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

[Christian Borntraeger]

On 13.02.20 19:02, Jason J. Herne wrote:
> On 2/6/20 5:09 AM, 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 <address@hidden>
>>> Signed-off-by: Jason J. Herne <address@hidden>
>>> ---
>>>   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;
>>
>>
>>
>> does something like
>>
>>
>> diff --git a/pc-bios/s390-ccw/jump2ipl.c b/pc-bios/s390-ccw/jump2ipl.c
>> index da13c43cc0..8839226803 100644
>> --- a/pc-bios/s390-ccw/jump2ipl.c
>> +++ b/pc-bios/s390-ccw/jump2ipl.c
>> @@ -18,6 +18,7 @@
>>   typedef struct ResetInfo {
>>       uint64_t ipl_psw;
>>       uint32_t ipl_continue;
>> +    uint32_t pad;
>>   } ResetInfo;
>>     static ResetInfo save;
>>
>>
>> also work? If yes, both variants are valid. Either packed or explicit 
>> padding.
>>
> 
> I don't believe this will work. I think the problem is that we're overwriting 
> too much memory when we cast address 0 as a ResetInfo and then overwrite it 
> (*current = save). I think we need the struct to be sized at 12-bytes instead 
> of 16.
> 

The idea of the code is that we _save_ the original content from address 0 to 
save and _restore_ it before jumping into final code. I do not yet understand 
why this does not work.