Messages in this thread | | | Date | Wed, 1 Aug 2012 16:31:32 +0200 | From | Oleg Nesterov <> | Subject | Re: Q: user_enable_single_step() && update_debugctlmsr() |
| |
On 08/01, Sebastian Andrzej Siewior wrote: > > 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.
OK, I was confuse by "kprobes" above.
And I think you missed my point. I'll try again.
We have the GDB process and the (stopped) tracee T. And we have another task X which have TIF_SINGLESTEP but not TIF_BLOCKSTEP. To simplify, suppose that X is already TASK_RUNNING but not on rq.
GDB does ptrace(PTRACE_SINGLEBLOCK, T). This sets X->TIF_BLOCKSTEP. Now suppose that GDB is preempted right after it does update_debugctlmsr(), and the scheduler choses X as the next task.
Both GDB and X do not have TIF_BLOCKSTEP, so __switch_to_extra() does not update DEBUGCTLMSR_BTF.
X returns to the user-mode with TIF_SINGLESTEP and TIF_BLOCKSTEP, the latter is wrong.
No?
Oleg.
| |