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 #ifdef ART_TARGET_ANDROID 18 #include <android/log.h> 19 #else 20 #include <stdarg.h> 21 #include <iostream> 22 #endif 23 24 #include <dlfcn.h> 25 #include <errno.h> 26 #include <pthread.h> 27 #include <signal.h> 28 #include <stdio.h> 29 #include <stdlib.h> 30 #include <string.h> 31 32 #include <initializer_list> 33 #include <mutex> 34 #include <utility> 35 36 #include "sigchain.h" 37 38 #if defined(__APPLE__) 39 #define _NSIG NSIG 40 #define sighandler_t sig_t 41 42 // Darwin has an #error when ucontext.h is included without _XOPEN_SOURCE defined. 43 #define _XOPEN_SOURCE 44 #endif 45 46 #include <ucontext.h> 47 48 // libsigchain provides an interception layer for signal handlers, to allow ART and others to give 49 // their signal handlers the first stab at handling signals before passing them on to user code. 50 // 51 // It implements wrapper functions for signal, sigaction, and sigprocmask, and a handler that 52 // forwards signals appropriately. 53 // 54 // In our handler, we start off with all signals blocked, fetch the original signal mask from the 55 // passed in ucontext, and then adjust our signal mask appropriately for the user handler. 56 // 57 // It's somewhat tricky for us to properly handle some flag cases: 58 // SA_NOCLDSTOP and SA_NOCLDWAIT: shouldn't matter, we don't have special handlers for SIGCHLD. 59 // SA_NODEFER: unimplemented, we can manually change the signal mask appropriately. 60 // ~SA_ONSTACK: always silently enable this 61 // SA_RESETHAND: unimplemented, but we can probably do this? 62 // ~SA_RESTART: unimplemented, maybe we can reserve an RT signal, register an empty handler that 63 // doesn't have SA_RESTART, and raise the signal to avoid restarting syscalls that are 64 // expected to be interrupted? 65 66 static void log(const char* format, ...) { 67 char buf[256]; 68 va_list ap; 69 va_start(ap, format); 70 vsnprintf(buf, sizeof(buf), format, ap); 71 #ifdef ART_TARGET_ANDROID 72 __android_log_write(ANDROID_LOG_ERROR, "libsigchain", buf); 73 #else 74 std::cout << buf << "\n"; 75 #endif 76 va_end(ap); 77 } 78 79 #define fatal(...) log(__VA_ARGS__); abort() 80 81 static int sigorset(sigset_t* dest, sigset_t* left, sigset_t* right) { 82 sigemptyset(dest); 83 for (size_t i = 0; i < sizeof(sigset_t) * CHAR_BIT; ++i) { 84 if (sigismember(left, i) == 1 || sigismember(right, i) == 1) { 85 sigaddset(dest, i); 86 } 87 } 88 return 0; 89 } 90 91 namespace art { 92 93 static decltype(&sigaction) linked_sigaction; 94 static decltype(&sigprocmask) linked_sigprocmask; 95 96 __attribute__((constructor)) static void InitializeSignalChain() { 97 static std::once_flag once; 98 std::call_once(once, []() { 99 void* linked_sigaction_sym = dlsym(RTLD_NEXT, "sigaction"); 100 if (linked_sigaction_sym == nullptr) { 101 linked_sigaction_sym = dlsym(RTLD_DEFAULT, "sigaction"); 102 if (linked_sigaction_sym == nullptr || 103 linked_sigaction_sym == reinterpret_cast<void*>(sigaction)) { 104 fatal("Unable to find next sigaction in signal chain"); 105 } 106 } 107 108 void* linked_sigprocmask_sym = dlsym(RTLD_NEXT, "sigprocmask"); 109 if (linked_sigprocmask_sym == nullptr) { 110 linked_sigprocmask_sym = dlsym(RTLD_DEFAULT, "sigprocmask"); 111 if (linked_sigprocmask_sym == nullptr || 112 linked_sigprocmask_sym == reinterpret_cast<void*>(sigprocmask)) { 113 fatal("Unable to find next sigprocmask in signal chain"); 114 } 115 } 116 117 linked_sigaction = 118 reinterpret_cast<decltype(linked_sigaction)>(linked_sigaction_sym); 119 linked_sigprocmask = 120 reinterpret_cast<decltype(linked_sigprocmask)>(linked_sigprocmask_sym); 121 }); 122 } 123 124 125 static pthread_key_t GetHandlingSignalKey() { 126 static pthread_key_t key; 127 static std::once_flag once; 128 std::call_once(once, []() { 129 int rc = pthread_key_create(&key, nullptr); 130 if (rc != 0) { 131 fatal("failed to create sigchain pthread key: %s", strerror(rc)); 132 } 133 }); 134 return key; 135 } 136 137 static bool GetHandlingSignal() { 138 void* result = pthread_getspecific(GetHandlingSignalKey()); 139 return reinterpret_cast<uintptr_t>(result); 140 } 141 142 static void SetHandlingSignal(bool value) { 143 pthread_setspecific(GetHandlingSignalKey(), 144 reinterpret_cast<void*>(static_cast<uintptr_t>(value))); 145 } 146 147 class ScopedHandlingSignal { 148 public: 149 ScopedHandlingSignal() : original_value_(GetHandlingSignal()) { 150 } 151 152 ~ScopedHandlingSignal() { 153 SetHandlingSignal(original_value_); 154 } 155 156 private: 157 bool original_value_; 158 }; 159 160 class SignalChain { 161 public: 162 SignalChain() : claimed_(false) { 163 } 164 165 bool IsClaimed() { 166 return claimed_; 167 } 168 169 void Claim(int signo) { 170 if (!claimed_) { 171 Register(signo); 172 claimed_ = true; 173 } 174 } 175 176 // Register the signal chain with the kernel if needed. 177 void Register(int signo) { 178 struct sigaction handler_action = {}; 179 handler_action.sa_sigaction = SignalChain::Handler; 180 handler_action.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK; 181 sigfillset(&handler_action.sa_mask); 182 linked_sigaction(signo, &handler_action, &action_); 183 } 184 185 void SetAction(const struct sigaction* action) { 186 action_ = *action; 187 } 188 189 struct sigaction GetAction() { 190 return action_; 191 } 192 193 void AddSpecialHandler(SigchainAction* sa) { 194 for (SigchainAction& slot : special_handlers_) { 195 if (slot.sc_sigaction == nullptr) { 196 slot = *sa; 197 return; 198 } 199 } 200 201 fatal("too many special signal handlers"); 202 } 203 204 void RemoveSpecialHandler(bool (*fn)(int, siginfo_t*, void*)) { 205 // This isn't thread safe, but it's unlikely to be a real problem. 206 size_t len = sizeof(special_handlers_)/sizeof(*special_handlers_); 207 for (size_t i = 0; i < len; ++i) { 208 if (special_handlers_[i].sc_sigaction == fn) { 209 for (size_t j = i; j < len - 1; ++j) { 210 special_handlers_[j] = special_handlers_[j + 1]; 211 } 212 special_handlers_[len - 1].sc_sigaction = nullptr; 213 return; 214 } 215 } 216 217 fatal("failed to find special handler to remove"); 218 } 219 220 221 static void Handler(int signo, siginfo_t* siginfo, void*); 222 223 private: 224 bool claimed_; 225 struct sigaction action_; 226 SigchainAction special_handlers_[2]; 227 }; 228 229 static SignalChain chains[_NSIG]; 230 231 void SignalChain::Handler(int signo, siginfo_t* siginfo, void* ucontext_raw) { 232 // Try the special handlers first. 233 // If one of them crashes, we'll reenter this handler and pass that crash onto the user handler. 234 if (!GetHandlingSignal()) { 235 for (const auto& handler : chains[signo].special_handlers_) { 236 if (handler.sc_sigaction == nullptr) { 237 break; 238 } 239 240 // The native bridge signal handler might not return. 241 // Avoid setting the thread local flag in this case, since we'll never 242 // get a chance to restore it. 243 bool handler_noreturn = (handler.sc_flags & SIGCHAIN_ALLOW_NORETURN); 244 sigset_t previous_mask; 245 linked_sigprocmask(SIG_SETMASK, &handler.sc_mask, &previous_mask); 246 247 ScopedHandlingSignal restorer; 248 if (!handler_noreturn) { 249 SetHandlingSignal(true); 250 } 251 252 if (handler.sc_sigaction(signo, siginfo, ucontext_raw)) { 253 return; 254 } 255 256 linked_sigprocmask(SIG_SETMASK, &previous_mask, nullptr); 257 } 258 } 259 260 // Forward to the user's signal handler. 261 int handler_flags = chains[signo].action_.sa_flags; 262 ucontext_t* ucontext = static_cast<ucontext_t*>(ucontext_raw); 263 sigset_t mask; 264 sigorset(&mask, &ucontext->uc_sigmask, &chains[signo].action_.sa_mask); 265 if (!(handler_flags & SA_NODEFER)) { 266 sigaddset(&mask, signo); 267 } 268 linked_sigprocmask(SIG_SETMASK, &mask, nullptr); 269 270 if ((handler_flags & SA_SIGINFO)) { 271 chains[signo].action_.sa_sigaction(signo, siginfo, ucontext_raw); 272 } else { 273 auto handler = chains[signo].action_.sa_handler; 274 if (handler == SIG_IGN) { 275 return; 276 } else if (handler == SIG_DFL) { 277 fatal("exiting due to SIG_DFL handler for signal %d", signo); 278 } else { 279 handler(signo); 280 } 281 } 282 } 283 284 extern "C" int sigaction(int signal, const struct sigaction* new_action, struct sigaction* old_action) { 285 InitializeSignalChain(); 286 287 // If this signal has been claimed as a signal chain, record the user's 288 // action but don't pass it on to the kernel. 289 // Note that we check that the signal number is in range here. An out of range signal 290 // number should behave exactly as the libc sigaction. 291 if (signal < 0 || signal >= _NSIG) { 292 errno = EINVAL; 293 return -1; 294 } 295 296 if (chains[signal].IsClaimed()) { 297 struct sigaction saved_action = chains[signal].GetAction(); 298 if (new_action != nullptr) { 299 chains[signal].SetAction(new_action); 300 } 301 if (old_action != nullptr) { 302 *old_action = saved_action; 303 } 304 return 0; 305 } 306 307 // Will only get here if the signal chain has not been claimed. We want 308 // to pass the sigaction on to the kernel via the real sigaction in libc. 309 return linked_sigaction(signal, new_action, old_action); 310 } 311 312 extern "C" sighandler_t signal(int signo, sighandler_t handler) { 313 InitializeSignalChain(); 314 315 if (signo < 0 || signo > _NSIG) { 316 errno = EINVAL; 317 return SIG_ERR; 318 } 319 320 struct sigaction sa = {}; 321 sigemptyset(&sa.sa_mask); 322 sa.sa_handler = handler; 323 sa.sa_flags = SA_RESTART | SA_ONSTACK; 324 sighandler_t oldhandler; 325 326 // If this signal has been claimed as a signal chain, record the user's 327 // action but don't pass it on to the kernel. 328 if (chains[signo].IsClaimed()) { 329 oldhandler = reinterpret_cast<sighandler_t>(chains[signo].GetAction().sa_handler); 330 chains[signo].SetAction(&sa); 331 return oldhandler; 332 } 333 334 // Will only get here if the signal chain has not been claimed. We want 335 // to pass the sigaction on to the kernel via the real sigaction in libc. 336 if (linked_sigaction(signo, &sa, &sa) == -1) { 337 return SIG_ERR; 338 } 339 340 return reinterpret_cast<sighandler_t>(sa.sa_handler); 341 } 342 343 #if !defined(__LP64__) 344 extern "C" sighandler_t bsd_signal(int signo, sighandler_t handler) { 345 InitializeSignalChain(); 346 347 return signal(signo, handler); 348 } 349 #endif 350 351 extern "C" int sigprocmask(int how, const sigset_t* bionic_new_set, sigset_t* bionic_old_set) { 352 InitializeSignalChain(); 353 354 // When inside a signal handler, forward directly to the actual sigprocmask. 355 if (GetHandlingSignal()) { 356 return linked_sigprocmask(how, bionic_new_set, bionic_old_set); 357 } 358 359 const sigset_t* new_set_ptr = bionic_new_set; 360 sigset_t tmpset; 361 if (bionic_new_set != nullptr) { 362 tmpset = *bionic_new_set; 363 364 if (how == SIG_BLOCK) { 365 // Don't allow claimed signals in the mask. If a signal chain has been claimed 366 // we can't allow the user to block that signal. 367 for (int i = 0 ; i < _NSIG; ++i) { 368 if (chains[i].IsClaimed() && sigismember(&tmpset, i)) { 369 sigdelset(&tmpset, i); 370 } 371 } 372 } 373 new_set_ptr = &tmpset; 374 } 375 376 return linked_sigprocmask(how, new_set_ptr, bionic_old_set); 377 } 378 379 extern "C" void AddSpecialSignalHandlerFn(int signal, SigchainAction* sa) { 380 InitializeSignalChain(); 381 382 if (signal <= 0 || signal >= _NSIG) { 383 fatal("Invalid signal %d", signal); 384 } 385 386 // Set the managed_handler. 387 chains[signal].AddSpecialHandler(sa); 388 chains[signal].Claim(signal); 389 } 390 391 extern "C" void RemoveSpecialSignalHandlerFn(int signal, bool (*fn)(int, siginfo_t*, void*)) { 392 InitializeSignalChain(); 393 394 if (signal <= 0 || signal >= _NSIG) { 395 fatal("Invalid signal %d", signal); 396 } 397 398 chains[signal].RemoveSpecialHandler(fn); 399 } 400 401 extern "C" void EnsureFrontOfChain(int signal) { 402 InitializeSignalChain(); 403 404 if (signal <= 0 || signal >= _NSIG) { 405 fatal("Invalid signal %d", signal); 406 } 407 408 // Read the current action without looking at the chain, it should be the expected action. 409 struct sigaction current_action; 410 linked_sigaction(signal, nullptr, ¤t_action); 411 412 // If the sigactions don't match then we put the current action on the chain and make ourself as 413 // the main action. 414 if (current_action.sa_sigaction != SignalChain::Handler) { 415 log("Warning: Unexpected sigaction action found %p\n", current_action.sa_sigaction); 416 chains[signal].Register(signal); 417 } 418 } 419 420 } // namespace art 421 422