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      1 /*
      2  * Copyright (C) 2014 The Android Open Source Project
      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  *      http://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 #include <link.h>
     18 #include <sys/auxv.h>
     19 #include <unistd.h>
     20 
     21 // x86 has a vdso, but there's nothing useful to us in it.
     22 #if defined(__aarch64__) || defined(__x86_64__)
     23 
     24 #if defined(__aarch64__)
     25 #define VDSO_CLOCK_GETTIME_SYMBOL "__kernel_clock_gettime"
     26 #define VDSO_GETTIMEOFDAY_SYMBOL  "__kernel_gettimeofday"
     27 #elif defined(__x86_64__)
     28 #define VDSO_CLOCK_GETTIME_SYMBOL "__vdso_clock_gettime"
     29 #define VDSO_GETTIMEOFDAY_SYMBOL  "__vdso_gettimeofday"
     30 #endif
     31 
     32 #include <time.h>
     33 
     34 extern "C" int __clock_gettime(int, timespec*);
     35 extern "C" int __gettimeofday(timeval*, struct timezone*);
     36 
     37 struct vdso_entry {
     38   const char* name;
     39   void* fn;
     40 };
     41 
     42 enum {
     43   VDSO_CLOCK_GETTIME = 0,
     44   VDSO_GETTIMEOFDAY,
     45   VDSO_END
     46 };
     47 
     48 static vdso_entry vdso_entries[] = {
     49   [VDSO_CLOCK_GETTIME] = { VDSO_CLOCK_GETTIME_SYMBOL, reinterpret_cast<void*>(__clock_gettime) },
     50   [VDSO_GETTIMEOFDAY] = { VDSO_GETTIMEOFDAY_SYMBOL, reinterpret_cast<void*>(__gettimeofday) },
     51 };
     52 
     53 int clock_gettime(int clock_id, timespec* tp) {
     54   static int (*vdso_clock_gettime)(int, timespec*) =
     55       (int (*)(int, timespec*)) vdso_entries[VDSO_CLOCK_GETTIME].fn;
     56   return vdso_clock_gettime(clock_id, tp);
     57 }
     58 
     59 int gettimeofday(timeval* tv, struct timezone* tz) {
     60   static int (*vdso_gettimeofday)(timeval*, struct timezone*) =
     61       (int (*)(timeval*, struct timezone*)) vdso_entries[VDSO_GETTIMEOFDAY].fn;
     62   return vdso_gettimeofday(tv, tz);
     63 }
     64 
     65 void __libc_init_vdso() {
     66   // Do we have a vdso?
     67   uintptr_t vdso_ehdr_addr = getauxval(AT_SYSINFO_EHDR);
     68   ElfW(Ehdr)* vdso_ehdr = reinterpret_cast<ElfW(Ehdr)*>(vdso_ehdr_addr);
     69   if (vdso_ehdr == NULL) {
     70     return;
     71   }
     72 
     73   // How many symbols does it have?
     74   size_t symbol_count = 0;
     75   ElfW(Shdr)* vdso_shdr = reinterpret_cast<ElfW(Shdr)*>(vdso_ehdr_addr + vdso_ehdr->e_shoff);
     76   for (size_t i = 0; i < vdso_ehdr->e_shnum; ++i) {
     77     if (vdso_shdr[i].sh_type == SHT_DYNSYM) {
     78       symbol_count = vdso_shdr[i].sh_size / sizeof(ElfW(Sym));
     79     }
     80   }
     81   if (symbol_count == 0) {
     82     return;
     83   }
     84 
     85   // Where's the dynamic table?
     86   ElfW(Addr) vdso_addr = 0;
     87   ElfW(Dyn)* vdso_dyn = NULL;
     88   ElfW(Phdr)* vdso_phdr = reinterpret_cast<ElfW(Phdr)*>(vdso_ehdr_addr + vdso_ehdr->e_phoff);
     89   for (size_t i = 0; i < vdso_ehdr->e_phnum; ++i) {
     90     if (vdso_phdr[i].p_type == PT_DYNAMIC) {
     91       vdso_dyn = reinterpret_cast<ElfW(Dyn)*>(vdso_ehdr_addr + vdso_phdr[i].p_offset);
     92     } else if (vdso_phdr[i].p_type == PT_LOAD) {
     93       vdso_addr = vdso_ehdr_addr + vdso_phdr[i].p_offset - vdso_phdr[i].p_vaddr;
     94     }
     95   }
     96   if (vdso_addr == 0 || vdso_dyn == NULL) {
     97     return;
     98   }
     99 
    100   // Where are the string and symbol tables?
    101   const char* strtab = NULL;
    102   ElfW(Sym)* symtab = NULL;
    103   for (ElfW(Dyn)* d = vdso_dyn; d->d_tag != DT_NULL; ++d) {
    104     if (d->d_tag == DT_STRTAB) {
    105       strtab = reinterpret_cast<const char*>(vdso_addr + d->d_un.d_ptr);
    106     } else if (d->d_tag == DT_SYMTAB) {
    107       symtab = reinterpret_cast<ElfW(Sym)*>(vdso_addr + d->d_un.d_ptr);
    108     }
    109   }
    110   if (strtab == NULL || symtab == NULL) {
    111     return;
    112   }
    113 
    114   // Are there any symbols we want?
    115   for (size_t i = 0; i < symbol_count; ++i) {
    116     for (size_t j = 0; j < VDSO_END; ++j) {
    117       if (strcmp(vdso_entries[j].name, strtab + symtab[i].st_name) == 0) {
    118         vdso_entries[j].fn = reinterpret_cast<void*>(vdso_addr + symtab[i].st_value);
    119       }
    120     }
    121   }
    122 }
    123 
    124 #else
    125 
    126 void __libc_init_vdso() {
    127 }
    128 
    129 #endif
    130