Re: [PATCH] ppc64: allow ptrace to set TM bits

[Cyril Bur <cyrilbur@gmail.com>]

On Tue, 2016-08-02 at 13:43 +0800, Simon Guo wrote:
> Hi Laurent,
> On Fri, Jul 29, 2016 at 11:51:22AM +0200, Laurent Dufour wrote:
> > 
> >  static int set_user_msr(struct task_struct *task, unsigned long
> > msr)
> >  {
> > +#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> > +	if (!(task->thread.regs->msr & MSR_TM)) {
> > +		/* If TM is not available, discard TM bits changes
> > */
> > +		msr &= ~(MSR_TM | MSR_TS_MASK);
> > +	}
> > +#endif
> 
> I am not sure whether following is an issue:
> Per PowerISA, any exception/interrupt will disable MSR[TM] bit 
> automatically and mark MSR_TS to be suspended when it is 
> transactional. It is possible that MSR[TM] = 0 and MSR[MSR_TS] != 0
> (suspended). 
> 
> Will set_user_msr() be able to escape from the above?
>  For example, one user space application encountered 
> page fault during transaction, its task->thread.regs->msr & MSR_TM ==
> 0
> and MSR[MSR_TS] == suspended.  Then it is being traced and 
> set_user_msr() is invoked on it. I think it will be incorrect to 
> clear its  MSR_TS_MASK bits.....
> 
> (suspended).ible that MSRTM] = 0 and MSR[MSR_TS] != 0> (suspended).

> Please correct me if I am wrong.

I'm not very familiar with ptracing and exactly what can happen but I
agree with Simon. Trying to change an MSR with that possible condition
stated ("It is possible that MSR[TM] = 0 and MSR[MSR_TS] !=
0> (suspended)") to MSR_TS and MSR_TS_MASK bits all 0 will cause a Bad
Thing.

Cyril

> 
> Thanks,
> - Simon
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