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Date: Fri, 18 Mar 2016 14:02:49 +0000
From: James Morse <james.morse@arm.com>
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To: Pratyush Anand <panand@redhat.com>
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Subject: Re: [PATCH v11 3/9] arm64: add copy_to/from_user to kprobes blacklist
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Hi Pratyush,

On 18/03/16 13:29, Pratyush Anand wrote:
> Probably, I can see why does not it work. So, when we are single stepping an
> instruction and page fault occurs, we will come to el1_da in entry.S. Here, we
> do enable_dbg. As soon as we will do this, we will start receiving single step
> exception after each instruction (not sure, probably for each alternate
> instruction). Since, there will not be any matching single step handler for
> these instructions, so we will see warning "Unexpected kernel single-step
> exception at EL1". 
> 
> So, I think, we should 
> 
> (1) may be do not enable debug for el1_da, or
> (2) enable_dbg only when single stepping is not enabled, or
> (3) or disable single stepping during el1_da execution.
> 
> (1) will solve the issue for sure, but not sure if it could be the best choice.

A variation on (3):

In kernel/entry.S when entered from EL0 we test for TIF_SINGLESTEP in the
thread_info flags, and use disable_step_tsk/enable_step_tsk to save/restore the
single-step state.

Could we do this regardless of which EL we came from?


Thanks,

James