linux-kernel - Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

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Date:	Wed, 23 Jul 2014 18:09:11 +0900
From:	AKASHI Takahiro <takahiro.akashi@...aro.org>
To:	Will Deacon <will.deacon@....com>
CC:	Kees Cook <keescook@...omium.org>, Will Drewry <wad@...omium.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"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 v5 1/3] arm64: ptrace: reload a syscall number after ptrace
 operations

On 07/23/2014 05:25 PM, Will Deacon wrote:
> On Wed, Jul 23, 2014 at 08:03:47AM +0100, AKASHI Takahiro wrote:
>> On 07/23/2014 05:15 AM, Kees Cook wrote:
>>> On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
>>> <takahiro.akashi@...aro.org> wrote:
>>>>    asmlinkage int syscall_trace_enter(struct pt_regs *regs)
>>>>    {
>>>> +       unsigned long saved_x0, saved_x8;
>>>> +
>>>> +       saved_x0 = regs->regs[0];
>>>> +       saved_x8 = regs->regs[8];
>>>> +
>>>>           if (test_thread_flag(TIF_SYSCALL_TRACE))
>>>>                   tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
>>>>
>>>> +       regs->syscallno = regs->regs[8];
>>>> +       if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
>>>> +               regs->regs[8] = saved_x8;
>>>> +               if (regs->regs[0] == saved_x0) /* not changed by user */
>>>> +                       regs->regs[0] = -ENOSYS;
>>>
>>> I'm not sure this is right compared to other architectures. Generally
>>> when a tracer performs a syscall skip, it's up to them to also adjust
>>> the return value. They may want to be faking a syscall, and what if
>>> the value they want to return happens to be what x0 was going into the
>>> tracer? It would have no way to avoid this -ENOSYS case. I think
>>> things are fine without this test.
>>
>> Yeah, I know this issue, but was not sure that setting a return value
>> is mandatory. (x86 seems to return -ENOSYS by default if not explicitly
>> specified.)
>> Is "fake a system call" a more appropriate word than "skip"?
>>
>> I will defer to Will.
>
> I agree with Kees -- iirc, I only suggested restoring x8.

OK.

-Takahiro AKASHI

> Will
>
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