Re: Q: user_enable_single_step() && update_debugctlmsr()

[Sebastian Andrzej Siewior]

On 08/01/2012 04:01 PM, Oleg Nesterov wrote:
> On 08/01, Sebastian Andrzej Siewior wrote:
> > On 08/01/2012 03:46 PM, Oleg Nesterov wrote:
> >
> > > > > But, worse, isn't it wrong? Suppose that debugger switches to
> > > > > another TIF_SINGLESTEP&&    !TIF_BLOCKSTEP task, in this case
> > > > > we "leak" DEBUGCTLMSR_BTF, no?
> > > >
> > > > __switch_to_xtra() should notice the difference in the TIF_BLOCKSTEP
> > > > flag and disable it.
> > >
> > > And how it can notice the difference if there is no difference?
> > >
> > > (unless, of course debugger is TIF_BLOCKSTEP'ed).
> >
> > Yes. enable_step() sets DEBUGCTLMSR_BTF along with TIF_BLOCKSTEP.
> > kprobes checks the same flag before touching DEBUGCTLMSR_BTF.
>
> It seems that you replied to the wrong email or I am confused ;)

No I think I replied to the correct one :)
enable_step() is the only place for ptrace/debugger which is touching
DEBUGCTLMSR_BTF. It always sets DEBUGCTLMSR_BTF and TIF_BLOCKSTEP in
sync so why should they both end up different? And once 
__switch_to_extra() notices that TIF_BLOCKSTEP from the previous task
is different from the next task is different, then the CPU flag has
to be changed.

> Let's ignore kprobes here.

done.

> Oleg.


Sebastian
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