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

[Guo Ren]

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.

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