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

[Sebastian Andrzej Siewior]

On 08/01/2012 03:01 PM, Oleg Nesterov wrote:
> Lets ignore uprobes which needs the changes anyway. This is
> only used by ptrace and the task is stopped. So, unless I missed
> something obvious, this update_debugctlmsr() is simply unneeded,
> __switch_to/__switch_to_xtra should notice _TIF_BLOCKSTEP and do
> update_debugctlmsr(DEBUGCTLMSR_BTF).

It looks like it unless a processes ptraces itself (which does not make
much sense anyway).

> 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.

> IOW, it seems to me we could safely remove update_debugctlmsr()
> arch/x86/kernel/step.c. However, if we want to re-use this code
> in uprobes, then we probably need to add "if (child == current)".
It looks that way.

> Or I am totally confused. Help!
>
> Oleg.

Sebastian
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