Re: [PATCH] riscv: Remove unnecessary path for syscall_trace

[Guo Ren]

Hi Tycho,

On Mon, May 25, 2020 at 10:36 PM Tycho Andersen <tycho@xxxxxxxx> wrote:
>
> On Mon, May 25, 2020 at 02:18:26PM +0000, guoren@xxxxxxxxxx wrote:
> > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> >
> > Obviously, there is no need to recover a0-a7 in reject path.
> >
> > Previous modification is from commit af33d243 by Tycho, to
> > fixup seccomp reject syscall code path.
>
> Doesn't this suffer from the same problem, though? a7 is clobbered, so
> the -ERESTARTSYS behavior won't work?

Look, the patch only affects the path of ret_from_syscall_rejected,
and there are two possible paths:
1. ret_from_syscall_rejected->handle_syscall_trace_exit->ret_from_exception
2. ret_from_syscall_rejected->ret_from_exception

All the above skip the check_syscall_nr and ignore the current a7, in
the C function they use the pt_regs in the stack to get proper reg's
value.

For the -ERESTARTSYS, we only process it in:
ret_from_exception->resume_userspace->work_notifysig->do_notify_resume:
do_signal & handle_signal:

                switch (regs->a0) {
                case -ERESTARTNOHAND:
                case -ERESTARTSYS:
                case -ERESTARTNOINTR:
                        regs->a0 = regs->orig_a0;
                        regs->epc -= 0x4;
                        break;

All above are done in pt_regs and when returning to userspace, a7 will
be recovered by restore_all in entry.S.

                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-- 
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/