linux-kernel - Re: [PATCH] riscv: Remove unnecessary path for syscall

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Date:   Tue, 26 May 2020 08:29:45 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     keescook@...omium.org, palmerdabbelt@...gle.com,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>
Subject: Re: [PATCH] riscv: Remove unnecessary path for syscall_trace

Hi Tycho,

On Mon, May 25, 2020 at 10:36 PM Tycho Andersen <tycho@...ho.ws> wrote:
>
> On Mon, May 25, 2020 at 02:18:26PM +0000, guoren@...nel.org wrote:
> > From: Guo Ren <guoren@...ux.alibaba.com>
> >
> > 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/