[PATCH v4 1/1] ntdll: Emulate mrs xN, CurrentEL instructions.

[Billy Laws]

From: Billy Laws

These trap on linux but under Windows userspace will write 0 to the
destination register. Fixes recent versions of VC redist that use
this unconditionally.
---
 dlls/ntdll/tests/exception.c   | 20 ++++++++++++++++++++
 dlls/ntdll/unix/signal_arm64.c | 15 +++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/dlls/ntdll/tests/exception.c b/dlls/ntdll/tests/exception.c
index 4a89cf6a5dc..9b5ca2ca508 100644
--- a/dlls/ntdll/tests/exception.c
+++ b/dlls/ntdll/tests/exception.c
@@ -8296,6 +8296,25 @@ static void test_restore_context(void)
         ok(0, "unexpected pass %ld\n", pass);
 }

+static void test_mrs_currentel(void)
+{
+    DWORD64 (*func_ptr)(void) = code_mem;
+    DWORD64 result;
+
+    static const DWORD call_func[] =
+    {
+        0xd5384240, /* mrs x0, CurrentEL */
+        0xd538425f, /* mrs xzr, CurrentEL */
+        0xd65f03c0, /* ret */
+    };
+
+    memcpy( func_ptr, call_func, sizeof(call_func) );
+    FlushInstructionCache( GetCurrentProcess(), func_ptr, sizeof(call_func) );
+    result = func_ptr();
+    ok( result == 0, "expected 0, got %llx\n", result );
+}
+
+
 #endif  /* __aarch64__ */

#if defined(__i386__) || defined(__x86_64__)
@@ -12024,6 +12043,7 @@ START_TEST(exception)
     test_nested_exception();
     test_collided_unwind();
     test_restore_context();
+    test_mrs_currentel();

#elif defined(__arm__)

diff --git a/dlls/ntdll/unix/signal_arm64.c b/dlls/ntdll/unix/signal_arm64.c
index 8ec4251feca..57d9c375076 100644
--- a/dlls/ntdll/unix/signal_arm64.c
+++ b/dlls/ntdll/unix/signal_arm64.c
@@ -1097,6 +1097,21 @@ static void segv_handler( int signal, siginfo_t *siginfo, void *sigcontext )
 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) {
+            ULONG reg = instr & 0x1f;
+            /* ignore writes to xzr */
+            if (reg != 31) REGn_sig(reg, context) = 0;
+            PC_sig(context) += 4;
+            return;
+        }
+    }

setup_exception( sigcontext, &rec );
 }