[PATCH 2/4] S390: Use own tbegin macro instead of __builtin_tbegin.

[Stefan Liebler]

On 01/12/2017 04:45 PM, Florian Weimer wrote:
> On 01/10/2017 05:34 PM, Torvald Riegel wrote:
>
>> (2) This introduces a facility to probe memory for being accessible or
>> not, considering that you say it masks segfaults.  It seems that this
>> probing may not be visible to the same extent as possible if a signal
>> handler were installed.  Is this relevant from a security perspective?
>
> If the fallback implementation has essentially the same behavior, I
> don't think there is a transaction-specific security problem.
>
> One thing to check is if anything in the transaction memory code writes
> unprotected function pointers/code addresses to memory.  I'm not
> familiar with z Systems machine code, so I don't know if that's the case.
>
> For example, it would be problematic to store the address of the
> transaction abort handler in a TLS variable.

The first tbegin instruction starts transactional execution mode.
Then the Transaction-Abort PSW is set to the instruction after the
tbegin instruction. If this transaction or multiple nested transactions 
are aborted, the instruction in Transaction-Abort PSW (after the 
outermost tbegin) is executed and the condition code set to 1-3.
The code behind tbegin has to determine what to do next.
There is no instruction to extract Transaction-Abort PSW.
>
>>> +               /* Begin transaction: save all gprs, allow    \
>>> +                  ar modification and fp operations.  Some    \
>>> +                  program-interruptions (e.g. a null    \
>>> +                  pointer access) are filtered and the    \
>>> +                  trancsaction will abort.  In this case    \
>
> Typo: “transaction”
okay.
>
> Thanks,
> Florian
>

I've attached the diff here and will later make one patch with changelog 
for this and the other two patches.




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