linux-kernel - Re: [PATCH v6 2/6] arm64: ptrace: allow tracer to skip a system call

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Message-ID: <20141003152328.GB32451@arm.com>
Date:	Fri, 3 Oct 2014 16:23:28 +0100
From:	Will Deacon <will.deacon@....com>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:	Kees Cook <keescook@...omium.org>,
	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 2/6] arm64: ptrace: allow tracer to skip a system call

On Wed, Oct 01, 2014 at 12:08:05PM +0100, AKASHI Takahiro wrote:
> Will,
> 
> When I was looking into syscall_trace_exit() more closely, I found
> another (big) problem.
> There are two system calls, execve() and rt_sigreturn(), which change
> 'syscallno' in pt_regs to -1 in start_thread() and restore_sigframe(),
> respectively.
> 
> Since syscallno is not valid anymore in syscall_trace_exit() for these
> system calls, we cannot create a correct syscall exit record for tracepoint
> in trace_sys_exit() (=> ftrace_syscall_exit()) and for audit in audit_syscall_exit().
> 
> This does not happen on arm because syscall numbers are kept in
> thread_info on arm.
> 
> How can we deal with this issue?

How is this handled on other architectures? x86, for example, seems to zero
orig_ax when restoring the sigcontext, but leaves it alone in start_thread.

What is the impact of this problem? AFAICT, we just miss some exits, right
(as opposed to an OOPs or the like)?

Will
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