vdso_support.h
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1 //
2 // Copyright 2017 The Abseil Authors.
3 //
4 // Licensed under the Apache License, Version 2.0 (the "License");
5 // you may not use this file except in compliance with the License.
6 // You may obtain a copy of the License at
7 //
8 // https://www.apache.org/licenses/LICENSE-2.0
9 //
10 // Unless required by applicable law or agreed to in writing, software
11 // distributed under the License is distributed on an "AS IS" BASIS,
12 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 // See the License for the specific language governing permissions and
14 // limitations under the License.
15 //
16 
17 // Allow dynamic symbol lookup in the kernel VDSO page.
18 //
19 // VDSO stands for "Virtual Dynamic Shared Object" -- a page of
20 // executable code, which looks like a shared library, but doesn't
21 // necessarily exist anywhere on disk, and which gets mmap()ed into
22 // every process by kernels which support VDSO, such as 2.6.x for 32-bit
23 // executables, and 2.6.24 and above for 64-bit executables.
24 //
25 // More details could be found here:
26 // http://www.trilithium.com/johan/2005/08/linux-gate/
27 //
28 // VDSOSupport -- a class representing kernel VDSO (if present).
29 //
30 // Example usage:
31 // VDSOSupport vdso;
32 // VDSOSupport::SymbolInfo info;
33 // typedef (*FN)(unsigned *, void *, void *);
34 // FN fn = nullptr;
35 // if (vdso.LookupSymbol("__vdso_getcpu", "LINUX_2.6", STT_FUNC, &info)) {
36 // fn = reinterpret_cast<FN>(info.address);
37 // }
38 
39 #ifndef ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_
40 #define ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_
41 
42 #include <atomic>
43 
44 #include "absl/base/attributes.h"
46 
47 #ifdef ABSL_HAVE_ELF_MEM_IMAGE
48 
49 #ifdef ABSL_HAVE_VDSO_SUPPORT
50 #error ABSL_HAVE_VDSO_SUPPORT cannot be directly set
51 #else
52 #define ABSL_HAVE_VDSO_SUPPORT 1
53 #endif
54 
55 namespace absl {
56 namespace debugging_internal {
57 
58 // NOTE: this class may be used from within tcmalloc, and can not
59 // use any memory allocation routines.
60 class VDSOSupport {
61  public:
62  VDSOSupport();
63 
64  typedef ElfMemImage::SymbolInfo SymbolInfo;
65  typedef ElfMemImage::SymbolIterator SymbolIterator;
66 
67  // On PowerPC64 VDSO symbols can either be of type STT_FUNC or STT_NOTYPE
68  // depending on how the kernel is built. The kernel is normally built with
69  // STT_NOTYPE type VDSO symbols. Let's make things simpler first by using a
70  // compile-time constant.
71 #ifdef __powerpc64__
72  enum { kVDSOSymbolType = STT_NOTYPE };
73 #else
74  enum { kVDSOSymbolType = STT_FUNC };
75 #endif
76 
77  // Answers whether we have a vdso at all.
78  bool IsPresent() const { return image_.IsPresent(); }
79 
80  // Allow to iterate over all VDSO symbols.
81  SymbolIterator begin() const { return image_.begin(); }
82  SymbolIterator end() const { return image_.end(); }
83 
84  // Look up versioned dynamic symbol in the kernel VDSO.
85  // Returns false if VDSO is not present, or doesn't contain given
86  // symbol/version/type combination.
87  // If info_out != nullptr, additional details are filled in.
88  bool LookupSymbol(const char *name, const char *version,
89  int symbol_type, SymbolInfo *info_out) const;
90 
91  // Find info about symbol (if any) which overlaps given address.
92  // Returns true if symbol was found; false if VDSO isn't present
93  // or doesn't have a symbol overlapping given address.
94  // If info_out != nullptr, additional details are filled in.
95  bool LookupSymbolByAddress(const void *address, SymbolInfo *info_out) const;
96 
97  // Used only for testing. Replace real VDSO base with a mock.
98  // Returns previous value of vdso_base_. After you are done testing,
99  // you are expected to call SetBase() with previous value, in order to
100  // reset state to the way it was.
101  const void *SetBase(const void *s);
102 
103  // Computes vdso_base_ and returns it. Should be called as early as
104  // possible; before any thread creation, chroot or setuid.
105  static const void *Init();
106 
107  private:
108  // image_ represents VDSO ELF image in memory.
109  // image_.ehdr_ == nullptr implies there is no VDSO.
110  ElfMemImage image_;
111 
112  // Cached value of auxv AT_SYSINFO_EHDR, computed once.
113  // This is a tri-state:
114  // kInvalidBase => value hasn't been determined yet.
115  // 0 => there is no VDSO.
116  // else => vma of VDSO Elf{32,64}_Ehdr.
117  //
118  // When testing with mock VDSO, low bit is set.
119  // The low bit is always available because vdso_base_ is
120  // page-aligned.
121  static std::atomic<const void *> vdso_base_;
122 
123  // NOLINT on 'long' because these routines mimic kernel api.
124  // The 'cache' parameter may be used by some versions of the kernel,
125  // and should be nullptr or point to a static buffer containing at
126  // least two 'long's.
127  static long InitAndGetCPU(unsigned *cpu, void *cache, // NOLINT 'long'.
128  void *unused);
129  static long GetCPUViaSyscall(unsigned *cpu, void *cache, // NOLINT 'long'.
130  void *unused);
131  typedef long (*GetCpuFn)(unsigned *cpu, void *cache, // NOLINT 'long'.
132  void *unused);
133 
134  // This function pointer may point to InitAndGetCPU,
135  // GetCPUViaSyscall, or __vdso_getcpu at different stages of initialization.
136  ABSL_CONST_INIT static std::atomic<GetCpuFn> getcpu_fn_;
137 
138  friend int GetCPU(void); // Needs access to getcpu_fn_.
139 
140  VDSOSupport(const VDSOSupport&) = delete;
141  VDSOSupport& operator=(const VDSOSupport&) = delete;
142 };
143 
144 // Same as sched_getcpu() on later glibc versions.
145 // Return current CPU, using (fast) __vdso_getcpu@LINUX_2.6 if present,
146 // otherwise use syscall(SYS_getcpu,...).
147 // May return -1 with errno == ENOSYS if the kernel doesn't
148 // support SYS_getcpu.
149 int GetCPU();
150 
151 } // namespace debugging_internal
152 } // namespace absl
153 
154 #endif // ABSL_HAVE_ELF_MEM_IMAGE
155 
156 #endif // ABSL_DEBUGGING_INTERNAL_VDSO_SUPPORT_H_
char * begin
#define ABSL_CONST_INIT
Definition: attributes.h:605
uint32_t version
Definition: graphcycles.cc:278
char * end
Definition: algorithm.h:29
char name[1]
Definition: mutex.cc:296


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autogenerated on Mon Feb 28 2022 21:31:21