linux-kernel - Re: [PATCH v4 13/16] ARM: Add an emulate flag to the kprobes/uprobes instruction decode functions

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Date:	Thu, 16 Jan 2014 13:12:32 -0500
From:	David Long <dave.long@...aro.org>
To:	"Jon Medhurst (Tixy)" <tixy@...aro.org>
CC:	linux-arm-kernel@...ts.infradead.org,
	Russell King <linux@....linux.org.uk>,
	Rabin Vincent <rabin@....in>, Oleg Nesterov <oleg@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	davem@...emloft.net, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 13/16] ARM: Add an emulate flag to the kprobes/uprobes
 instruction decode functions

On 01/16/14 04:18, Jon Medhurst (Tixy) wrote:
> On Wed, 2014-01-15 at 14:31 -0500, David Long wrote:
>> On 12/20/13 09:58, Jon Medhurst (Tixy) wrote:
>>> On Sun, 2013-12-15 at 23:08 -0500, David Long wrote:
> [...]
>>>>    {
>>>>    #ifdef CONFIG_THUMB2_KERNEL
>>>>    	if (thumb) {
>>>> @@ -253,7 +253,7 @@ set_emulated_insn(probes_opcode_t insn, struct arch_specific_insn *asi,
>>>>     * non-zero value, the corresponding nibble in pinsn is validated and modified
>>>>     * according to the type.
>>>>     */
>>>> -static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs)
>>>> +static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs, bool modify)
>>>>    {
>>>>    	probes_opcode_t insn = *pinsn;
>>>>    	probes_opcode_t mask = 0xf; /* Start at least significant nibble */
>>>> @@ -317,9 +317,16 @@ static bool __kprobes decode_regs(probes_opcode_t *pinsn, u32 regs)
>>>>    		/* Replace value of nibble with new register number... */
>>>>    		insn &= ~mask;
>>>>    		insn |= new_bits & mask;
>>>> +		if (modify) {
>>>> +			/* Replace value of nibble with new register number */
>>>> +			insn &= ~mask;
>>>> +			insn |= new_bits & mask;
>>>> +		}
>>>
>>> Huh? As is, the above addition doesn't do anything because insn has
>>> already been modified. I guess you played with the idea that you needed
>>> to avoid changing insn (you don't) and then didn't undo the experiment
>>> quite right. :-)
>>>
>>
>> The conditional modification of the instruction was part of Rabin's
>> original work for uprobes, but I messed up the merge from an earlier
>> working version of my patches.  My intention was/is to delete the old
>> unconditional code.  Sounds like maybe you disagree though.  The intent
>> is to only modify the instruction in the kprobes case.
>
> 'insn' is the local variable containing the instruction value we're
> processing. It doesn't matter if we change that, we just need to avoid
> updating the instruction in memory, which the code in the next chunk
> already correctly checks for...
>
>>>>    	}
>>>>
>>>> -	*pinsn = insn;
>>>> +	if (modify)
>>>> +		*pinsn = insn;
>>>> +
>>>>    	return true;
>>>>
>
> So only one of these 'if (modify)' checks is required for code
> correctness, and I suggest keeping the second one as it's more explicit
> and defensive.
>
>

OK, I see your point.  I shall simplify the code as you have suggested.

-dl


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