// 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.

// Allow dynamic symbol lookup in the kernel VDSO page.

//

// VDSOSupport -- a class representing kernel VDSO (if present).

#include "absl/debugging/internal/vdso_support.h"

#include "absl/base/attributes.h"

#ifdef ABSL_HAVE_VDSO_SUPPORT     // defined in vdso_support.h

#if !defined(__has_include)

#define __has_include(header) 0

#endif

#include <errno.h>

#include <fcntl.h>

#if __has_include(<syscall.h>)

#include <syscall.h>

#elif __has_include(<sys/syscall.h>)

#include <sys/syscall.h>

#endif

#include <unistd.h>

#if !defined(__UCLIBC__) && defined(__GLIBC__) && \

    (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 16))

#define ABSL_HAVE_GETAUXVAL

#endif

#ifdef ABSL_HAVE_GETAUXVAL

#include <sys/auxv.h>

#endif

#include "absl/base/dynamic_annotations.h"

#include "absl/base/internal/raw_logging.h"

#include "absl/base/port.h"

#ifndef AT_SYSINFO_EHDR

#define AT_SYSINFO_EHDR 33  // for crosstoolv10

#endif

#if defined(__NetBSD__)

using Elf32_auxv_t = Aux32Info;

using Elf64_auxv_t = Aux64Info;

#endif

#if defined(__FreeBSD__)

#if defined(__ELF_WORD_SIZE) && __ELF_WORD_SIZE == 64

using Elf64_auxv_t = Elf64_Auxinfo;

#endif

using Elf32_auxv_t = Elf32_Auxinfo;

#endif

namespace absl {

ABSL_NAMESPACE_BEGIN

namespace debugging_internal {

ABSL_CONST_INIT

std::atomic<const void *> VDSOSupport::vdso_base_(

    debugging_internal::ElfMemImage::kInvalidBase);

ABSL_CONST_INIT std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(

    &InitAndGetCPU);

VDSOSupport::VDSOSupport()

    // If vdso_base_ is still set to kInvalidBase, we got here

    // before VDSOSupport::Init has been called. Call it now.

    : image_(vdso_base_.load(std::memory_order_relaxed) ==

                     debugging_internal::ElfMemImage::kInvalidBase

                 ? Init()

                 : vdso_base_.load(std::memory_order_relaxed)) {}

// NOTE: we can't use GoogleOnceInit() below, because we can be

// called by tcmalloc, and none of the *once* stuff may be functional yet.

//

// In addition, we hope that the VDSOSupportHelper constructor

// causes this code to run before there are any threads, and before

// InitGoogle() has executed any chroot or setuid calls.

//

// Finally, even if there is a race here, it is harmless, because

// the operation should be idempotent.

const void *VDSOSupport::Init() {

  const auto kInvalidBase = debugging_internal::ElfMemImage::kInvalidBase;

#ifdef ABSL_HAVE_GETAUXVAL

  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {

    errno = 0;

    const void *const sysinfo_ehdr =

        reinterpret_cast<const void *>(getauxval(AT_SYSINFO_EHDR));

    if (errno == 0) {

      vdso_base_.store(sysinfo_ehdr, std::memory_order_relaxed);

    }

  }

#endif  // ABSL_HAVE_GETAUXVAL

  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {

    int fd = open("/proc/self/auxv", O_RDONLY);

    if (fd == -1) {

      // Kernel too old to have a VDSO.

      vdso_base_.store(nullptr, std::memory_order_relaxed);

      getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);

      return nullptr;

    }

    ElfW(auxv_t) aux;

    while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {

      if (aux.a_type == AT_SYSINFO_EHDR) {

#if defined(__NetBSD__)

        vdso_base_.store(reinterpret_cast<void *>(aux.a_v),

                         std::memory_order_relaxed);

#else

        vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),

                         std::memory_order_relaxed);

#endif

        break;

      }

    }

    close(fd);

    if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {

      // Didn't find AT_SYSINFO_EHDR in auxv[].

      vdso_base_.store(nullptr, std::memory_order_relaxed);

    }

  }

  GetCpuFn fn = &GetCPUViaSyscall;  // default if VDSO not present.

  if (vdso_base_.load(std::memory_order_relaxed)) {

    VDSOSupport vdso;

    SymbolInfo info;

    if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {

      fn = reinterpret_cast<GetCpuFn>(const_cast<void *>(info.address));

    }

  }

  // Subtle: this code runs outside of any locks; prevent compiler

  // from assigning to getcpu_fn_ more than once.

  getcpu_fn_.store(fn, std::memory_order_relaxed);

  return vdso_base_.load(std::memory_order_relaxed);

}

const void *VDSOSupport::SetBase(const void *base) {

  ABSL_RAW_CHECK(base != debugging_internal::ElfMemImage::kInvalidBase,

                 "internal error");

  const void *old_base = vdso_base_.load(std::memory_order_relaxed);

  vdso_base_.store(base, std::memory_order_relaxed);

  image_.Init(base);

  // Also reset getcpu_fn_, so GetCPU could be tested with simulated VDSO.

  getcpu_fn_.store(&InitAndGetCPU, std::memory_order_relaxed);

  return old_base;

}

bool VDSOSupport::LookupSymbol(const char *name,

                               const char *version,

                               int type,

                               SymbolInfo *info) const {

  return image_.LookupSymbol(name, version, type, info);

}

bool VDSOSupport::LookupSymbolByAddress(const void *address,

                                        SymbolInfo *info_out) const {

  return image_.LookupSymbolByAddress(address, info_out);

}

// NOLINT on 'long' because this routine mimics kernel api.

long VDSOSupport::GetCPUViaSyscall(unsigned *cpu,  // NOLINT(runtime/int)

                                   void *, void *) {

#ifdef SYS_getcpu

  return syscall(SYS_getcpu, cpu, nullptr, nullptr);

#else

  // x86_64 never implemented sys_getcpu(), except as a VDSO call.

  static_cast<void>(cpu);  // Avoid an unused argument compiler warning.

  errno = ENOSYS;

  return -1;

#endif

}

// Use fast __vdso_getcpu if available.

long VDSOSupport::InitAndGetCPU(unsigned *cpu,  // NOLINT(runtime/int)

                                void *x, void *y) {

  Init();

  GetCpuFn fn = getcpu_fn_.load(std::memory_order_relaxed);

  ABSL_RAW_CHECK(fn != &InitAndGetCPU, "Init() did not set getcpu_fn_");

  return (*fn)(cpu, x, y);

}

// This function must be very fast, and may be called from very

// low level (e.g. tcmalloc). Hence I avoid things like

// GoogleOnceInit() and ::operator new.

// The destination in VDSO is unknown to CFI and VDSO does not set MSAN

// shadow for the return value.

ABSL_ATTRIBUTE_NO_SANITIZE_CFI

ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY

int GetCPU() {

  unsigned cpu;

  long ret_code =  // NOLINT(runtime/int)

      (*VDSOSupport::getcpu_fn_)(&cpu, nullptr, nullptr);

  return ret_code == 0 ? static_cast<int>(cpu) : static_cast<int>(ret_code);

}

}  // namespace debugging_internal

ABSL_NAMESPACE_END

}  // namespace absl

#endif  // ABSL_HAVE_VDSO_SUPPORT