Re: [PATCH] pc-bios/s390x: Pack ResetInfo struct

[Christian Borntraeger <borntraeger@de.ibm.com>]

Jason,

can you run objdump -Sdr on jump2ipl.o on a broken variant?


On 06.02.20 12:00, Thomas Huth wrote:
> On 06/02/2020 11.09, Christian Borntraeger wrote:
>>
>>
>> On 05.02.20 19:21, Jason J. Herne wrote:
>>> This fixes vfio-ccw when booting non-Linux operating systems. Without this
>>> struct being packed, a few extra bytes of low core memory get overwritten when
>>> we  assign a value to memory address 0 in jump_to_IPL_2. This is enough to
>>> cause some non-Linux OSes of fail when booting.
>>>
>>> The problem was introduced by:
>>> 5c6f0d5f46a77d77 "pc-bios/s390x: Fix reset psw mask".
>>>
>>> The fix is to pack the struct thereby removing the 4 bytes of padding that get
>>> added at the end, likely to allow an array of these structs to naturally align
>>> on an 8-byte boundary.
>>>
>>> Fixes: 5c6f0d5f46a7 ("pc-bios/s390x: Fix reset psw mask")
>>> CC: Janosch Frank <frankja@linux.ibm.com>
>>> Signed-off-by: Jason J. Herne <jjherne@linux.ibm.com>
>>> ---
>>>  pc-bios/s390-ccw/jump2ipl.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
>>> index da13c43cc0..1e9eaa037f 100644
>>> --- a/pc-bios/s390-ccw/jump2ipl.c
>>> +++ b/pc-bios/s390-ccw/jump2ipl.c
>>> @@ -18,7 +18,7 @@
>>>  typedef struct ResetInfo {
>>>      uint64_t ipl_psw;
>>>      uint32_t ipl_continue;
>>> -} ResetInfo;
>>> +} __attribute__((packed)) ResetInfo;
>>>  
>>>  static ResetInfo save;
>>
>> Just looked into that.
>>
>> We do save the old content in "save" and restore the old memory content.
>>
>> static void jump_to_IPL_2(void)
>> {
>>     ResetInfo *current = 0;
>>
>>     void (*ipl)(void) = (void *) (uint64_t) current->ipl_continue;
>> --->*current = save;
>>     ipl(); /* should not return */
>> }
>>
>> void jump_to_IPL_code(uint64_t address)
>> {
>>     /* store the subsystem information _after_ the bootmap was loaded */
>>     write_subsystem_identification();
>>
>>     /* prevent unknown IPL types in the guest */
>>     if (iplb.pbt == S390_IPL_TYPE_QEMU_SCSI) {
>>         iplb.pbt = S390_IPL_TYPE_CCW;
>>         set_iplb(&iplb);
>>     }
>>
>>     /*
>>      * The IPL PSW is at address 0. We also must not overwrite the
>>      * content of non-BIOS memory after we loaded the guest, so we
>>      * save the original content and restore it in jump_to_IPL_2.
>>      */
>>     ResetInfo *current = 0;
>>
>> --->save = *current;
> 
> Right, and this should also work without your modification. I've stared
> at the code a couple of weeks ago, looking for a very similar issue:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2019-12/msg03484.html
> 
> ... but in the end, the problem was something else:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2019-12/msg03520.html
> 
> and the fix had been done in the startup code of the test:
> 
>  https://lists.gnu.org/archive/html/qemu-devel/2019-12/msg04225.html
> 
> So I'd guess that you face the very same problem here. That means, you
> either have to convince the non-Linux OS to check their startup code
> whether they depend on zeroed registers somewhere, or we fix this issue
> for good in jump_to_IPL_2() by clearing the registers there before
> jumping into the OS code (which we likely should do anyway since the OS
> may expect a clean state).
> 
>  Thomas
> 
>