

>>
>> Could you please also look at the other backends?
>>
>> I saw from git history that ppc once had the same bug.
>> The sparc backend (and maybe others) might still have it.
>
> Confirmed for Sparc.
>
>>
>> Regards,
>> Stefan W.
>>
>>
>>>
>>> Signed-off-by: Scott Wood<scottwood@freescale.com>
>>> ---
>>>  tcg/tci/tcg-target.c |    2 +-
>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>
>>> diff --git a/tcg/tci/tcg-target.c b/tcg/tci/tcg-target.c
>>> index 453f187..3c6b0f5 100644
>>> --- a/tcg/tci/tcg-target.c
>>> +++ b/tcg/tci/tcg-target.c
>>> @@ -487,7 +487,7 @@ static void tci_out_label(TCGContext *s, TCGArg arg)
>>>          assert(label->u.value);
>>>      } else {
>>>          tcg_out_reloc(s, s->code_ptr, sizeof(tcg_target_ulong), 
>>> arg, 0);
>>> -        tcg_out_i(s, 0);
>>> +        s->code_ptr += sizeof(tcg_target_ulong);
>
> I like this fix. Other similar fixes rewrote the fixed part of the
> opcode and not the label, but the fixed part may cross byte
> boundaries.
>
>>>      }
>>>  }
>>>
>>
>