Messages in this thread | | | Date | Mon, 10 Nov 2014 15:36:56 +0900 | From | AKASHI Takahiro <> | Subject | Re: [RFC] ptrace: add generic SET_SYSCALL request |
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On 11/07/2014 09:27 PM, Will Deacon wrote: > On Fri, Nov 07, 2014 at 12:03:00PM +0000, Arnd Bergmann wrote: >> On Friday 07 November 2014 11:55:51 Will Deacon wrote: >>> On Fri, Nov 07, 2014 at 09:30:53AM +0000, Arnd Bergmann wrote: >>>> On Friday 07 November 2014 16:47:23 AKASHI Takahiro wrote: >>>>> This patch adds a new generic ptrace request, PTRACE_SET_SYSCALL. >>>>> It can be used to change a system call number as follows: >>>>> ret = ptrace(pid, PTRACE_SET_SYSCALL, null, new_syscall_no); >>>>> 'new_syscall_no' can be -1 to skip this system call, you need to modify >>>>> a register's value, in arch-specific way, as return value though. >>>>> >>>>> Please note that we can't define PTRACE_SET_SYSCALL macro in >>>>> uapi/linux/ptrace.h partly because its value on arm, 23, is used as another >>>>> request on sparc. >>>>> >>>>> This patch also contains an example of change on arch side, arm. >>>>> Only syscall_set_nr() is required to be defined in asm/syscall.h. >>>>> >>>>> Currently only arm has this request, while arm64 would also have it >>>>> once my patch series of seccomp for arm64 is merged. It will also be >>>>> usable for most of other arches. >>>>> See the discussions in lak-ml: >>>>> http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/300167.html >>>>> >>>>> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org> >>>>> >>>> >>>> Can you describe why you are moving the implementation? Is this a feature >>>> that we want to have on all architectures in the future? As you say, >>>> only arm32 implements is at the moment. >>> >>> We need this for arm64 and, since all architectures seem to have a mechanism >>> for setting a system call via ptrace, moving it to generic code should make >>> sense for new architectures too, no? >> >> It makes a little more sense now, but I still don't understand why you >> need to set the system call number via ptrace. What is this used for, >> and why doesn't any other architecture have this? > > I went through the same thought process back in August, and Akashi > eventually convinced me that this was the best thing to do: > > http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/278692.html > > It comes down to a debugger (which could be GDB, seccomp, tracer ...) > wanting to change the system call number. This is also used as a mechanism > to skip a system call by setting it to '-1' (yeah, it's gross, and the > interaction between all of these syscall hooks is horrible too). > > If we update w8 directly instead, we run into a couple of issues: > > - Needing to restore the original w8 if the value is set to '-1' for > skip, but continuing to return -ENOSYS for syscall(-1) when not on a > tracer path
Yeah, this restriction still exists on my recent patch, v7. (this is because arm64 uses the same register, x0, as the first argument and a return value.)
> - seccomp assumes that syscall_get_nr will return the version set by > the most recent tracer, so then we need hacks in ptrace to route > register writes to w8 to syscallno in pt_regs, but again, only in the > case that we're tracing.
The problem here is that, if we had a hack of replacinging syscallno with w8 in ptrace (ptrace_syscall_enter()), secure_computing() (actually, seccomp_phase2() on v3.18-rc) would have no chance of seeing a modified syscall number because the hack would be executed after secure_computing(). (Please note that a tracer simply modifies w8, not syscallno directly).
This eventually results in missing a special case of -1 (skipping this system call). http://lists.infradead.org/pipermail/linux-arm-kernel/2014-August/280846.html
That is why we needed to have a dedicated new interface.
-Takahiro AKASHI
> Akashi might be able to elaborate on other problems, since this was a > couple of months ago and I take every opportunity I can to avoid looking > at this part of the kernel. > > Will >
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