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Subject: Re: [qemu-s390x] s390 qemu boot failure in -next
To:     Christian Borntraeger <borntraeger@de.ibm.com>,
        Guenter Roeck <linux@roeck-us.net>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc:     linux-s390@vger.kernel.org, Vasily Gorbik <gor@linux.ibm.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        linux-kernel@vger.kernel.org, qemu-devel <qemu-devel@nongnu.org>,
        qemu-s390x <qemu-s390x@nongnu.org>
References: <20180622194736.GA5794@roeck-us.net>
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From:   Thomas Huth <thuth@redhat.com>
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Message-ID: <7416298e-8f8a-fa1a-759f-da6526eab6ca@redhat.com>
Date:   Mon, 25 Jun 2018 10:27:26 +0200
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Sender: linux-kernel-owner@vger.kernel.org
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On 25.06.2018 10:02, Christian Borntraeger wrote:
> 
> 
> On 06/25/2018 09:27 AM, Christian Borntraeger wrote:
>> Also adding QEMU.
>>
>> On 06/25/2018 09:10 AM, Christian Borntraeger wrote:
>>>
>>>
>>> On 06/22/2018 09:47 PM, Guenter Roeck wrote:
>>>> Hi,
>>>>
>>>> starting with commit 's390/boot: make head.S and als.c be part of the
>>>> decompressor only' in -next, s390 immages no longer boot in qemu.
>>>> As far as I can see, the reason is that the command line is no longer
>>>> passed from qemu to the kernel, which results in a panic because the
>>>> root file system can not be mounted.
>>>>
>>>> Was this change made on purpose ? If so, is there a way to get qemu
>>>> back to working ?
>>>
>>> Certainly not on purpose.
>>>
>>> Vasily, I can reproduce this with KVM and an external kernel boot of the vmlinux file (the elf file)
>>>
>>> e.g.
>>>
>>> qemu-system-s390 -enable-kvm -nographic -kernel vmlinux -append "this string no longer is command line"
>>>
>>> The compressed image (bzImage) seems to work fine though.
>>>
>>> This seems to be an unfortunate side effect of QEMUs ways to "guess" its Linux (checking for start
>>> address 0x10000, which is no longer true for the vmlinux file). With the pure vmlinux elf file
>>> the load address is 0x100000 as there is no unpacker.
>>>
>>> Guenter, can you check if arch/s390/boot/bzImage works for you as a workaround?
>>
>> Something like this in QEMU 
>>
>> diff --git a/hw/s390x/ipl.c b/hw/s390x/ipl.c
>> index f278036fa7..14153ce880 100644
>> --- a/hw/s390x/ipl.c
>> +++ b/hw/s390x/ipl.c
>> @@ -187,11 +187,13 @@ static void s390_ipl_realize(DeviceState *dev, Error **errp)
>>           */
>>          if (pentry == KERN_IMAGE_START || pentry == 0x800) {
>>              ipl->start_addr = KERN_IMAGE_START;
>> -            /* Overwrite parameters in the kernel image, which are "rom" */
>> -            strcpy(rom_ptr(KERN_PARM_AREA), ipl->cmdline);
>>          } else {
>>              ipl->start_addr = pentry;
>>          }
>> +       if (ipl->cmdline) {
>> +            /* If there is a command line, put it in the right place */
>> +            strcpy(rom_ptr(KERN_PARM_AREA), ipl->cmdline);

You definitely need to check for rom_ptr() != NULL first before calling
strcpy. Otherwise QEMU segfaults in case the user tried to load a kernel
to a different location.

See also:
https://lists.gnu.org/archive/html/qemu-devel/2018-06/msg04227.html

 Thomas