third_party/tcmalloc/chromium/src/base/vdso_support.cc

   1 // Copyright (c) 2008, Google Inc.
   2 // All rights reserved.
   3 //
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  10 //     * Redistributions in binary form must reproduce the above
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  29
  30 // ---
  31 // Author: Paul Pluzhnikov
  32 //
  33 // Allow dynamic symbol lookup in the kernel VDSO page.
  34 //
  35 // VDSOSupport -- a class representing kernel VDSO (if present).
  36 //
  37
  38 #include "base/vdso_support.h"
  39
  40 #ifdef HAVE_VDSO_SUPPORT     // defined in vdso_support.h
  41
  42 #include <fcntl.h>
  43 #include <stddef.h>   // for ptrdiff_t
  44
  45 #include "base/atomicops.h"  // for MemoryBarrier
  46 #include "base/linux_syscall_support.h"
  47 #include "base/logging.h"
  48 #include "base/dynamic_annotations.h"
  49 #include "base/basictypes.h"  // for COMPILE_ASSERT
  50
  51 using base::subtle::MemoryBarrier;
  52
  53 #ifndef AT_SYSINFO_EHDR
  54 #define AT_SYSINFO_EHDR 33
  55 #endif
  56
  57 namespace base {
  58
  59 const void *VDSOSupport::vdso_base_ = ElfMemImage::kInvalidBase;
  60 VDSOSupport::GetCpuFn VDSOSupport::getcpu_fn_ = &InitAndGetCPU;
  61 VDSOSupport::VDSOSupport()
  62     // If vdso_base_ is still set to kInvalidBase, we got here
  63     // before VDSOSupport::Init has been called. Call it now.
  64     : image_(vdso_base_ == ElfMemImage::kInvalidBase ? Init() : vdso_base_) {
  65 }
  66
  67 // NOTE: we can't use GoogleOnceInit() below, because we can be
  68 // called by tcmalloc, and none of the *once* stuff may be functional yet.
  69 //
  70 // In addition, we hope that the VDSOSupportHelper constructor
  71 // causes this code to run before there are any threads, and before
  72 // InitGoogle() has executed any chroot or setuid calls.
  73 //
  74 // Finally, even if there is a race here, it is harmless, because
  75 // the operation should be idempotent.
  76 const void *VDSOSupport::Init() {
  77   if (vdso_base_ == ElfMemImage::kInvalidBase) {
  78     // Valgrind zaps AT_SYSINFO_EHDR and friends from the auxv[]
  79     // on stack, and so glibc works as if VDSO was not present.
  80     // But going directly to kernel via /proc/self/auxv below bypasses
  81     // Valgrind zapping. So we check for Valgrind separately.
  82     if (RunningOnValgrind()) {
  83       vdso_base_ = NULL;
  84       getcpu_fn_ = &GetCPUViaSyscall;
  85       return NULL;
  86     }
  87     int fd = open("/proc/self/auxv", O_RDONLY);
  88     if (fd == -1) {
  89       // Kernel too old to have a VDSO.
  90       vdso_base_ = NULL;
  91       getcpu_fn_ = &GetCPUViaSyscall;
  92       return NULL;
  93     }
  94     ElfW(auxv_t) aux;
  95     while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
  96       if (aux.a_type == AT_SYSINFO_EHDR) {
  97         COMPILE_ASSERT(sizeof(vdso_base_) == sizeof(aux.a_un.a_val),
  98                        unexpected_sizeof_pointer_NE_sizeof_a_val);
  99         vdso_base_ = reinterpret_cast<void *>(aux.a_un.a_val);
 100         break;
 101       }
 102     }
 103     close(fd);
 104     if (vdso_base_ == ElfMemImage::kInvalidBase) {
 105       // Didn't find AT_SYSINFO_EHDR in auxv[].
 106       vdso_base_ = NULL;
 107     }
 108   }
 109   GetCpuFn fn = &GetCPUViaSyscall;  // default if VDSO not present.
 110   if (vdso_base_) {
 111     VDSOSupport vdso;
 112     SymbolInfo info;
 113     if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {
 114       // Casting from an int to a pointer is not legal C++.  To emphasize
 115       // this, we use a C-style cast rather than a C++-style cast.
 116       fn = (GetCpuFn)(info.address);
 117     }
 118   }
 119   // Subtle: this code runs outside of any locks; prevent compiler
 120   // from assigning to getcpu_fn_ more than once.
 121   base::subtle::MemoryBarrier();
 122   getcpu_fn_ = fn;
 123   return vdso_base_;
 124 }
 125
 126 const void *VDSOSupport::SetBase(const void *base) {
 127   CHECK(base != ElfMemImage::kInvalidBase);
 128   const void *old_base = vdso_base_;
 129   vdso_base_ = base;
 130   image_.Init(base);
 131   // Also reset getcpu_fn_, so GetCPU could be tested with simulated VDSO.
 132   getcpu_fn_ = &InitAndGetCPU;
 133   return old_base;
 134 }
 135
 136 bool VDSOSupport::LookupSymbol(const char *name,
 137                                const char *version,
 138                                int type,
 139                                SymbolInfo *info) const {
 140   return image_.LookupSymbol(name, version, type, info);
 141 }
 142
 143 bool VDSOSupport::LookupSymbolByAddress(const void *address,
 144                                         SymbolInfo *info_out) const {
 145   return image_.LookupSymbolByAddress(address, info_out);
 146 }
 147
 148 // NOLINT on 'long' because this routine mimics kernel api.
 149 long VDSOSupport::GetCPUViaSyscall(unsigned *cpu, void *, void *) { // NOLINT
 150 #if defined(__NR_getcpu)
 151   return sys_getcpu(cpu, NULL, NULL);
 152 #else
 153   // x86_64 never implemented sys_getcpu(), except as a VDSO call.
 154   errno = ENOSYS;
 155   return -1;
 156 #endif
 157 }
 158
 159 // Use fast __vdso_getcpu if available.
 160 long VDSOSupport::InitAndGetCPU(unsigned *cpu, void *x, void *y) { // NOLINT
 161   Init();
 162   CHECK_NE(getcpu_fn_, &InitAndGetCPU); // << "Init() did not set getcpu_fn_";
 163   return (*getcpu_fn_)(cpu, x, y);
 164 }
 165
 166 // This function must be very fast, and may be called from very
 167 // low level (e.g. tcmalloc). Hence I avoid things like
 168 // GoogleOnceInit() and ::operator new.
 169 int GetCPU(void) {
 170   unsigned cpu;
 171   int ret_code = (*VDSOSupport::getcpu_fn_)(&cpu, NULL, NULL);
 172   return ret_code == 0 ? cpu : ret_code;
 173 }
 174
 175 // We need to make sure VDSOSupport::Init() is called before
 176 // the main() runs, since it might do something like setuid or
 177 // chroot.  If VDSOSupport
 178 // is used in any global constructor, this will happen, since
 179 // VDSOSupport's constructor calls Init.  But if not, we need to
 180 // ensure it here, with a global constructor of our own.  This
 181 // is an allowed exception to the normal rule against non-trivial
 182 // global constructors.
 183 static class VDSOInitHelper {
 184  public:
 185   VDSOInitHelper() { VDSOSupport::Init(); }
 186 } vdso_init_helper;
 187 }
 188
 189 #endif  // HAVE_VDSO_SUPPORT