sigchainlib/sigchain.cc - LeafOS-Project/android_art - Gitiles

 /*
  * Copyright (C) 2014 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include <dlfcn.h>
 #include <errno.h>
 #include <pthread.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 #if defined(__BIONIC__)
 #include <bionic/macros.h>
 #endif

 #include <algorithm>
 #include <initializer_list>
 #include <mutex>
 #include <type_traits>
 #include <utility>

 #include "log.h"
 #include "sigchain.h"

 #if defined(__APPLE__)
 #define _NSIG NSIG
 #define sighandler_t sig_t

 // Darwin has an #error when ucontext.h is included without _XOPEN_SOURCE defined.
 #define _XOPEN_SOURCE
 #endif

 #define SA_UNSUPPORTED 0x00000400
 #define SA_EXPOSE_TAGBITS 0x00000800

 #include <ucontext.h>

 // libsigchain provides an interception layer for signal handlers, to allow ART and others to give
 // their signal handlers the first stab at handling signals before passing them on to user code.
 //
 // It implements wrapper functions for signal, sigaction, and sigprocmask, and a handler that
 // forwards signals appropriately.
 //
 // In our handler, we start off with all signals blocked, fetch the original signal mask from the
 // passed in ucontext, and then adjust our signal mask appropriately for the user handler.
 //
 // It's somewhat tricky for us to properly handle some flag cases:
 //   SA_NOCLDSTOP and SA_NOCLDWAIT: shouldn't matter, we don't have special handlers for SIGCHLD.
 //   SA_NODEFER: unimplemented, we can manually change the signal mask appropriately.
 //  ~SA_ONSTACK: always silently enable this
 //   SA_RESETHAND: unimplemented, but we can probably do this?
 //  ~SA_RESTART: unimplemented, maybe we can reserve an RT signal, register an empty handler that
 //               doesn't have SA_RESTART, and raise the signal to avoid restarting syscalls that are
 //               expected to be interrupted?

 #if defined(__BIONIC__) && !defined(__LP64__)
 static int sigismember(const sigset64_t* sigset, int signum) {
   return sigismember64(sigset, signum);
 }

 static int sigemptyset(sigset64_t* sigset) {
   return sigemptyset64(sigset);
 }

 static int sigaddset(sigset64_t* sigset, int signum) {
   return sigaddset64(sigset, signum);
 }

 static int sigdelset(sigset64_t* sigset, int signum) {
   return sigdelset64(sigset, signum);
 }
 #endif

 template<typename SigsetType>
 static int sigorset(SigsetType* dest, SigsetType* left, SigsetType* right) {
   sigemptyset(dest);
   for (size_t i = 0; i < sizeof(SigsetType) * CHAR_BIT; ++i) {
     if (sigismember(left, i) == 1 || sigismember(right, i) == 1) {
       sigaddset(dest, i);
     }
   }
   return 0;
 }

 namespace art {

 static decltype(&sigaction) linked_sigaction;
 static decltype(&sigprocmask) linked_sigprocmask;

 #if defined(__BIONIC__)
 static decltype(&sigaction64) linked_sigaction64;
 static decltype(&sigprocmask64) linked_sigprocmask64;
 #endif

 template <typename T>
 static void lookup_libc_symbol(T* output, T wrapper, const char* name) {
 #if defined(__BIONIC__)
   constexpr const char* libc_name = "libc.so";
 #elif defined(__GLIBC__)
 #if __GNU_LIBRARY__ != 6
 #error unsupported glibc version
 #endif
   constexpr const char* libc_name = "libc.so.6";
 #else
 #error unsupported libc: not bionic or glibc?
 #endif

   static void* libc = []() {
     void* result = dlopen(libc_name, RTLD_LOCAL | RTLD_LAZY);
     if (!result) {
       fatal("failed to dlopen %s: %s", libc_name, dlerror());
     }
     return result;
   }();

   void* sym = dlsym(libc, name);  // NOLINT glibc triggers cert-dcl16-c with RTLD_NEXT.
   if (sym == nullptr) {
     sym = dlsym(RTLD_DEFAULT, name);
     if (sym == wrapper || sym == sigaction) {
       fatal("Unable to find next %s in signal chain", name);
     }
   }
   *output = reinterpret_cast<T>(sym);
 }

 __attribute__((constructor)) static void InitializeSignalChain() {
   static std::once_flag once;
   std::call_once(once, []() {
     lookup_libc_symbol(&linked_sigaction, sigaction, "sigaction");
     lookup_libc_symbol(&linked_sigprocmask, sigprocmask, "sigprocmask");

 #if defined(__BIONIC__)
     lookup_libc_symbol(&linked_sigaction64, sigaction64, "sigaction64");
     lookup_libc_symbol(&linked_sigprocmask64, sigprocmask64, "sigprocmask64");
 #endif
   });
 }

 static pthread_key_t GetHandlingSignalKey() {
   static pthread_key_t key;
   static std::once_flag once;
   std::call_once(once, []() {
     int rc = pthread_key_create(&key, nullptr);
     if (rc != 0) {
       fatal("failed to create sigchain pthread key: %s", strerror(rc));
     }
   });
   return key;
 }

 static bool GetHandlingSignal() {
   void* result = pthread_getspecific(GetHandlingSignalKey());
   return reinterpret_cast<uintptr_t>(result);
 }

 static void SetHandlingSignal(bool value) {
   pthread_setspecific(GetHandlingSignalKey(),
                       reinterpret_cast<void*>(static_cast<uintptr_t>(value)));
 }

 class ScopedHandlingSignal {
  public:
   ScopedHandlingSignal() : original_value_(GetHandlingSignal()) {
   }

   ~ScopedHandlingSignal() {
     SetHandlingSignal(original_value_);
   }

  private:
   bool original_value_;
 };

 class SignalChain {
  public:
   SignalChain() : claimed_(false) {
   }

   bool IsClaimed() {
     return claimed_;
   }

   void Claim(int signo) {
     if (!claimed_) {
       Register(signo);
       claimed_ = true;
     }
   }

   // Register the signal chain with the kernel if needed.
   void Register(int signo) {
 #if defined(__BIONIC__)
     struct sigaction64 handler_action = {};
     sigfillset64(&handler_action.sa_mask);
 #else
     struct sigaction handler_action = {};
     sigfillset(&handler_action.sa_mask);
 #endif

     handler_action.sa_sigaction = SignalChain::Handler;
     handler_action.sa_flags = SA_RESTART | SA_SIGINFO | SA_ONSTACK |
                               SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;

 #if defined(__BIONIC__)
     linked_sigaction64(signo, &handler_action, &action_);
     linked_sigaction64(signo, nullptr, &handler_action);
 #else
     linked_sigaction(signo, &handler_action, &action_);
     linked_sigaction(signo, nullptr, &handler_action);
 #endif

     // Newer kernels clear unknown flags from sigaction.sa_flags in order to
     // allow userspace to determine which flag bits are supported. We use this
     // behavior in turn to implement the same flag bit support detection
     // protocol regardless of kernel version. Due to the lack of a flag bit
     // support detection protocol in older kernels we assume support for a base
     // set of flags that have been supported since at least 2003 [1]. No flags
     // were introduced since then until the introduction of SA_EXPOSE_TAGBITS
     // handled below. glibc headers do not define SA_RESTORER so we define it
     // ourselves.
     //
     // TODO(pcc): The new kernel behavior has been implemented in a kernel
     // patch [2] that has not yet landed. Update the code if necessary once it
     // lands.
     //
     // [1] https://github.com/mpe/linux-fullhistory/commit/c0f806c86fc8b07ad426df023f1a4bb0e53c64f6
     // [2] https://lore.kernel.org/linux-arm-kernel/cover.1605235762.git.pcc@google.com/
 #if !defined(__BIONIC__)
 #define SA_RESTORER 0x04000000
 #endif
     kernel_supported_flags_ = SA_NOCLDSTOP | SA_NOCLDWAIT | SA_SIGINFO |
                               SA_ONSTACK | SA_RESTART | SA_NODEFER |
                               SA_RESETHAND | SA_RESTORER;

     // Determine whether the kernel supports SA_EXPOSE_TAGBITS. For newer
     // kernels we use the flag support detection protocol described above. In
     // order to allow userspace to distinguish old and new kernels,
     // SA_UNSUPPORTED has been reserved as an unsupported flag. If the kernel
     // did not clear it then we know that we have an old kernel that would not
     // support SA_EXPOSE_TAGBITS anyway.
     if (!(handler_action.sa_flags & SA_UNSUPPORTED) &&
         (handler_action.sa_flags & SA_EXPOSE_TAGBITS)) {
       kernel_supported_flags_ |= SA_EXPOSE_TAGBITS;
     }
   }

   template <typename SigactionType>
   SigactionType GetAction() {
     if constexpr (std::is_same_v<decltype(action_), SigactionType>) {
       return action_;
     } else {
       SigactionType result;
       result.sa_flags = action_.sa_flags;
       result.sa_handler = action_.sa_handler;
 #if defined(SA_RESTORER)
       result.sa_restorer = action_.sa_restorer;
 #endif
       memcpy(&result.sa_mask, &action_.sa_mask,
              std::min(sizeof(action_.sa_mask), sizeof(result.sa_mask)));
       return result;
     }
   }

   template <typename SigactionType>
   void SetAction(const SigactionType* new_action) {
     if constexpr (std::is_same_v<decltype(action_), SigactionType>) {
       action_ = *new_action;
     } else {
       action_.sa_flags = new_action->sa_flags;
       action_.sa_handler = new_action->sa_handler;
 #if defined(SA_RESTORER)
       action_.sa_restorer = new_action->sa_restorer;
 #endif
       sigemptyset(&action_.sa_mask);
       memcpy(&action_.sa_mask, &new_action->sa_mask,
              std::min(sizeof(action_.sa_mask), sizeof(new_action->sa_mask)));
     }
     action_.sa_flags &= kernel_supported_flags_;
   }

   void AddSpecialHandler(SigchainAction* sa) {
     for (SigchainAction& slot : special_handlers_) {
       if (slot.sc_sigaction == nullptr) {
         slot = *sa;
         return;
       }
     }

     fatal("too many special signal handlers");
   }

   void RemoveSpecialHandler(bool (*fn)(int, siginfo_t*, void*)) {
     // This isn't thread safe, but it's unlikely to be a real problem.
     size_t len = sizeof(special_handlers_)/sizeof(*special_handlers_);
     for (size_t i = 0; i < len; ++i) {
       if (special_handlers_[i].sc_sigaction == fn) {
         for (size_t j = i; j < len - 1; ++j) {
           special_handlers_[j] = special_handlers_[j + 1];
         }
         special_handlers_[len - 1].sc_sigaction = nullptr;
         return;
       }
     }

     fatal("failed to find special handler to remove");
   }


   static void Handler(int signo, siginfo_t* siginfo, void*);

  private:
   bool claimed_;
   int kernel_supported_flags_;
 #if defined(__BIONIC__)
   struct sigaction64 action_;
 #else
   struct sigaction action_;
 #endif
   SigchainAction special_handlers_[2];
 };

 // _NSIG is 1 greater than the highest valued signal, but signals start from 1.
 // Leave an empty element at index 0 for convenience.
 static SignalChain chains[_NSIG + 1];

 static bool is_signal_hook_debuggable = false;

 void SignalChain::Handler(int signo, siginfo_t* siginfo, void* ucontext_raw) {
   // Try the special handlers first.
   // If one of them crashes, we'll reenter this handler and pass that crash onto the user handler.
   if (!GetHandlingSignal()) {
     for (const auto& handler : chains[signo].special_handlers_) {
       if (handler.sc_sigaction == nullptr) {
         break;
       }

       // The native bridge signal handler might not return.
       // Avoid setting the thread local flag in this case, since we'll never
       // get a chance to restore it.
       bool handler_noreturn = (handler.sc_flags & SIGCHAIN_ALLOW_NORETURN);
       sigset_t previous_mask;
       linked_sigprocmask(SIG_SETMASK, &handler.sc_mask, &previous_mask);

       ScopedHandlingSignal restorer;
       if (!handler_noreturn) {
         SetHandlingSignal(true);
       }

       if (handler.sc_sigaction(signo, siginfo, ucontext_raw)) {
         return;
       }

       linked_sigprocmask(SIG_SETMASK, &previous_mask, nullptr);
     }
   }

   // Forward to the user's signal handler.
   int handler_flags = chains[signo].action_.sa_flags;
   ucontext_t* ucontext = static_cast<ucontext_t*>(ucontext_raw);
 #if defined(__BIONIC__)
   sigset64_t mask;
   sigorset(&mask, &ucontext->uc_sigmask64, &chains[signo].action_.sa_mask);
 #else
   sigset_t mask;
   sigorset(&mask, &ucontext->uc_sigmask, &chains[signo].action_.sa_mask);
 #endif
   if (!(handler_flags & SA_NODEFER)) {
     sigaddset(&mask, signo);
   }

 #if defined(__BIONIC__)
   linked_sigprocmask64(SIG_SETMASK, &mask, nullptr);
 #else
   linked_sigprocmask(SIG_SETMASK, &mask, nullptr);
 #endif

   if ((handler_flags & SA_SIGINFO)) {
     // If the chained handler is not expecting tag bits in the fault address,
     // mask them out now.
 #if defined(__BIONIC__)
     if (!(handler_flags & SA_EXPOSE_TAGBITS) &&
         (signo == SIGILL || signo == SIGFPE || signo == SIGSEGV ||
          signo == SIGBUS || signo == SIGTRAP) &&
         siginfo->si_code > SI_USER && siginfo->si_code < SI_KERNEL &&
         !(signo == SIGTRAP && siginfo->si_code == TRAP_HWBKPT)) {
       siginfo->si_addr = untag_address(siginfo->si_addr);
     }
 #endif
     chains[signo].action_.sa_sigaction(signo, siginfo, ucontext_raw);
   } else {
     auto handler = chains[signo].action_.sa_handler;
     if (handler == SIG_IGN) {
       return;
     } else if (handler == SIG_DFL) {
       fatal("exiting due to SIG_DFL handler for signal %d, ucontext %p", signo, ucontext);
     } else {
       handler(signo);
     }
   }
 }

 template <typename SigactionType>
 static int __sigaction(int signal, const SigactionType* new_action,
                        SigactionType* old_action,
                        int (*linked)(int, const SigactionType*,
                                      SigactionType*)) {
   if (is_signal_hook_debuggable) {
     return 0;
   }

   // If this signal has been claimed as a signal chain, record the user's
   // action but don't pass it on to the kernel.
   // Note that we check that the signal number is in range here.  An out of range signal
   // number should behave exactly as the libc sigaction.
   if (signal <= 0 || signal >= _NSIG) {
     errno = EINVAL;
     return -1;
   }

   if (chains[signal].IsClaimed()) {
     SigactionType saved_action = chains[signal].GetAction<SigactionType>();
     if (new_action != nullptr) {
       chains[signal].SetAction(new_action);
     }
     if (old_action != nullptr) {
       *old_action = saved_action;
     }
     return 0;
   }

   // Will only get here if the signal chain has not been claimed.  We want
   // to pass the sigaction on to the kernel via the real sigaction in libc.
   return linked(signal, new_action, old_action);
 }

 extern "C" int sigaction(int signal, const struct sigaction* new_action,
                          struct sigaction* old_action) {
   InitializeSignalChain();
   return __sigaction(signal, new_action, old_action, linked_sigaction);
 }

 #if defined(__BIONIC__)
 extern "C" int sigaction64(int signal, const struct sigaction64* new_action,
                            struct sigaction64* old_action) {
   InitializeSignalChain();
   return __sigaction(signal, new_action, old_action, linked_sigaction64);
 }
 #endif

 extern "C" sighandler_t signal(int signo, sighandler_t handler) {
   InitializeSignalChain();

   if (signo <= 0 || signo >= _NSIG) {
     errno = EINVAL;
     return SIG_ERR;
   }

   struct sigaction sa = {};
   sigemptyset(&sa.sa_mask);
   sa.sa_handler = handler;
   sa.sa_flags = SA_RESTART | SA_ONSTACK;
   sighandler_t oldhandler;

   // If this signal has been claimed as a signal chain, record the user's
   // action but don't pass it on to the kernel.
   if (chains[signo].IsClaimed()) {
     oldhandler = reinterpret_cast<sighandler_t>(
         chains[signo].GetAction<struct sigaction>().sa_handler);
     chains[signo].SetAction(&sa);
     return oldhandler;
   }

   // Will only get here if the signal chain has not been claimed.  We want
   // to pass the sigaction on to the kernel via the real sigaction in libc.
   if (linked_sigaction(signo, &sa, &sa) == -1) {
     return SIG_ERR;
   }

   return reinterpret_cast<sighandler_t>(sa.sa_handler);
 }

 #if !defined(__LP64__)
 extern "C" sighandler_t bsd_signal(int signo, sighandler_t handler) {
   InitializeSignalChain();

   return signal(signo, handler);
 }
 #endif

 template <typename SigsetType>
 int __sigprocmask(int how, const SigsetType* new_set, SigsetType* old_set,
                   int (*linked)(int, const SigsetType*, SigsetType*)) {
   // When inside a signal handler, forward directly to the actual sigprocmask.
   if (GetHandlingSignal()) {
     return linked(how, new_set, old_set);
   }

   const SigsetType* new_set_ptr = new_set;
   SigsetType tmpset;
   if (new_set != nullptr) {
     tmpset = *new_set;

     if (how == SIG_BLOCK || how == SIG_SETMASK) {
       // Don't allow claimed signals in the mask.  If a signal chain has been claimed
       // we can't allow the user to block that signal.
       for (int i = 1; i < _NSIG; ++i) {
         if (chains[i].IsClaimed() && sigismember(&tmpset, i)) {
           sigdelset(&tmpset, i);
         }
       }
     }
     new_set_ptr = &tmpset;
   }

   return linked(how, new_set_ptr, old_set);
 }

 extern "C" int sigprocmask(int how, const sigset_t* new_set,
                            sigset_t* old_set) {
   InitializeSignalChain();
   return __sigprocmask(how, new_set, old_set, linked_sigprocmask);
 }

 #if defined(__BIONIC__)
 extern "C" int sigprocmask64(int how, const sigset64_t* new_set,
                              sigset64_t* old_set) {
   InitializeSignalChain();
   return __sigprocmask(how, new_set, old_set, linked_sigprocmask64);
 }
 #endif

 extern "C" void AddSpecialSignalHandlerFn(int signal, SigchainAction* sa) {
   InitializeSignalChain();

   if (signal <= 0 || signal >= _NSIG) {
     fatal("Invalid signal %d", signal);
   }

   // Set the managed_handler.
   chains[signal].AddSpecialHandler(sa);
   chains[signal].Claim(signal);
 }

 extern "C" void RemoveSpecialSignalHandlerFn(int signal, bool (*fn)(int, siginfo_t*, void*)) {
   InitializeSignalChain();

   if (signal <= 0 || signal >= _NSIG) {
     fatal("Invalid signal %d", signal);
   }

   chains[signal].RemoveSpecialHandler(fn);
 }

 extern "C" void EnsureFrontOfChain(int signal) {
   InitializeSignalChain();

   if (signal <= 0 || signal >= _NSIG) {
     fatal("Invalid signal %d", signal);
   }

   // Read the current action without looking at the chain, it should be the expected action.
 #if defined(__BIONIC__)
   struct sigaction64 current_action;
   linked_sigaction64(signal, nullptr, &current_action);
 #else
   struct sigaction current_action;
   linked_sigaction(signal, nullptr, &current_action);
 #endif

   // If the sigactions don't match then we put the current action on the chain and make ourself as
   // the main action.
   if (current_action.sa_sigaction != SignalChain::Handler) {
     log("Warning: Unexpected sigaction action found %p\n", current_action.sa_sigaction);
     chains[signal].Register(signal);
   }
 }

 extern "C" void SkipAddSignalHandler(bool value) {
   is_signal_hook_debuggable = value;
 }

 }   // namespace art
	/*
	* Copyright (C) 2014 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include <dlfcn.h>
	#include <errno.h>
	#include <pthread.h>
	#include <signal.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <string.h>

	#if defined(__BIONIC__)
	#include <bionic/macros.h>
	#endif

	#include <algorithm>
	#include <initializer_list>
	#include <mutex>
	#include <type_traits>
	#include <utility>

	#include "log.h"
	#include "sigchain.h"

	#if defined(__APPLE__)
	#define _NSIG NSIG
	#define sighandler_t sig_t

	// Darwin has an #error when ucontext.h is included without _XOPEN_SOURCE defined.
	#define _XOPEN_SOURCE
	#endif

	#define SA_UNSUPPORTED 0x00000400
	#define SA_EXPOSE_TAGBITS 0x00000800

	#include <ucontext.h>

	// libsigchain provides an interception layer for signal handlers, to allow ART and others to give
	// their signal handlers the first stab at handling signals before passing them on to user code.
	//
	// It implements wrapper functions for signal, sigaction, and sigprocmask, and a handler that
	// forwards signals appropriately.
	//
	// In our handler, we start off with all signals blocked, fetch the original signal mask from the
	// passed in ucontext, and then adjust our signal mask appropriately for the user handler.
	//
	// It's somewhat tricky for us to properly handle some flag cases:
	// SA_NOCLDSTOP and SA_NOCLDWAIT: shouldn't matter, we don't have special handlers for SIGCHLD.
	// SA_NODEFER: unimplemented, we can manually change the signal mask appropriately.
	// ~SA_ONSTACK: always silently enable this
	// SA_RESETHAND: unimplemented, but we can probably do this?
	// ~SA_RESTART: unimplemented, maybe we can reserve an RT signal, register an empty handler that
	// doesn't have SA_RESTART, and raise the signal to avoid restarting syscalls that are
	// expected to be interrupted?

	#if defined(__BIONIC__) && !defined(__LP64__)
	static int sigismember(const sigset64_t* sigset, int signum) {
	return sigismember64(sigset, signum);
	}

	static int sigemptyset(sigset64_t* sigset) {
	return sigemptyset64(sigset);
	}

	static int sigaddset(sigset64_t* sigset, int signum) {
	return sigaddset64(sigset, signum);
	}

	static int sigdelset(sigset64_t* sigset, int signum) {
	return sigdelset64(sigset, signum);
	}
	#endif

	template<typename SigsetType>
	static int sigorset(SigsetType* dest, SigsetType* left, SigsetType* right) {
	sigemptyset(dest);
	for (size_t i = 0; i < sizeof(SigsetType) * CHAR_BIT; ++i) {
	if (sigismember(left, i) == 1 \|\| sigismember(right, i) == 1) {
	sigaddset(dest, i);
	}
	}
	return 0;
	}

	namespace art {

	static decltype(&sigaction) linked_sigaction;
	static decltype(&sigprocmask) linked_sigprocmask;

	#if defined(__BIONIC__)
	static decltype(&sigaction64) linked_sigaction64;
	static decltype(&sigprocmask64) linked_sigprocmask64;
	#endif

	template <typename T>
	static void lookup_libc_symbol(T* output, T wrapper, const char* name) {
	#if defined(__BIONIC__)
	constexpr const char* libc_name = "libc.so";
	#elif defined(__GLIBC__)
	#if __GNU_LIBRARY__ != 6
	#error unsupported glibc version
	#endif
	constexpr const char* libc_name = "libc.so.6";
	#else
	#error unsupported libc: not bionic or glibc?
	#endif

	static void* libc = []() {
	void* result = dlopen(libc_name, RTLD_LOCAL \| RTLD_LAZY);
	if (!result) {
	fatal("failed to dlopen %s: %s", libc_name, dlerror());
	}
	return result;
	}();

	void* sym = dlsym(libc, name); // NOLINT glibc triggers cert-dcl16-c with RTLD_NEXT.
	if (sym == nullptr) {
	sym = dlsym(RTLD_DEFAULT, name);
	if (sym == wrapper \|\| sym == sigaction) {
	fatal("Unable to find next %s in signal chain", name);
	}
	}
	*output = reinterpret_cast<T>(sym);
	}

	__attribute__((constructor)) static void InitializeSignalChain() {
	static std::once_flag once;
	std::call_once(once, []() {
	lookup_libc_symbol(&linked_sigaction, sigaction, "sigaction");
	lookup_libc_symbol(&linked_sigprocmask, sigprocmask, "sigprocmask");

	#if defined(__BIONIC__)
	lookup_libc_symbol(&linked_sigaction64, sigaction64, "sigaction64");
	lookup_libc_symbol(&linked_sigprocmask64, sigprocmask64, "sigprocmask64");
	#endif
	});
	}

	static pthread_key_t GetHandlingSignalKey() {
	static pthread_key_t key;
	static std::once_flag once;
	std::call_once(once, []() {
	int rc = pthread_key_create(&key, nullptr);
	if (rc != 0) {
	fatal("failed to create sigchain pthread key: %s", strerror(rc));
	}
	});
	return key;
	}

	static bool GetHandlingSignal() {
	void* result = pthread_getspecific(GetHandlingSignalKey());
	return reinterpret_cast<uintptr_t>(result);
	}

	static void SetHandlingSignal(bool value) {
	pthread_setspecific(GetHandlingSignalKey(),
	reinterpret_cast<void*>(static_cast<uintptr_t>(value)));
	}

	class ScopedHandlingSignal {
	public:
	ScopedHandlingSignal() : original_value_(GetHandlingSignal()) {
	}

	~ScopedHandlingSignal() {
	SetHandlingSignal(original_value_);
	}

	private:
	bool original_value_;
	};

	class SignalChain {
	public:
	SignalChain() : claimed_(false) {
	}

	bool IsClaimed() {
	return claimed_;
	}

	void Claim(int signo) {
	if (!claimed_) {
	Register(signo);
	claimed_ = true;
	}
	}

	// Register the signal chain with the kernel if needed.
	void Register(int signo) {
	#if defined(__BIONIC__)
	struct sigaction64 handler_action = {};
	sigfillset64(&handler_action.sa_mask);
	#else
	struct sigaction handler_action = {};
	sigfillset(&handler_action.sa_mask);
	#endif

	handler_action.sa_sigaction = SignalChain::Handler;
	handler_action.sa_flags = SA_RESTART \| SA_SIGINFO \| SA_ONSTACK \|
	SA_UNSUPPORTED \| SA_EXPOSE_TAGBITS;

	#if defined(__BIONIC__)
	linked_sigaction64(signo, &handler_action, &action_);
	linked_sigaction64(signo, nullptr, &handler_action);
	#else
	linked_sigaction(signo, &handler_action, &action_);
	linked_sigaction(signo, nullptr, &handler_action);
	#endif

	// Newer kernels clear unknown flags from sigaction.sa_flags in order to
	// allow userspace to determine which flag bits are supported. We use this
	// behavior in turn to implement the same flag bit support detection
	// protocol regardless of kernel version. Due to the lack of a flag bit
	// support detection protocol in older kernels we assume support for a base
	// set of flags that have been supported since at least 2003 [1]. No flags
	// were introduced since then until the introduction of SA_EXPOSE_TAGBITS
	// handled below. glibc headers do not define SA_RESTORER so we define it
	// ourselves.
	//
	// TODO(pcc): The new kernel behavior has been implemented in a kernel
	// patch [2] that has not yet landed. Update the code if necessary once it
	// lands.
	//
	// [1] https://github.com/mpe/linux-fullhistory/commit/c0f806c86fc8b07ad426df023f1a4bb0e53c64f6
	// [2] https://lore.kernel.org/linux-arm-kernel/cover.1605235762.git.pcc@google.com/
	#if !defined(__BIONIC__)
	#define SA_RESTORER 0x04000000
	#endif
	kernel_supported_flags_ = SA_NOCLDSTOP \| SA_NOCLDWAIT \| SA_SIGINFO \|
	SA_ONSTACK \| SA_RESTART \| SA_NODEFER \|
	SA_RESETHAND \| SA_RESTORER;

	// Determine whether the kernel supports SA_EXPOSE_TAGBITS. For newer
	// kernels we use the flag support detection protocol described above. In
	// order to allow userspace to distinguish old and new kernels,
	// SA_UNSUPPORTED has been reserved as an unsupported flag. If the kernel
	// did not clear it then we know that we have an old kernel that would not
	// support SA_EXPOSE_TAGBITS anyway.
	if (!(handler_action.sa_flags & SA_UNSUPPORTED) &&
	(handler_action.sa_flags & SA_EXPOSE_TAGBITS)) {
	kernel_supported_flags_ \|= SA_EXPOSE_TAGBITS;
	}
	}

	template <typename SigactionType>
	SigactionType GetAction() {
	if constexpr (std::is_same_v<decltype(action_), SigactionType>) {
	return action_;
	} else {
	SigactionType result;
	result.sa_flags = action_.sa_flags;
	result.sa_handler = action_.sa_handler;
	#if defined(SA_RESTORER)
	result.sa_restorer = action_.sa_restorer;
	#endif
	memcpy(&result.sa_mask, &action_.sa_mask,
	std::min(sizeof(action_.sa_mask), sizeof(result.sa_mask)));
	return result;
	}
	}

	template <typename SigactionType>
	void SetAction(const SigactionType* new_action) {
	if constexpr (std::is_same_v<decltype(action_), SigactionType>) {
	action_ = *new_action;
	} else {
	action_.sa_flags = new_action->sa_flags;
	action_.sa_handler = new_action->sa_handler;
	#if defined(SA_RESTORER)
	action_.sa_restorer = new_action->sa_restorer;
	#endif
	sigemptyset(&action_.sa_mask);
	memcpy(&action_.sa_mask, &new_action->sa_mask,
	std::min(sizeof(action_.sa_mask), sizeof(new_action->sa_mask)));
	}
	action_.sa_flags &= kernel_supported_flags_;
	}

	void AddSpecialHandler(SigchainAction* sa) {
	for (SigchainAction& slot : special_handlers_) {
	if (slot.sc_sigaction == nullptr) {
	slot = *sa;
	return;
	}
	}

	fatal("too many special signal handlers");
	}

	void RemoveSpecialHandler(bool (fn)(int, siginfo_t, void*)) {
	// This isn't thread safe, but it's unlikely to be a real problem.
	size_t len = sizeof(special_handlers_)/sizeof(*special_handlers_);
	for (size_t i = 0; i < len; ++i) {
	if (special_handlers_[i].sc_sigaction == fn) {
	for (size_t j = i; j < len - 1; ++j) {
	special_handlers_[j] = special_handlers_[j + 1];
	}
	special_handlers_[len - 1].sc_sigaction = nullptr;
	return;
	}
	}

	fatal("failed to find special handler to remove");
	}


	static void Handler(int signo, siginfo_t* siginfo, void*);

	private:
	bool claimed_;
	int kernel_supported_flags_;
	#if defined(__BIONIC__)
	struct sigaction64 action_;
	#else
	struct sigaction action_;
	#endif
	SigchainAction special_handlers_[2];
	};

	// _NSIG is 1 greater than the highest valued signal, but signals start from 1.
	// Leave an empty element at index 0 for convenience.
	static SignalChain chains[_NSIG + 1];

	static bool is_signal_hook_debuggable = false;

	void SignalChain::Handler(int signo, siginfo_t* siginfo, void* ucontext_raw) {
	// Try the special handlers first.
	// If one of them crashes, we'll reenter this handler and pass that crash onto the user handler.
	if (!GetHandlingSignal()) {
	for (const auto& handler : chains[signo].special_handlers_) {
	if (handler.sc_sigaction == nullptr) {
	break;
	}

	// The native bridge signal handler might not return.
	// Avoid setting the thread local flag in this case, since we'll never
	// get a chance to restore it.
	bool handler_noreturn = (handler.sc_flags & SIGCHAIN_ALLOW_NORETURN);
	sigset_t previous_mask;
	linked_sigprocmask(SIG_SETMASK, &handler.sc_mask, &previous_mask);

	ScopedHandlingSignal restorer;
	if (!handler_noreturn) {
	SetHandlingSignal(true);
	}

	if (handler.sc_sigaction(signo, siginfo, ucontext_raw)) {
	return;
	}

	linked_sigprocmask(SIG_SETMASK, &previous_mask, nullptr);
	}
	}

	// Forward to the user's signal handler.
	int handler_flags = chains[signo].action_.sa_flags;
	ucontext_t* ucontext = static_cast<ucontext_t*>(ucontext_raw);
	#if defined(__BIONIC__)
	sigset64_t mask;
	sigorset(&mask, &ucontext->uc_sigmask64, &chains[signo].action_.sa_mask);
	#else
	sigset_t mask;
	sigorset(&mask, &ucontext->uc_sigmask, &chains[signo].action_.sa_mask);
	#endif
	if (!(handler_flags & SA_NODEFER)) {
	sigaddset(&mask, signo);
	}

	#if defined(__BIONIC__)
	linked_sigprocmask64(SIG_SETMASK, &mask, nullptr);
	#else
	linked_sigprocmask(SIG_SETMASK, &mask, nullptr);
	#endif

	if ((handler_flags & SA_SIGINFO)) {
	// If the chained handler is not expecting tag bits in the fault address,
	// mask them out now.
	#if defined(__BIONIC__)
	if (!(handler_flags & SA_EXPOSE_TAGBITS) &&
	(signo == SIGILL \|\| signo == SIGFPE \|\| signo == SIGSEGV \|\|
	signo == SIGBUS \|\| signo == SIGTRAP) &&
	siginfo->si_code > SI_USER && siginfo->si_code < SI_KERNEL &&
	!(signo == SIGTRAP && siginfo->si_code == TRAP_HWBKPT)) {
	siginfo->si_addr = untag_address(siginfo->si_addr);
	}
	#endif
	chains[signo].action_.sa_sigaction(signo, siginfo, ucontext_raw);
	} else {
	auto handler = chains[signo].action_.sa_handler;
	if (handler == SIG_IGN) {
	return;
	} else if (handler == SIG_DFL) {
	fatal("exiting due to SIG_DFL handler for signal %d, ucontext %p", signo, ucontext);
	} else {
	handler(signo);
	}
	}
	}

	template <typename SigactionType>
	static int __sigaction(int signal, const SigactionType* new_action,
	SigactionType* old_action,
	int (linked)(int, const SigactionType,
	SigactionType*)) {
	if (is_signal_hook_debuggable) {
	return 0;
	}

	// If this signal has been claimed as a signal chain, record the user's
	// action but don't pass it on to the kernel.
	// Note that we check that the signal number is in range here. An out of range signal
	// number should behave exactly as the libc sigaction.
	if (signal <= 0 \|\| signal >= _NSIG) {
	errno = EINVAL;
	return -1;
	}

	if (chains[signal].IsClaimed()) {
	SigactionType saved_action = chains[signal].GetAction<SigactionType>();
	if (new_action != nullptr) {
	chains[signal].SetAction(new_action);
	}
	if (old_action != nullptr) {
	*old_action = saved_action;
	}
	return 0;
	}

	// Will only get here if the signal chain has not been claimed. We want
	// to pass the sigaction on to the kernel via the real sigaction in libc.
	return linked(signal, new_action, old_action);
	}

	extern "C" int sigaction(int signal, const struct sigaction* new_action,
	struct sigaction* old_action) {
	InitializeSignalChain();
	return __sigaction(signal, new_action, old_action, linked_sigaction);
	}

	#if defined(__BIONIC__)
	extern "C" int sigaction64(int signal, const struct sigaction64* new_action,
	struct sigaction64* old_action) {
	InitializeSignalChain();
	return __sigaction(signal, new_action, old_action, linked_sigaction64);
	}
	#endif

	extern "C" sighandler_t signal(int signo, sighandler_t handler) {
	InitializeSignalChain();

	if (signo <= 0 \|\| signo >= _NSIG) {
	errno = EINVAL;
	return SIG_ERR;
	}

	struct sigaction sa = {};
	sigemptyset(&sa.sa_mask);
	sa.sa_handler = handler;
	sa.sa_flags = SA_RESTART \| SA_ONSTACK;
	sighandler_t oldhandler;

	// If this signal has been claimed as a signal chain, record the user's
	// action but don't pass it on to the kernel.
	if (chains[signo].IsClaimed()) {
	oldhandler = reinterpret_cast<sighandler_t>(
	chains[signo].GetAction<struct sigaction>().sa_handler);
	chains[signo].SetAction(&sa);
	return oldhandler;
	}

	// Will only get here if the signal chain has not been claimed. We want
	// to pass the sigaction on to the kernel via the real sigaction in libc.
	if (linked_sigaction(signo, &sa, &sa) == -1) {
	return SIG_ERR;
	}

	return reinterpret_cast<sighandler_t>(sa.sa_handler);
	}

	#if !defined(__LP64__)
	extern "C" sighandler_t bsd_signal(int signo, sighandler_t handler) {
	InitializeSignalChain();

	return signal(signo, handler);
	}
	#endif

	template <typename SigsetType>
	int __sigprocmask(int how, const SigsetType* new_set, SigsetType* old_set,
	int (linked)(int, const SigsetType, SigsetType*)) {
	// When inside a signal handler, forward directly to the actual sigprocmask.
	if (GetHandlingSignal()) {
	return linked(how, new_set, old_set);
	}

	const SigsetType* new_set_ptr = new_set;
	SigsetType tmpset;
	if (new_set != nullptr) {
	tmpset = *new_set;

	if (how == SIG_BLOCK \|\| how == SIG_SETMASK) {
	// Don't allow claimed signals in the mask. If a signal chain has been claimed
	// we can't allow the user to block that signal.
	for (int i = 1; i < _NSIG; ++i) {
	if (chains[i].IsClaimed() && sigismember(&tmpset, i)) {
	sigdelset(&tmpset, i);
	}
	}
	}
	new_set_ptr = &tmpset;
	}

	return linked(how, new_set_ptr, old_set);
	}

	extern "C" int sigprocmask(int how, const sigset_t* new_set,
	sigset_t* old_set) {
	InitializeSignalChain();
	return __sigprocmask(how, new_set, old_set, linked_sigprocmask);
	}

	#if defined(__BIONIC__)
	extern "C" int sigprocmask64(int how, const sigset64_t* new_set,
	sigset64_t* old_set) {
	InitializeSignalChain();
	return __sigprocmask(how, new_set, old_set, linked_sigprocmask64);
	}
	#endif

	extern "C" void AddSpecialSignalHandlerFn(int signal, SigchainAction* sa) {
	InitializeSignalChain();

	if (signal <= 0 \|\| signal >= _NSIG) {
	fatal("Invalid signal %d", signal);
	}

	// Set the managed_handler.
	chains[signal].AddSpecialHandler(sa);
	chains[signal].Claim(signal);
	}

	extern "C" void RemoveSpecialSignalHandlerFn(int signal, bool (fn)(int, siginfo_t, void*)) {
	InitializeSignalChain();

	if (signal <= 0 \|\| signal >= _NSIG) {
	fatal("Invalid signal %d", signal);
	}

	chains[signal].RemoveSpecialHandler(fn);
	}

	extern "C" void EnsureFrontOfChain(int signal) {
	InitializeSignalChain();

	if (signal <= 0 \|\| signal >= _NSIG) {
	fatal("Invalid signal %d", signal);
	}

	// Read the current action without looking at the chain, it should be the expected action.
	#if defined(__BIONIC__)
	struct sigaction64 current_action;
	linked_sigaction64(signal, nullptr, &current_action);
	#else
	struct sigaction current_action;
	linked_sigaction(signal, nullptr, &current_action);
	#endif

	// If the sigactions don't match then we put the current action on the chain and make ourself as
	// the main action.
	if (current_action.sa_sigaction != SignalChain::Handler) {
	log("Warning: Unexpected sigaction action found %p\n", current_action.sa_sigaction);
	chains[signal].Register(signal);
	}
	}

	extern "C" void SkipAddSignalHandler(bool value) {
	is_signal_hook_debuggable = value;
	}

	} // namespace art