From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Fri, 15 Aug 2014 15:33:46 +0100 Subject: [PATCH v5 3/3] arm64: Add seccomp support In-Reply-To: <53E9F7E1.9090405@linaro.org> References: <53D082E9.8090303@amacapital.net> <53D09C4F.9010104@linaro.org> <53D2255C.9050006@linaro.org> <53E47DCE.4080902@linaro.org> <20140811092409.GE15344@arm.com> <53E9BAD5.3080201@linaro.org> <20140812094058.GD29013@arm.com> <53E9F7E1.9090405@linaro.org> Message-ID: <20140815143346.GV27466@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Tue, Aug 12, 2014 at 12:17:53PM +0100, AKASHI Takahiro wrote: > On 08/12/2014 06:40 PM, Will Deacon wrote: > > On Tue, Aug 12, 2014 at 07:57:25AM +0100, AKASHI Takahiro wrote: > >> > >> > case SECCOMP_MODE_FILTER: > >> > case SECCOMP_RET_TRACE: > >> > ... > >> > if (syscall_get_nr(current, regs) < 0) > >> > goto skip; > >> > >> This implies that we should modify syscallno *before* __secure_computing() > >> returns. > > > > Why does it imply that? There are four competing entities here: > > > > - seccomp > > - tracehook > > - ftrace (trace_sys_*) > > - audit > > > > With the exception of ftrace, they can all potentially rewrite the pt_regs > > (the code you cite above is just below a ptrace_event call), so we have > > to choose some order in which to call them. > > (audit won't change registers.) Sorry, you're quite right. > > On entry, x86 and arm call them in the order I listed above, so it seems > > sensible to follow that. > > Right, but as far as I understand, ptrace_event() in __secure_computing() > calls ptrace_notify(), and eventually executes ptrace_stop(), which can > be stopped while tracer runs (until ptrace(PTRACE_CONT)?). > So syscall_get_nr() is expected to return -1 if trace changes a syscall number to -1 > (as far as sycall_get_nr() refers to syscallno in pt_regs). > > That is why I think we should have PTRACE_SET_SYSCALL. Gotcha, yeah that looks like the cleanest approach after all. Thanks for the explanation. Will