Re: [PATCH] riscv: entry: set a0 prior to syscall_handler

[Guo Ren]

On Thu, Jul 13, 2023 at 11:28 AM Celeste Liu <coelacanthushex@xxxxxxxxx> wrote:
>
>
> On 2023/7/13 08:00, Guo Ren wrote:
> > On Tue, Jul 11, 2023 at 2:22 AM Celeste Liu <coelacanthushex@xxxxxxxxx> wrote:
> >>
> >> When we test seccomp with 6.4 kernel, we found errno has wrong value.
> >> If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
> >> get ENOSYS. We got same result with 9c2598d43510 ("riscv: entry: Save a0
> >> prior syscall_enter_from_user_mode()").
> >>
> >> Compared with x86 and loongarch's implementation of this part of the
> >> function, we think that regs->a0 = -ENOSYS should be advanced before
> >> syscall_handler to fix this problem. We have written the following patch,
> >> which can fix this problem after testing. But we don't know enough about
> >> this part of the code to explain the root cause. Hope someone can find
> >> a reasonable explanation. And we'd like to reword this commit message
> >> according to the explanation in v2
> >>
> >> Fixes: f0bddf50586d ("riscv: entry: Convert to generic entry")
> >> Reported-by: Felix Yan <felixonmars@xxxxxxxxxxxxx>
> >> Co-developed-by: Ruizhe Pan <c141028@xxxxxxxxx>
> >> Signed-off-by: Ruizhe Pan <c141028@xxxxxxxxx>
> >> Co-developed-by: Shiqi Zhang <shiqi@xxxxxxxxxxxxxxxx>
> >> Signed-off-by: Shiqi Zhang <shiqi@xxxxxxxxxxxxxxxx>
> >> Signed-off-by: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
> >> Tested-by: Felix Yan <felixonmars@xxxxxxxxxxxxx>
> >> ---
> >>  arch/riscv/kernel/traps.c | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> >> index f910dfccbf5d2..ccadb5ffd063c 100644
> >> --- a/arch/riscv/kernel/traps.c
> >> +++ b/arch/riscv/kernel/traps.c
> >> @@ -301,6 +301,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
> >>
> >>                 regs->epc += 4;
> >>                 regs->orig_a0 = regs->a0;
> >> +               regs->a0 = -ENOSYS;
> > Oh, no. You destroyed the a0 for syscall_handler, right? It's not
> > reasonable. Let's see which syscall_handler needs a0=-ENOSYS.
>
> syscall_handler always use orig_a0, not a0.
> And I have a mistake in original email, corret one is
> syscall_enter_from_user_mode not syscall_handler.
I misunderstood. Yes, a0 would be replaced by orig_a0:
syscall_enter_from_user_mode_work ->  syscall_rollback

If the syscall was denied by syscall_enter_from_user_mode(), the
return number is forced to be -ENOSYS. Maybe regs->a0 has already been
updated by SYSCALL_WORK_SECCOMP. eg:

__seccomp_filter() {
...
        case SECCOMP_RET_TRAP:
                /* Show the handler the original registers. */
                syscall_rollback(current, current_pt_regs());
                /* Let the filter pass back 16 bits of data. */
                force_sig_seccomp(this_syscall, data, false);
                goto skip;

>
> > Could you give out more detail on your test case?
> >
> >>
> >>                 riscv_v_vstate_discard(regs);
> >>
> >> @@ -308,8 +309,6 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
> >>
> >>                 if (syscall < NR_syscalls)
> >>                         syscall_handler(regs, syscall);
> >> -               else
> >> -                       regs->a0 = -ENOSYS;
> >>
> >>                 syscall_exit_to_user_mode(regs);
> >>         } else {
> >> --
> >> 2.41.0
> >>
> >
> >
>


-- 
Best Regards
 Guo Ren