[llvm-branch-commits] [compiler-rt] compiler-rt: Introduce runtime functions for emulated PAC. (PR #133530)

[Anatoly Trosinenko via llvm-branch-commits]

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@@ -0,0 +1,133 @@
+//===--- emupac.cpp - Emulated PAC implementation -------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+//  This file implements Emulated PAC using SipHash_1_3 as the IMPDEF hashing
+//  scheme.
+//
+//===----------------------------------------------------------------------===//
+
+#include <stdint.h>
+
+#include "siphash/SipHash.h"
+
+// EmuPAC implements runtime emulation of PAC instructions. If the current
+// CPU supports PAC, EmuPAC uses real PAC instructions. Otherwise, it uses the
+// emulation, which is effectively an implementation of PAC with an IMPDEF
+// hashing scheme based on SipHash_1_3.
+//
+// The purpose of the emulation is to allow programs to be built to be portable
+// to machines without PAC support, with some performance loss and increased
+// probability of false positives (due to not being able to portably determine
+// the VA size), while being functionally almost equivalent to running on a
+// machine with PAC support. One example of a use case is if PAC is used in
+// production as a security mitigation, but the testing environment is
+// heterogeneous (i.e. some machines lack PAC support). In this case we would
+// like the testing machines to be able to detect issues resulting
+// from the use of PAC instructions that would affect production by running
+// tests. This can be achieved by building test binaries with EmuPAC and
+// production binaries with real PAC.
+//
+// The emulation assumes that the VA size is at most 48 bits. The architecture
+// as of ARMv8.2, which was the last architecture version in which PAC was not
+// mandatory, permitted VA size up to 52 bits via ARMv8.2-LVA, but we are
+// unaware of an ARMv8.2 CPU that implemented ARMv8.2-LVA.
+
+const uint64_t kMaxVASize = 48;
+const uint64_t kPACMask = ((1ULL << 55) - 1) & ~((1ULL << kMaxVASize) - 1);
+const uint64_t kTTBR1Mask = 1ULL << 55;
+
+// Determine whether PAC is supported without accessing memory. This utilizes
+// the XPACLRI instruction which will copy bit 55 of x30 into at least bit 54 if
+// PAC is supported and acts as a NOP if PAC is not supported.
+static bool pac_supported() {
----------------
atrosinenko wrote:

[nit] It looks like functions under `compiler-rt/lib/builtins` which are not part of the public interface generally follow the standard naming conventions.
```suggestion
static bool pacSupported() {
```

https://github.com/llvm/llvm-project/pull/133530