linux-kernel - Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET

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Date:	Wed, 27 Aug 2014 14:32:44 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Kees Cook <keescook@...omium.org>
CC:	Will Deacon <will.deacon@....com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Deepak Saxena <dsaxena@...aro.org>,
	"arndb@...db.de" <arndb@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

Kees,

On 08/27/2014 02:46 AM, Will Deacon wrote:
> On Fri, Aug 22, 2014 at 01:19:13AM +0100, AKASHI Takahiro wrote:
>> On 08/22/2014 01:47 AM, Kees Cook wrote:
>>> On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
>>> <takahiro.akashi@...aro.org> wrote:
>>>> To allow tracer to be able to change/skip a system call by re-writing
>>>> a syscall number, there are several approaches:
>>>>
>>>> (1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this case
>>>>       later on in syscall_trace_enter(), or
>>>> (2) support ptrace(PTRACE_SET_SYSCALL) as on arm
>>>>
>>>> Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
>>>> tracer as well as that secure_computing() expects a changed syscall number
>>>> to be visible, especially case of -1, before this function returns in
>>>> syscall_trace_enter(), we'd better take (2).
>>>>
>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
>>>
>>> Thanks, I like having this on both arm and arm64.
>>
>> Yeah, having this simplified the code of syscall_trace_enter() a bit, but
>> also imposes some restriction on arm64, too.
>>
>>   > I wonder if other archs should add this option too.
>>
>> Do you think so? I assumed that SET_SYSCALL is to be avoided if possible.
>>
>> I also think that SET_SYSCALL should take an extra argument for a return value
>> just in case of -1 (or we have SKIP_SYSCALL?).
>
> I think we should propose this as a new request in the generic ptrace code.
> We can have an architecture-hook for actually setting the syscall, and allow
> architectures to define their own implementation of the request so they can
> be moved over one by one.

What do you think about this request?

-Takahiro AKASHI

> Will
>
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