path: root/absl/random/internal/randen_detect.cc

// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the"License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an"AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// HERMETIC NOTE: The randen_hwaes target must not introduce duplicate
// symbols from arbitrary system and other headers, since it may be built
// with different flags from other targets, using different levels of
// optimization, potentially introducing ODR violations.

#include "absl/random/internal/randen_detect.h"

#include <cstdint>
#include <cstring>

#include "absl/random/internal/platform.h"

#if defined(ABSL_ARCH_X86_64)
#define ABSL_INTERNAL_USE_X86_CPUID
#elif defined(ABSL_ARCH_PPC) || defined(ABSL_ARCH_ARM) || \
    defined(ABSL_ARCH_AARCH64)
#if defined(__ANDROID__)
#define ABSL_INTERNAL_USE_ANDROID_GETAUXVAL
#define ABSL_INTERNAL_USE_GETAUXVAL
#elif defined(__linux__)
#define ABSL_INTERNAL_USE_LINUX_GETAUXVAL
#define ABSL_INTERNAL_USE_GETAUXVAL
#endif
#endif

#if defined(ABSL_INTERNAL_USE_X86_CPUID)
#if defined(_WIN32) || defined(_WIN64)
#include <intrin.h>  // NOLINT(build/include_order)
#pragma intrinsic(__cpuid)
#else
// MSVC-equivalent __cpuid intrinsic function.
static void __cpuid(int cpu_info[4], int info_type) {
  __asm__ volatile("cpuid \n\t"
                   : "=a"(cpu_info[0]), "=b"(cpu_info[1]), "=c"(cpu_info[2]),
                     "=d"(cpu_info[3])
                   : "a"(info_type), "c"(0));
}
#endif
#endif  // ABSL_INTERNAL_USE_X86_CPUID

// On linux, just use the c-library getauxval call.
#if defined(ABSL_INTERNAL_USE_LINUX_GETAUXVAL)

extern "C" unsigned long getauxval(unsigned long type);  // NOLINT(runtime/int)

static uint32_t GetAuxval(uint32_t hwcap_type) {
  return static_cast<uint32_t>(getauxval(hwcap_type));
}

#endif

// On android, probe the system's C library for getauxval().
// This is the same technique used by the android NDK cpu features library
// as well as the google open-source cpu_features library.
//
// TODO(absl-team): Consider implementing a fallback of directly reading
// /proc/self/auxval.
#if defined(ABSL_INTERNAL_USE_ANDROID_GETAUXVAL)
#include <dlfcn.h>

static uint32_t GetAuxval(uint32_t hwcap_type) {
  // NOLINTNEXTLINE(runtime/int)
  typedef unsigned long (*getauxval_func_t)(unsigned long);

  dlerror();  // Cleaning error state before calling dlopen.
  void* libc_handle = dlopen("libc.so", RTLD_NOW);
  if (!libc_handle) {
    return 0;
  }
  uint32_t result = 0;
  void* sym = dlsym(libc_handle, "getauxval");
  if (sym) {
    getauxval_func_t func;
    memcpy(&func, &sym, sizeof(func));
    result = static_cast<uint32_t>((*func)(hwcap_type));
  }
  dlclose(libc_handle);
  return result;
}

#endif

namespace absl {
namespace random_internal {

// The default return at the end of the function might be unreachable depending
// on the configuration. Ignore that warning.
#if defined(__clang__)
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wunreachable-code-return"
#endif

// CPUSupportsRandenHwAes returns whether the CPU is a microarchitecture
// which supports the crpyto/aes instructions or extensions necessary to use the
// accelerated RandenHwAes implementation.
//
// 1. For x86 it is sufficient to use the CPUID instruction to detect whether
//    the cpu supports AES instructions. Done.
//
// Fon non-x86 it is much more complicated.
//
// 2. When ABSL_INTERNAL_USE_GETAUXVAL is defined, use getauxval() (either
//    the direct c-library version, or the android probing version which loads
//    libc), and read the hardware capability bits.
//    This is based on the technique used by boringssl uses to detect
//    cpu capabilities, and should allow us to enable crypto in the android
//    builds where it is supported.
//
// 3. Use the default for the compiler architecture.
//

bool CPUSupportsRandenHwAes() {
#if defined(ABSL_INTERNAL_USE_X86_CPUID)
  // 1. For x86: Use CPUID to detect the required AES instruction set.
  int regs[4];
  __cpuid(reinterpret_cast<int*>(regs), 1);
  return regs[2] & (1 << 25);  // AES

#elif defined(ABSL_INTERNAL_USE_GETAUXVAL)
  // 2. Use getauxval() to read the hardware bits and determine
  // cpu capabilities.

#define AT_HWCAP 16
#define AT_HWCAP2 26
#if defined(ABSL_ARCH_PPC)
  // For Power / PPC: Expect that the cpu supports VCRYPTO
  // See https://members.openpowerfoundation.org/document/dl/576
  // VCRYPTO should be present in POWER8 >= 2.07.
  // Uses Linux kernel constants from arch/powerpc/include/uapi/asm/cputable.h
  static const uint32_t kVCRYPTO = 0x02000000;
  const uint32_t hwcap = GetAuxval(AT_HWCAP2);
  return (hwcap & kVCRYPTO) != 0;

#elif defined(ABSL_ARCH_ARM)
  // For ARM: Require crypto+neon
  // http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500f/CIHBIBBA.html
  // Uses Linux kernel constants from arch/arm64/include/asm/hwcap.h
  static const uint32_t kNEON = 1 << 12;
  uint32_t hwcap = GetAuxval(AT_HWCAP);
  if ((hwcap & kNEON) == 0) {
    return false;
  }

  // And use it again to detect AES.
  static const uint32_t kAES = 1 << 0;
  const uint32_t hwcap2 = GetAuxval(AT_HWCAP2);
  return (hwcap2 & kAES) != 0;

#elif defined(ABSL_ARCH_AARCH64)
  // For AARCH64: Require crypto+neon
  // http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0500f/CIHBIBBA.html
  static const uint32_t kNEON = 1 << 1;
  static const uint32_t kAES = 1 << 3;
  const uint32_t hwcap = GetAuxval(AT_HWCAP);
  return ((hwcap & kNEON) != 0) && ((hwcap & kAES) != 0);
#endif

#else  // ABSL_INTERNAL_USE_GETAUXVAL
  // 3. By default, assume that the compiler default.
  return ABSL_HAVE_ACCELERATED_AES ? true : false;

#endif
  // NOTE: There are some other techniques that may be worth trying:
  //
  // * Use an environment variable: ABSL_RANDOM_USE_HWAES
  //
  // * Rely on compiler-generated target-based dispatch.
  // Using x86/gcc it might look something like this:
  //
  // int __attribute__((target("aes"))) HasAes() { return 1; }
  // int __attribute__((target("default"))) HasAes() { return 0; }
  //
  // This does not work on all architecture/compiler combinations.
  //
  // * On Linux consider reading /proc/cpuinfo and/or /proc/self/auxv.
  // These files have lines which are easy to parse; for ARM/AARCH64 it is quite
  // easy to find the Features: line and extract aes / neon. Likewise for
  // PPC.
  //
  // * Fork a process and test for SIGILL:
  //
  // * Many architectures have instructions to read the ISA. Unfortunately
  //   most of those require that the code is running in ring 0 /
  //   protected-mode.
  //
  //   There are several examples. e.g. Valgrind detects PPC ISA 2.07:
  //   https://github.com/lu-zero/valgrind/blob/master/none/tests/ppc64/test_isa_2_07_part1.c
  //
  //   MRS <Xt>, ID_AA64ISAR0_EL1 ; Read ID_AA64ISAR0_EL1 into Xt
  //
  //   uint64_t val;
  //   __asm __volatile("mrs %0, id_aa64isar0_el1" :"=&r" (val));
  //
  // * Use a CPUID-style heuristic database.
  //
  // * On Apple (__APPLE__), AES is available on Arm v8.
  //   https://stackoverflow.com/questions/45637888/how-to-determine-armv8-features-at-runtime-on-ios
}

#if defined(__clang__)
#pragma clang diagnostic pop
#endif

}  // namespace random_internal
}  // namespace absl