linux-kernel - Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call

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Message-ID: <20141125135626.GB8541@arm.com>
Date:	Tue, 25 Nov 2014 13:56:26 +0000
From:	Will Deacon <will.deacon@....com>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:	"keescook@...omium.org" <keescook@...omium.org>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"dsaxena@...aro.org" <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>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 2/6] arm64: ptrace: allow tracer to skip a system call

On Thu, Nov 20, 2014 at 05:52:34AM +0000, AKASHI Takahiro wrote:
> On 11/20/2014 02:13 PM, AKASHI Takahiro wrote:
> > On 11/20/2014 04:06 AM, Will Deacon wrote:
> >> Ok, but now userspace sees -ENOSYS for a skipped system call in that case,
> >> whereas it would usually see whatever the trace put in x0, right?
> >
> > Yes.
> > If you don't really like this behavior, how about this patch instead of my [2/6] patch?
> >
> > diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> > index 726b910..1ef57d0 100644
> > --- a/arch/arm64/kernel/entry.S
> > +++ b/arch/arm64/kernel/entry.S
> > @@ -668,8 +668,15 @@ ENDPROC(el0_svc)
> >           * switches, and waiting for our parent to respond.
> >           */
> >   __sys_trace:
> > +       cmp     w8, #-1                         // default errno for invalid
> 
> I needed to correct the code here:
> w8 should be w26, thinking of compat syscalls.
> 
> > +       b.ne    1f                              // system call
> > +       mov     x0, #-ENOSYS
> > +       str     x0, [sp, #S_X0]
> > +1:
> 
> and this part might better be generalized like the following:
> 
> __sys_trace:
> 	cmp	w26, w25	// cannot use x26 and x25 here
> 	b.hs	1f		// scno > sc_nr || scno < 0
> 	b	2f
> 1:
> 	mov	x0, #-ENOSYS
> 	str	x0, [sp, #S_X0]
> 2:
> 
> If you will be comfortable, I will submit a new patch soon.

Yes, please send a new series including this change.

Will
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