Re: [PATCH 0/1] MR7009: ntdll: Emulate mrs xN, CurrentEL instructions.

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about dlls/ntdll/unix/signal_arm64.c:
> static void ill_handler( int signal, siginfo_t *siginfo, void *sigcontext )
>  {
>      EXCEPTION_RECORD rec = { EXCEPTION_ILLEGAL_INSTRUCTION };
> - ucontext_t *context = sigcontext;
> -
> - if (!(PSTATE_sig( context ) & 0x10) && /* AArch64 (not WoW) */
> -
>    !(PC_sig( context ) & 3))
> - {
> -
>    ULONG instr = *(ULONG *)PC_sig( context );
> -
>    /* emulate mrs xN, CurrentEL */
> -
>    if ((instr & ~0x1f) == 0xd5384240) {
> -
>        REGn_sig(instr & 0x1f, context) = 0;

For MRS, register 31 is XZR, not SP:

```suggestion:-0+0
            ULONG reg = instr & 0x1f;
            if (reg != 31)  /* 31: XZR */
                REGn_sig(reg, context) = 0;
```

`mrs xzr, CurrentEL` could be emitted by volatile inline asm `mrs %0, CurrentEL` where the compiler optimized out the destination. It could also be part of incorrect JIT code, and such deviation could be difficult to debug.