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      1 /*
      2  * Copyright (C) 2016 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 <errno.h>
     18 #include <fcntl.h>
     19 #include <inttypes.h>
     20 #include <pthread.h>
     21 #include <sched.h>
     22 #include <stddef.h>
     23 #include <stdio.h>
     24 #include <stdlib.h>
     25 #include <string.h>
     26 #include <sys/mman.h>
     27 #include <sys/syscall.h>
     28 #include <sys/types.h>
     29 #include <sys/wait.h>
     30 #include <unistd.h>
     31 
     32 #include "android-base/macros.h"
     33 
     34 #include "PtracerThread.h"
     35 #include "anon_vma_naming.h"
     36 #include "log.h"
     37 
     38 namespace android {
     39 
     40 class Stack {
     41  public:
     42   explicit Stack(size_t size) : size_(size) {
     43     int prot = PROT_READ | PROT_WRITE;
     44     int flags = MAP_PRIVATE | MAP_ANONYMOUS;
     45     page_size_ = sysconf(_SC_PAGE_SIZE);
     46     size_ += page_size_ * 2;  // guard pages
     47     base_ = mmap(NULL, size_, prot, flags, -1, 0);
     48     if (base_ == MAP_FAILED) {
     49       base_ = NULL;
     50       size_ = 0;
     51       return;
     52     }
     53     prctl(PR_SET_VMA, PR_SET_VMA_ANON_NAME, base_, size_, "libmemunreachable stack");
     54     mprotect(base_, page_size_, PROT_NONE);
     55     mprotect(top(), page_size_, PROT_NONE);
     56   };
     57   ~Stack() { munmap(base_, size_); };
     58   void* top() {
     59     return reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(base_) + size_ - page_size_);
     60   };
     61 
     62  private:
     63   DISALLOW_COPY_AND_ASSIGN(Stack);
     64 
     65   void* base_;
     66   size_t size_;
     67   size_t page_size_;
     68 };
     69 
     70 PtracerThread::PtracerThread(const std::function<int()>& func) : child_pid_(0) {
     71   stack_ = std::make_unique<Stack>(PTHREAD_STACK_MIN);
     72   if (stack_->top() == nullptr) {
     73     MEM_LOG_ALWAYS_FATAL("failed to mmap child stack: %s", strerror(errno));
     74   }
     75 
     76   func_ = std::function<int()>{[&, func]() -> int {
     77     // In the child thread, lock and unlock the mutex to wait for the parent
     78     // to finish setting up for the child thread
     79     std::unique_lock<std::mutex> lk(m_);
     80     lk.unlock();
     81     _exit(func());
     82   }};
     83 }
     84 
     85 PtracerThread::~PtracerThread() {
     86   Kill();
     87   Join();
     88   ClearTracer();
     89   stack_ = nullptr;
     90 }
     91 
     92 bool PtracerThread::Start() {
     93   std::unique_lock<std::mutex> lk(m_);
     94 
     95   // Convert from void(*)(void*) to lambda with captures
     96   auto proxy = [](void* arg) -> int {
     97     prctl(PR_SET_NAME, "libmemunreachable ptrace thread");
     98     return (*reinterpret_cast<std::function<int()>*>(arg))();
     99   };
    100 
    101   // See README.md for why we create the child process this way
    102   child_pid_ = clone(proxy, stack_->top(), CLONE_VM | CLONE_FS | CLONE_FILES /*|CLONE_UNTRACED*/,
    103                      reinterpret_cast<void*>(&func_));
    104   if (child_pid_ < 0) {
    105     MEM_ALOGE("failed to clone child: %s", strerror(errno));
    106     return false;
    107   }
    108 
    109   SetTracer(child_pid_);
    110 
    111   lk.unlock();
    112 
    113   return true;
    114 }
    115 
    116 int PtracerThread::Join() {
    117   if (child_pid_ == -1) {
    118     return -1;
    119   }
    120   int status;
    121   int ret = TEMP_FAILURE_RETRY(waitpid(child_pid_, &status, __WALL));
    122   if (ret < 0) {
    123     MEM_ALOGE("waitpid %d failed: %s", child_pid_, strerror(errno));
    124     return -1;
    125   }
    126 
    127   child_pid_ = -1;
    128 
    129   if (WIFEXITED(status)) {
    130     return WEXITSTATUS(status);
    131   } else if (WIFSIGNALED(status)) {
    132     return -WTERMSIG(status);
    133   } else {
    134     MEM_ALOGE("unexpected status %x", status);
    135     return -1;
    136   }
    137 }
    138 
    139 void PtracerThread::Kill() {
    140   if (child_pid_ == -1) {
    141     return;
    142   }
    143 
    144   syscall(SYS_tkill, child_pid_, SIGKILL);
    145 }
    146 
    147 void PtracerThread::SetTracer(pid_t tracer_pid) {
    148   prctl(PR_SET_PTRACER, tracer_pid);
    149 }
    150 
    151 void PtracerThread::ClearTracer() {
    152   prctl(PR_SET_PTRACER, 0);
    153 }
    154 
    155 }  // namespace android
    156