vdso_support.cc
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1 // Copyright 2017 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14 
15 // Allow dynamic symbol lookup in the kernel VDSO page.
16 //
17 // VDSOSupport -- a class representing kernel VDSO (if present).
18 
20 
21 #ifdef ABSL_HAVE_VDSO_SUPPORT // defined in vdso_support.h
22 
23 #include <errno.h>
24 #include <fcntl.h>
25 #include <sys/syscall.h>
26 #include <unistd.h>
27 
28 #if __GLIBC_PREREQ(2, 16) // GLIBC-2.16 implements getauxval.
29 #include <sys/auxv.h>
30 #endif
31 
34 #include "absl/base/port.h"
35 
36 #ifndef AT_SYSINFO_EHDR
37 #define AT_SYSINFO_EHDR 33 // for crosstoolv10
38 #endif
39 
40 namespace absl {
41 namespace debugging_internal {
42 
44 std::atomic<const void *> VDSOSupport::vdso_base_(
45  debugging_internal::ElfMemImage::kInvalidBase);
46 
47 std::atomic<VDSOSupport::GetCpuFn> VDSOSupport::getcpu_fn_(&InitAndGetCPU);
48 VDSOSupport::VDSOSupport()
49  // If vdso_base_ is still set to kInvalidBase, we got here
50  // before VDSOSupport::Init has been called. Call it now.
51  : image_(vdso_base_.load(std::memory_order_relaxed) ==
52  debugging_internal::ElfMemImage::kInvalidBase
53  ? Init()
54  : vdso_base_.load(std::memory_order_relaxed)) {}
55 
56 // NOTE: we can't use GoogleOnceInit() below, because we can be
57 // called by tcmalloc, and none of the *once* stuff may be functional yet.
58 //
59 // In addition, we hope that the VDSOSupportHelper constructor
60 // causes this code to run before there are any threads, and before
61 // InitGoogle() has executed any chroot or setuid calls.
62 //
63 // Finally, even if there is a race here, it is harmless, because
64 // the operation should be idempotent.
65 const void *VDSOSupport::Init() {
66  const auto kInvalidBase = debugging_internal::ElfMemImage::kInvalidBase;
67 #if __GLIBC_PREREQ(2, 16)
68  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
69  errno = 0;
70  const void *const sysinfo_ehdr =
71  reinterpret_cast<const void *>(getauxval(AT_SYSINFO_EHDR));
72  if (errno == 0) {
73  vdso_base_.store(sysinfo_ehdr, std::memory_order_relaxed);
74  }
75  }
76 #endif // __GLIBC_PREREQ(2, 16)
77  if (vdso_base_.load(std::memory_order_relaxed) == 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_.store(nullptr, std::memory_order_relaxed);
84  getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
85  return nullptr;
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_.store(nullptr, std::memory_order_relaxed);
91  getcpu_fn_.store(&GetCPUViaSyscall, std::memory_order_relaxed);
92  return nullptr;
93  }
94  ElfW(auxv_t) aux;
95  while (read(fd, &aux, sizeof(aux)) == sizeof(aux)) {
96  if (aux.a_type == AT_SYSINFO_EHDR) {
97  vdso_base_.store(reinterpret_cast<void *>(aux.a_un.a_val),
98  std::memory_order_relaxed);
99  break;
100  }
101  }
102  close(fd);
103  if (vdso_base_.load(std::memory_order_relaxed) == kInvalidBase) {
104  // Didn't find AT_SYSINFO_EHDR in auxv[].
105  vdso_base_.store(nullptr, std::memory_order_relaxed);
106  }
107  }
108  GetCpuFn fn = &GetCPUViaSyscall; // default if VDSO not present.
109  if (vdso_base_.load(std::memory_order_relaxed)) {
110  VDSOSupport vdso;
111  SymbolInfo info;
112  if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {
113  fn = reinterpret_cast<GetCpuFn>(const_cast<void *>(info.address));
114  }
115  }
116  // Subtle: this code runs outside of any locks; prevent compiler
117  // from assigning to getcpu_fn_ more than once.
118  getcpu_fn_.store(fn, std::memory_order_relaxed);
119  return vdso_base_.load(std::memory_order_relaxed);
120 }
121 
122 const void *VDSOSupport::SetBase(const void *base) {
123  ABSL_RAW_CHECK(base != debugging_internal::ElfMemImage::kInvalidBase,
124  "internal error");
125  const void *old_base = vdso_base_.load(std::memory_order_relaxed);
126  vdso_base_.store(base, std::memory_order_relaxed);
127  image_.Init(base);
128  // Also reset getcpu_fn_, so GetCPU could be tested with simulated VDSO.
129  getcpu_fn_.store(&InitAndGetCPU, std::memory_order_relaxed);
130  return old_base;
131 }
132 
133 bool VDSOSupport::LookupSymbol(const char *name,
134  const char *version,
135  int type,
136  SymbolInfo *info) const {
137  return image_.LookupSymbol(name, version, type, info);
138 }
139 
140 bool VDSOSupport::LookupSymbolByAddress(const void *address,
141  SymbolInfo *info_out) const {
142  return image_.LookupSymbolByAddress(address, info_out);
143 }
144 
145 // NOLINT on 'long' because this routine mimics kernel api.
146 long VDSOSupport::GetCPUViaSyscall(unsigned *cpu, // NOLINT(runtime/int)
147  void *, void *) {
148 #ifdef SYS_getcpu
149  return syscall(SYS_getcpu, cpu, nullptr, nullptr);
150 #else
151  // x86_64 never implemented sys_getcpu(), except as a VDSO call.
152  static_cast<void>(cpu); // Avoid an unused argument compiler warning.
153  errno = ENOSYS;
154  return -1;
155 #endif
156 }
157 
158 // Use fast __vdso_getcpu if available.
159 long VDSOSupport::InitAndGetCPU(unsigned *cpu, // NOLINT(runtime/int)
160  void *x, void *y) {
161  Init();
162  GetCpuFn fn = getcpu_fn_.load(std::memory_order_relaxed);
163  ABSL_RAW_CHECK(fn != &InitAndGetCPU, "Init() did not set getcpu_fn_");
164  return (*fn)(cpu, x, y);
165 }
166 
167 // This function must be very fast, and may be called from very
168 // low level (e.g. tcmalloc). Hence I avoid things like
169 // GoogleOnceInit() and ::operator new.
171 int GetCPU() {
172  unsigned cpu;
173  int ret_code = (*VDSOSupport::getcpu_fn_)(&cpu, nullptr, nullptr);
174  return ret_code == 0 ? cpu : ret_code;
175 }
176 
177 // We need to make sure VDSOSupport::Init() is called before
178 // InitGoogle() does any setuid or chroot calls. If VDSOSupport
179 // is used in any global constructor, this will happen, since
180 // VDSOSupport's constructor calls Init. But if not, we need to
181 // ensure it here, with a global constructor of our own. This
182 // is an allowed exception to the normal rule against non-trivial
183 // global constructors.
184 static class VDSOInitHelper {
185  public:
186  VDSOInitHelper() { VDSOSupport::Init(); }
187 } vdso_init_helper;
188 
189 } // namespace debugging_internal
190 } // namespace absl
191 
192 #endif // ABSL_HAVE_VDSO_SUPPORT
#define ABSL_CONST_INIT
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#define ABSL_RAW_CHECK(condition, message)
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char name[1]
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autogenerated on Mon Feb 28 2022 21:31:21