[clang] Define ptrauth_string_discriminator builtin.

This exposes the ABI-stable hash function that allows computing a 16-bit
discriminator from a constant string.

This allows manually matching the implicit string discriminators
computed in the ABI (e.g., from mangled names for vtable pointer/entry
signing), as well as enabling the use of interesting discriminators when
manually annotating specific pointers with the __ptrauth qualifier.

Co-Authored-By: John McCall <[email protected]>

Author: ahmedbougacha

clang/include/clang/Basic/Builtins.td

@@ -4411,6 +4411,12 @@ def PtrauthAuth : Builtin {
   let Prototype = "void*(void*,int,void*)";
 }
 
+def PtrauthStringDiscriminator : Builtin {
+  let Spellings = ["__builtin_ptrauth_string_discriminator"];
+  let Attributes = [NoThrow, Const, Constexpr];
+  let Prototype = "size_t(char const*)";
+}
+
 // OpenCL v2.0 s6.13.16, s9.17.3.5 - Pipe functions.
 // We need the generic prototype, since the packet type could be anything.
 def ReadPipe : OCLPipeLangBuiltin {

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -928,6 +928,8 @@ def warn_ptrauth_sign_null_pointer :
 def warn_ptrauth_auth_null_pointer :
   Warning<"authenticating a null pointer will almost certainly trap">,
   InGroup<PtrAuthNullPointers>;
+def err_ptrauth_string_not_literal : Error<
+  "argument must be a string literal%select{| of char type}0">;
 
 /// main()
 // static main() is not an error in C, just in C++.

clang/lib/AST/ExprConstant.cpp

@@ -58,6 +58,7 @@
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/SaveAndRestore.h"
+#include "llvm/Support/SipHash.h"
 #include "llvm/Support/TimeProfiler.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cstring>
@@ -12583,6 +12584,12 @@ bool IntExprEvaluator::VisitBuiltinCallExpr(const CallExpr *E,
   case Builtin::BI__builtin_expect_with_probability:
     return Visit(E->getArg(0));
 
+  case Builtin::BI__builtin_ptrauth_string_discriminator: {
+    auto literal = cast<StringLiteral>(E->getArg(0)->IgnoreParenImpCasts());
+    auto result = getPointerAuthStableSipHash16(literal->getString());
+    return Success(result, E);
+  }
+
   case Builtin::BI__builtin_ffs:
   case Builtin::BI__builtin_ffsl:
   case Builtin::BI__builtin_ffsll: {

clang/lib/Headers/ptrauth.h

@@ -135,6 +135,17 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
 #define ptrauth_auth_data(__value, __old_key, __old_data)                      \
   __builtin_ptrauth_auth(__value, __old_key, __old_data)
 
+/* Compute a constant discriminator from the given string.
+
+   The result can be used as the second argument to
+   ptrauth_blend_discriminator or the third argument to the
+   __ptrauth qualifier.  It has type size_t.
+
+   The argument must be a string literal.
+   A call to this function is an integer constant expression. */
+#define ptrauth_string_discriminator(__string)                                 \
+  __builtin_ptrauth_string_discriminator(__string)
+
 /* Compute a signature for the given pair of pointer-sized values.
    The order of the arguments is significant.
 
@@ -196,6 +207,12 @@ typedef __UINTPTR_TYPE__ ptrauth_generic_signature_t;
     __value;                                                                   \
   })
 
+#define ptrauth_string_discriminator(__string)                                 \
+  ({                                                                           \
+    (void)__string;                                                            \
+    ((ptrauth_extra_data_t)0);                                                 \
+  })
+
 #define ptrauth_sign_generic_data(__value, __data)                             \
   ({                                                                           \
     (void)__value;                                                             \

clang/lib/Sema/SemaChecking.cpp

@@ -2156,6 +2156,24 @@ static ExprResult PointerAuthAuthAndResign(Sema &S, CallExpr *Call) {
   return Call;
 }
 
+static ExprResult PointerAuthStringDiscriminator(Sema &S, CallExpr *call) {
+  if (checkPointerAuthEnabled(S, call)) return ExprError();
+
+  // We've already performed normal call type-checking.
+  Expr *arg = call->getArgs()[0]->IgnoreParenImpCasts();
+
+  // Operand must be an ordinary or UTF-8 string literal.
+  auto literal = dyn_cast<StringLiteral>(arg);
+  if (!literal || literal->getCharByteWidth() != 1) {
+    S.Diag(arg->getExprLoc(), diag::err_ptrauth_string_not_literal)
+      << (literal ? 1 : 0)
+      << arg->getSourceRange();
+    return ExprError();
+  }
+
+  return call;
+}
+
 static ExprResult BuiltinLaunder(Sema &S, CallExpr *TheCall) {
   if (S.checkArgCount(TheCall, 1))
     return ExprError();
@@ -2922,6 +2940,8 @@ Sema::CheckBuiltinFunctionCall(FunctionDecl *FDecl, unsigned BuiltinID,
     return PointerAuthSignGenericData(*this, TheCall);
   case Builtin::BI__builtin_ptrauth_auth_and_resign:
     return PointerAuthAuthAndResign(*this, TheCall);
+  case Builtin::BI__builtin_ptrauth_string_discriminator:
+    return PointerAuthStringDiscriminator(*this, TheCall);
   // OpenCL v2.0, s6.13.16 - Pipe functions
   case Builtin::BIread_pipe:
   case Builtin::BIwrite_pipe:

clang/test/CodeGen/ptrauth-intrinsics.c

@@ -71,3 +71,16 @@ void test_sign_generic_data() {
   // CHECK-NEXT: store i64 [[RESULT]], ptr @signature,
   signature = __builtin_ptrauth_sign_generic_data(fnptr, ptr_discriminator);
 }
+
+// CHECK-LABEL: define void @test_string_discriminator()
+void test_string_discriminator() {
+  // CHECK:      [[X:%.*]] = alloca i32
+
+  // Check a couple of random discriminators used by Swift.
+
+  // CHECK:      store i32 58298, ptr [[X]],
+  int x = __builtin_ptrauth_string_discriminator("InitializeWithCopy");
+
+  // CHECK:      store i32 9112, ptr [[X]],
+  x = __builtin_ptrauth_string_discriminator("DestroyArray");
+}

clang/test/Sema/ptrauth-intrinsics-macro.c

@@ -32,3 +32,8 @@ void test(int *dp, int value) {
   int t2 = ptrauth_sign_generic_data(dp, 0);
   (void)t2;
 }
+
+void test_string_discriminator(int *dp) {
+  ptrauth_extra_data_t t0 = ptrauth_string_discriminator("string");
+  (void)t0;
+}