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

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Date:	Wed, 19 Nov 2014 19:06:01 +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 Wed, Nov 19, 2014 at 08:46:19AM +0000, AKASHI Takahiro wrote:
> On 11/18/2014 11:04 PM, Will Deacon wrote:
> > On Tue, Nov 18, 2014 at 01:10:34AM +0000, AKASHI Takahiro wrote:
> >>
> >> +	if (((int)regs->syscallno == -1) && (orig_syscallno == -1)) {
> >> +		/*
> >> +		 * user-issued syscall(-1):
> >> +		 * RESTRICTION: We always return ENOSYS whatever value is
> >> +		 *   stored in x0 (a return value) at this point.
> >> +		 * Normally, with ptrace off, syscall(-1) returns -ENOSYS.
> >> +		 * With ptrace on, however, if a tracer didn't pay any
> >> +		 * attention to user-issued syscall(-1) and just let it go
> >> +		 * without a hack here, it would return a value in x0 as in
> >> +		 * other system call cases. This means that this system call
> >> +		 * might succeed and see any bogus return value.
> >> +		 * This should be definitely avoided.
> >> +		 */
> >> +		regs->regs[0] = -ENOSYS;
> >> +	}
> >
> > I'm still really uncomfortable with this, and it doesn't seem to match what
> > arch/arm/ does either.
> 
> Yeah, I know but
> as I mentioned before, syscall(-1) will be signaled on arm, and so we don't
> have to care about a return value :)

What does x86 do?

> > Doesn't it also prevent a tracer from skipping syscall(-1)?
> 
> Syscall(-1) will return -ENOSYS whether or not a syscallno is explicitly
> replaced with -1 by a tracer, and, in this sense, it is *skipped*.

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?

Will
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