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

[Celeste Liu <coelacanthushex@gmail.com>]

On 2023/7/13 08:00, Guo Ren wrote:
> On Tue, Jul 11, 2023 at 2:22 AM Celeste Liu <coelacanthushex@gmail.com> 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@archlinux.org>
>> Co-developed-by: Ruizhe Pan <c141028@gmail.com>
>> Signed-off-by: Ruizhe Pan <c141028@gmail.com>
>> Co-developed-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
>> Signed-off-by: Shiqi Zhang <shiqi@isrc.iscas.ac.cn>
>> Signed-off-by: Celeste Liu <CoelacanthusHex@gmail.com>
>> Tested-by: Felix Yan <felixonmars@archlinux.org>
>> ---
>>  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?
> 

Our test case is here:

int main() {
  scmp_filter_ctx ctx;
  ctx = seccomp_init(SCMP_ACT_ALLOW);

  seccomp_rule_add_exact(ctx, SCMP_ACT_ERRNO(EAFNOSUPPORT), SCMP_SYS(socket), 2,
                         SCMP_CMP(0, SCMP_CMP_EQ, AF_NETLINK),
                         SCMP_CMP(2, SCMP_CMP_EQ, NETLINK_AUDIT));

  if (seccomp_load(ctx) < 0) {
    perror("seccomp_load");
    exit(EXIT_FAILURE);
  }

  int sock_fd1 = socket(AF_NETLINK, SOCK_RAW, NETLINK_GENERIC);
  if (sock_fd1 < 0) {
    printf("1st socket syscall failed with return value %d and errno %d (%s), which is unexpected\n",
           sock_fd1, errno, strerror(errno));
    seccomp_release(ctx);
    return 1;
  }
  printf("1st socket created successfully, as expected.\n");

  int sock_fd2 = socket(AF_NETLINK, SOCK_RAW, NETLINK_AUDIT);
  if (sock_fd2 < 0) {
    printf("2nd socket syscall failed with return value %d and errno %d (%s).\n", sock_fd2, errno,
           strerror(errno));

    if (errno == EAFNOSUPPORT) {
      printf("2nd socket syscall failed with EAFNOSUPPORT, as expected.\n");
      seccomp_release(ctx);
      return 0;
    } else {
      printf("2nd socket syscall failed with unexpected errno, which is unexpected.\n");
      seccomp_release(ctx);
      return 1;
    }
  }
  printf("2nd socket created successfully, which is unexpected.\n");
  seccomp_release(ctx);

  return 2;
}

>>
>>                 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
>>
> 
>