Re: [PATCH v5 1/3] arm64: ptrace: reload a syscall number after ptrace operations

[Will Deacon <will.deacon@arm.com>]

On Wed, Jul 23, 2014 at 08:03:47AM +0100, AKASHI Takahiro wrote:
> On 07/23/2014 05:15 AM, Kees Cook wrote:
> > On Tue, Jul 22, 2014 at 2:14 AM, AKASHI Takahiro
> > <takahiro.akashi@linaro.org> wrote:
> >>   asmlinkage int syscall_trace_enter(struct pt_regs *regs)
> >>   {
> >> +       unsigned long saved_x0, saved_x8;
> >> +
> >> +       saved_x0 = regs->regs[0];
> >> +       saved_x8 = regs->regs[8];
> >> +
> >>          if (test_thread_flag(TIF_SYSCALL_TRACE))
> >>                  tracehook_report_syscall(regs, PTRACE_SYSCALL_ENTER);
> >>
> >> +       regs->syscallno = regs->regs[8];
> >> +       if ((long)regs->syscallno == ~0UL) { /* skip this syscall */
> >> +               regs->regs[8] = saved_x8;
> >> +               if (regs->regs[0] == saved_x0) /* not changed by user */
> >> +                       regs->regs[0] = -ENOSYS;
> >
> > I'm not sure this is right compared to other architectures. Generally
> > when a tracer performs a syscall skip, it's up to them to also adjust
> > the return value. They may want to be faking a syscall, and what if
> > the value they want to return happens to be what x0 was going into the
> > tracer? It would have no way to avoid this -ENOSYS case. I think
> > things are fine without this test.
> 
> Yeah, I know this issue, but was not sure that setting a return value
> is mandatory. (x86 seems to return -ENOSYS by default if not explicitly
> specified.)
> Is "fake a system call" a more appropriate word than "skip"?
> 
> I will defer to Will.

I agree with Kees -- iirc, I only suggested restoring x8.

Will