Hi Akashi,
On Tue, Aug 12, 2014 at 07:57:25AM +0100, AKASHI Takahiro wrote:
On 08/11/2014 06:24 PM, Will Deacon wrote:
On Fri, Aug 08, 2014 at 08:35:42AM +0100, AKASHI Takahiro wrote:
As discussed in a few weeks ago, aarch64 won't support PTRACE_SET_SYSCALL.
Well, I don't think anything was set in stone. If you have a compelling reason why adding the new request gives you something over setting w8 directly, then we can extend ptrace.
Yeah, I think I may have to change my mind. Looking into __secure_computing(), I found the code below:
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.
On entry, x86 and arm call them in the order I listed above, so it seems sensible to follow that.
I assumed, in my next version, we could skip a system call by overwriting syscallno with x8 in syscall_trace_enter() after __secure_computing() returns 0, and it actually works.
Why does overwriting the syscallno with x8 skip the syscall?
I thought the idea was that we would save w8 prior to each call that could change the pt_regs, then if it was changed to -1 we would replace it with the saved value and return -1? The only confusion I have is whether we should call the exit hooks after skipping a syscall. I *think* x86 does call them, but ARM doesn't. Andy says this can trigger an OOPs:
http://lists.infradead.org/pipermail/linux-arm-kernel/2014-July/274988.html
so we should fix that for ARM while we're here.
Will