// -*- Mode: C++; c-basic-offset: 2; indent-tabs-mode: nil -*-
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// ---
// Author: Paul Pluzhnikov
//
// Allow dynamic symbol lookup in the kernel VDSO page.
//
// VDSOSupport -- a class representing kernel VDSO (if present).
//

#include "base/vdso_support.h"

#ifdef HAVE_VDSO_SUPPORT     // defined in vdso_support.h

#include <fcntl.h>
#include <stddef.h>   // for ptrdiff_t

#include "base/logging.h"
#include "base/dynamic_annotations.h"
#include "base/basictypes.h"

#ifndef AT_SYSINFO_EHDR
#define AT_SYSINFO_EHDR 33
#endif

namespace base {

const void *VDSOSupport::vdso_base_ = ElfMemImage::kInvalidBase;
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_ == ElfMemImage::kInvalidBase ? Init() : vdso_base_) {
}

// 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() {
  if (vdso_base_ == ElfMemImage::kInvalidBase) {
    // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
    // on stack, and so glibc works as if VDSO was not present.
    // But going directly to kernel via /proc/self/auxv below bypasses
    // Valgrind zapping. So we check for Valgrind separately.
    if (RunningOnValgrind()) {
      vdso_base_ = nullptr;
      return nullptr;
    }
    int fd = open("/proc/self/auxv", O_RDONLY);
    if (fd == -1) {
      // Kernel too old to have a VDSO.
      vdso_base_ = nullptr;
      return nullptr;
    }
    ElfW(auxv_t) aux;
    while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
      if (aux.a_type == AT_SYSINFO_EHDR) {
        static_assert(sizeof(vdso_base_) == sizeof(aux.a_un.a_val),
                      "unexpected sizeof(pointer) != sizeof(a_val)");
        vdso_base_ = reinterpret_cast<void *>(aux.a_un.a_val);
        break;
      }
    }
    close(fd);
    if (vdso_base_ == ElfMemImage::kInvalidBase) {
      // Didn't find AT_SYSINFO_EHDR in auxv[].
      vdso_base_ = nullptr;
    }
  }
  return vdso_base_;
}

const void *VDSOSupport::SetBase(const void *base) {
  CHECK(base != ElfMemImage::kInvalidBase);
  const void *old_base = vdso_base_;
  vdso_base_ = base;
  image_.Init(base);
  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);
}

// We need to make sure VDSOSupport::Init() is called before
// the main() runs, since it might do something like setuid or
// chroot.  If VDSOSupport
// is used in any global constructor, this will happen, since
// VDSOSupport's constructor calls Init.  But if not, we need to
// ensure it here, with a global constructor of our own.  This
// is an allowed exception to the normal rule against non-trivial
// global constructors.
static class VDSOInitHelper {
 public:
  VDSOInitHelper() { VDSOSupport::Init(); }
} vdso_init_helper;
}

#endif  // HAVE_VDSO_SUPPORT
