On Fri, Aug 22, 2014 at 01:19:13AM +0100, AKASHI Takahiro wrote:

On 08/22/2014 01:47 AM, Kees Cook wrote:

quoted

On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
[off-list ref] wrote:

quoted

To allow tracer to be able to change/skip a system call by re-writing
a syscall number, there are several approaches:

(1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this case
     later on in syscall_trace_enter(), or
(2) support ptrace(PTRACE_SET_SYSCALL) as on arm

Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
tracer as well as that secure_computing() expects a changed syscall number
to be visible, especially case of -1, before this function returns in
syscall_trace_enter(), we'd better take (2).

Signed-off-by: AKASHI Takahiro <redacted>

Thanks, I like having this on both arm and arm64.

Yeah, having this simplified the code of syscall_trace_enter() a bit, but
also imposes some restriction on arm64, too.

 > I wonder if other archs should add this option too.

Do you think so? I assumed that SET_SYSCALL is to be avoided if possible.

I also think that SET_SYSCALL should take an extra argument for a return value
just in case of -1 (or we have SKIP_SYSCALL?).

I think we should propose this as a new request in the generic ptrace code.
We can have an architecture-hook for actually setting the syscall, and allow
architectures to define their own implementation of the request so they can
be moved over one by one.

Will