[kvm-unit-tests PATCH 03/32] powerpc: Fix stack backtrace termination

[Thomas Huth]

On 27/02/2024 09.50, Thomas Huth wrote:
> On 26/02/2024 11.11, Nicholas Piggin wrote:
>> The backtrace handler terminates when it sees a NULL caller address,
>> but the powerpc stack setup does not keep such a NULL caller frame
>> at the start of the stack.
>>
>> This happens to work on pseries because the memory at 0 is mapped and
>> it contains 0 at the location of the return address pointer if it
>> were a stack frame. But this is fragile, and does not work with powernv
>> where address 0 contains firmware instructions.
>>
>> Use the existing dummy frame on stack as the NULL caller, and create a
>> new frame on stack for the entry code.
>>
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>>   powerpc/cstart64.S | 12 ++++++++++--
>>   1 file changed, 10 insertions(+), 2 deletions(-)
> 
> Thanks for tackling this! ... however, not doing powerpc work since years 
> anymore, I have some ignorant questions below...
> 
>> diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
>> index e18ae9a22..14ab0c6c8 100644
>> --- a/powerpc/cstart64.S
>> +++ b/powerpc/cstart64.S
>> @@ -46,8 +46,16 @@ start:
>>       add    r1, r1, r31
>>       add    r2, r2, r31
>> +    /* Zero backpointers in initial stack frame so backtrace() stops */
>> +    li    r0,0
>> +    std    r0,0(r1)
> 
> 0(r1) is the back chain pointer ...
> 
>> +    std    r0,16(r1)
> 
> ... but what is 16(r1) ? I suppose that should be the "LR save word" ? But 
> isn't that at 8(r1) instead?? (not sure whether I'm looking at the right ELF 
> abi spec right now...)

Ok, I was looking at the wrong ELF spec, indeed (it was an ancient 32-bit 
spec, not the 64-bit ABI). Sorry for the confusion. Having a proper #define 
or a comment for the 16 here would still be helpful, though.

  Thomas