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31 // Author: Paul Pluzhnikov
33 // Allow dynamic symbol lookup in the kernel VDSO page.
35 // VDSOSupport -- a class representing kernel VDSO (if present).
38 #include "base/vdso_support.h"
40 #ifdef HAVE_VDSO_SUPPORT // defined in vdso_support.h
43 #include <stddef.h> // for ptrdiff_t
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
51 using base::subtle::MemoryBarrier;
53 #ifndef AT_SYSINFO_EHDR
54 #define AT_SYSINFO_EHDR 33
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_) {
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.
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.
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()) {
84 getcpu_fn_ = &GetCPUViaSyscall;
87 int fd = open("/proc/self/auxv", O_RDONLY);
89 // Kernel too old to have a VDSO.
91 getcpu_fn_ = &GetCPUViaSyscall;
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);
104 if (vdso_base_ == ElfMemImage::kInvalidBase) {
105 // Didn't find AT_SYSINFO_EHDR in auxv[].
109 GetCpuFn fn = &GetCPUViaSyscall; // default if VDSO not present.
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);
119 // Subtle: this code runs outside of any locks; prevent compiler
120 // from assigning to getcpu_fn_ more than once.
121 base::subtle::MemoryBarrier();
126 const void *VDSOSupport::SetBase(const void *base) {
127 CHECK(base != ElfMemImage::kInvalidBase);
128 const void *old_base = vdso_base_;
131 // Also reset getcpu_fn_, so GetCPU could be tested with simulated VDSO.
132 getcpu_fn_ = &InitAndGetCPU;
136 bool VDSOSupport::LookupSymbol(const char *name,
139 SymbolInfo *info) const {
140 return image_.LookupSymbol(name, version, type, info);
143 bool VDSOSupport::LookupSymbolByAddress(const void *address,
144 SymbolInfo *info_out) const {
145 return image_.LookupSymbolByAddress(address, info_out);
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);
153 // x86_64 never implemented sys_getcpu(), except as a VDSO call.
159 // Use fast __vdso_getcpu if available.
160 long VDSOSupport::InitAndGetCPU(unsigned *cpu, void *x, void *y) { // NOLINT
162 CHECK_NE(getcpu_fn_, &InitAndGetCPU); // << "Init() did not set getcpu_fn_";
163 return (*getcpu_fn_)(cpu, x, y);
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.
171 int ret_code = (*VDSOSupport::getcpu_fn_)(&cpu, NULL, NULL);
172 return ret_code == 0 ? cpu : ret_code;
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 {
185 VDSOInitHelper() { VDSOSupport::Init(); }
189 #endif // HAVE_VDSO_SUPPORT