1//===- Signals.cpp - Generic Unix Signals Implementation -----*- C++ -*-===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file defines some helpful functions for dealing with the possibility of
10// Unix signals occurring while your program is running.
11//
12//===----------------------------------------------------------------------===//
13//
14// This file is extremely careful to only do signal-safe things while in a
15// signal handler. In particular, memory allocation and acquiring a mutex
16// while in a signal handler should never occur. ManagedStatic isn't usable from
17// a signal handler for 2 reasons:
18//
19//  1. Creating a new one allocates.
20//  2. The signal handler could fire while llvm_shutdown is being processed, in
21//     which case the ManagedStatic is in an unknown state because it could
22//     already have been destroyed, or be in the process of being destroyed.
23//
24// Modifying the behavior of the signal handlers (such as registering new ones)
25// can acquire a mutex, but all this guarantees is that the signal handler
26// behavior is only modified by one thread at a time. A signal handler can still
27// fire while this occurs!
28//
29// Adding work to a signal handler requires lock-freedom (and assume atomics are
30// always lock-free) because the signal handler could fire while new work is
31// being added.
32//
33//===----------------------------------------------------------------------===//
34
35#include "Unix.h"
36#include "llvm/ADT/STLExtras.h"
37#include "llvm/Config/config.h"
38#include "llvm/Demangle/Demangle.h"
39#include "llvm/Support/FileSystem.h"
40#include "llvm/Support/FileUtilities.h"
41#include "llvm/Support/Format.h"
42#include "llvm/Support/MemoryBuffer.h"
43#include "llvm/Support/Mutex.h"
44#include "llvm/Support/Program.h"
45#include "llvm/Support/UniqueLock.h"
46#include "llvm/Support/raw_ostream.h"
47#include <algorithm>
48#include <string>
49#include <sysexits.h>
50#ifdef HAVE_BACKTRACE
51# include BACKTRACE_HEADER         // For backtrace().
52#endif
53#if HAVE_SIGNAL_H
54#include <signal.h>
55#endif
56#if HAVE_SYS_STAT_H
57#include <sys/stat.h>
58#endif
59#if HAVE_DLFCN_H
60#include <dlfcn.h>
61#endif
62#if HAVE_MACH_MACH_H
63#include <mach/mach.h>
64#endif
65#if HAVE_LINK_H
66#include <link.h>
67#endif
68#ifdef HAVE__UNWIND_BACKTRACE
69// FIXME: We should be able to use <unwind.h> for any target that has an
70// _Unwind_Backtrace function, but on FreeBSD the configure test passes
71// despite the function not existing, and on Android, <unwind.h> conflicts
72// with <link.h>.
73#ifdef __GLIBC__
74#include <unwind.h>
75#else
76#undef HAVE__UNWIND_BACKTRACE
77#endif
78#endif
79
80using namespace llvm;
81
82static RETSIGTYPE SignalHandler(int Sig);  // defined below.
83
84/// The function to call if ctrl-c is pressed.
85using InterruptFunctionType = void (*)();
86static std::atomic<InterruptFunctionType> InterruptFunction =
87    ATOMIC_VAR_INIT(nullptr);
88
89namespace {
90/// Signal-safe removal of files.
91/// Inserting and erasing from the list isn't signal-safe, but removal of files
92/// themselves is signal-safe. Memory is freed when the head is freed, deletion
93/// is therefore not signal-safe either.
94class FileToRemoveList {
95  std::atomic<char *> Filename = ATOMIC_VAR_INIT(nullptr);
96  std::atomic<FileToRemoveList *> Next = ATOMIC_VAR_INIT(nullptr);
97
98  FileToRemoveList() = default;
99  // Not signal-safe.
100  FileToRemoveList(const std::string &str) : Filename(strdup(str.c_str())) {}
101
102public:
103  // Not signal-safe.
104  ~FileToRemoveList() {
105    if (FileToRemoveList *N = Next.exchange(nullptr))
106      delete N;
107    if (char *F = Filename.exchange(nullptr))
108      free(F);
109  }
110
111  // Not signal-safe.
112  static void insert(std::atomic<FileToRemoveList *> &Head,
113                     const std::string &Filename) {
114    // Insert the new file at the end of the list.
115    FileToRemoveList *NewHead = new FileToRemoveList(Filename);
116    std::atomic<FileToRemoveList *> *InsertionPoint = &Head;
117    FileToRemoveList *OldHead = nullptr;
118    while (!InsertionPoint->compare_exchange_strong(OldHead, NewHead)) {
119      InsertionPoint = &OldHead->Next;
120      OldHead = nullptr;
121    }
122  }
123
124  // Not signal-safe.
125  static void erase(std::atomic<FileToRemoveList *> &Head,
126                    const std::string &Filename) {
127    // Use a lock to avoid concurrent erase: the comparison would access
128    // free'd memory.
129    static ManagedStatic<sys::SmartMutex<true>> Lock;
130    sys::SmartScopedLock<true> Writer(*Lock);
131
132    for (FileToRemoveList *Current = Head.load(); Current;
133         Current = Current->Next.load()) {
134      if (char *OldFilename = Current->Filename.load()) {
135        if (OldFilename != Filename)
136          continue;
137        // Leave an empty filename.
138        OldFilename = Current->Filename.exchange(nullptr);
139        // The filename might have become null between the time we
140        // compared it and we exchanged it.
141        if (OldFilename)
142          free(OldFilename);
143      }
144    }
145  }
146
147  // Signal-safe.
148  static void removeAllFiles(std::atomic<FileToRemoveList *> &Head) {
149    // If cleanup were to occur while we're removing files we'd have a bad time.
150    // Make sure we're OK by preventing cleanup from doing anything while we're
151    // removing files. If cleanup races with us and we win we'll have a leak,
152    // but we won't crash.
153    FileToRemoveList *OldHead = Head.exchange(nullptr);
154
155    for (FileToRemoveList *currentFile = OldHead; currentFile;
156         currentFile = currentFile->Next.load()) {
157      // If erasing was occuring while we're trying to remove files we'd look
158      // at free'd data. Take away the path and put it back when done.
159      if (char *path = currentFile->Filename.exchange(nullptr)) {
160        // Get the status so we can determine if it's a file or directory. If we
161        // can't stat the file, ignore it.
162        struct stat buf;
163        if (stat(path, &buf) != 0)
164          continue;
165
166        // If this is not a regular file, ignore it. We want to prevent removal
167        // of special files like /dev/null, even if the compiler is being run
168        // with the super-user permissions.
169        if (!S_ISREG(buf.st_mode))
170          continue;
171
172        // Otherwise, remove the file. We ignore any errors here as there is
173        // nothing else we can do.
174        unlink(path);
175
176        // We're done removing the file, erasing can safely proceed.
177        currentFile->Filename.exchange(path);
178      }
179    }
180
181    // We're done removing files, cleanup can safely proceed.
182    Head.exchange(OldHead);
183  }
184};
185static std::atomic<FileToRemoveList *> FilesToRemove = ATOMIC_VAR_INIT(nullptr);
186
187/// Clean up the list in a signal-friendly manner.
188/// Recall that signals can fire during llvm_shutdown. If this occurs we should
189/// either clean something up or nothing at all, but we shouldn't crash!
190struct FilesToRemoveCleanup {
191  // Not signal-safe.
192  ~FilesToRemoveCleanup() {
193    FileToRemoveList *Head = FilesToRemove.exchange(nullptr);
194    if (Head)
195      delete Head;
196  }
197};
198} // namespace
199
200static StringRef Argv0;
201
202// Signals that represent requested termination. There's no bug or failure, or
203// if there is, it's not our direct responsibility. For whatever reason, our
204// continued execution is no longer desirable.
205static const int IntSigs[] = {
206  SIGHUP, SIGINT, SIGPIPE, SIGTERM, SIGUSR1, SIGUSR2
207};
208
209// Signals that represent that we have a bug, and our prompt termination has
210// been ordered.
211static const int KillSigs[] = {
212  SIGILL, SIGTRAP, SIGABRT, SIGFPE, SIGBUS, SIGSEGV, SIGQUIT
213#ifdef SIGSYS
214  , SIGSYS
215#endif
216#ifdef SIGXCPU
217  , SIGXCPU
218#endif
219#ifdef SIGXFSZ
220  , SIGXFSZ
221#endif
222#ifdef SIGEMT
223  , SIGEMT
224#endif
225};
226
227static std::atomic<unsigned> NumRegisteredSignals = ATOMIC_VAR_INIT(0);
228static struct {
229  struct sigaction SA;
230  int SigNo;
231} RegisteredSignalInfo[array_lengthof(IntSigs) + array_lengthof(KillSigs)];
232
233#if defined(HAVE_SIGALTSTACK)
234// Hold onto both the old and new alternate signal stack so that it's not
235// reported as a leak. We don't make any attempt to remove our alt signal
236// stack if we remove our signal handlers; that can't be done reliably if
237// someone else is also trying to do the same thing.
238static stack_t OldAltStack;
239static void* NewAltStackPointer;
240
241static void CreateSigAltStack() {
242  const size_t AltStackSize = MINSIGSTKSZ + 64 * 1024;
243
244  // If we're executing on the alternate stack, or we already have an alternate
245  // signal stack that we're happy with, there's nothing for us to do. Don't
246  // reduce the size, some other part of the process might need a larger stack
247  // than we do.
248  if (sigaltstack(nullptr, &OldAltStack) != 0 ||
249      OldAltStack.ss_flags & SS_ONSTACK ||
250      (OldAltStack.ss_sp && OldAltStack.ss_size >= AltStackSize))
251    return;
252
253  stack_t AltStack = {};
254  AltStack.ss_sp = static_cast<char *>(safe_malloc(AltStackSize));
255  NewAltStackPointer = AltStack.ss_sp; // Save to avoid reporting a leak.
256  AltStack.ss_size = AltStackSize;
257  if (sigaltstack(&AltStack, &OldAltStack) != 0)
258    free(AltStack.ss_sp);
259}
260#else
261static void CreateSigAltStack() {}
262#endif
263
264static void RegisterHandlers() { // Not signal-safe.
265  // The mutex prevents other threads from registering handlers while we're
266  // doing it. We also have to protect the handlers and their count because
267  // a signal handler could fire while we're registeting handlers.
268  static ManagedStatic<sys::SmartMutex<true>> SignalHandlerRegistrationMutex;
269  sys::SmartScopedLock<true> Guard(*SignalHandlerRegistrationMutex);
270
271  // If the handlers are already registered, we're done.
272  if (NumRegisteredSignals.load() != 0)
273    return;
274
275  // Create an alternate stack for signal handling. This is necessary for us to
276  // be able to reliably handle signals due to stack overflow.
277  CreateSigAltStack();
278
279  auto registerHandler = [&](int Signal) {
280    unsigned Index = NumRegisteredSignals.load();
281    assert(Index < array_lengthof(RegisteredSignalInfo) &&
282           "Out of space for signal handlers!");
283
284    struct sigaction NewHandler;
285
286    NewHandler.sa_handler = SignalHandler;
287    NewHandler.sa_flags = SA_NODEFER | SA_RESETHAND | SA_ONSTACK;
288    sigemptyset(&NewHandler.sa_mask);
289
290    // Install the new handler, save the old one in RegisteredSignalInfo.
291    sigaction(Signal, &NewHandler, &RegisteredSignalInfo[Index].SA);
292    RegisteredSignalInfo[Index].SigNo = Signal;
293    ++NumRegisteredSignals;
294  };
295
296  for (auto S : IntSigs)
297    registerHandler(S);
298  for (auto S : KillSigs)
299    registerHandler(S);
300}
301
302static void UnregisterHandlers() {
303  // Restore all of the signal handlers to how they were before we showed up.
304  for (unsigned i = 0, e = NumRegisteredSignals.load(); i != e; ++i) {
305    sigaction(RegisteredSignalInfo[i].SigNo,
306              &RegisteredSignalInfo[i].SA, nullptr);
307    --NumRegisteredSignals;
308  }
309}
310
311/// Process the FilesToRemove list.
312static void RemoveFilesToRemove() {
313  FileToRemoveList::removeAllFiles(FilesToRemove);
314}
315
316// The signal handler that runs.
317static RETSIGTYPE SignalHandler(int Sig) {
318  // Restore the signal behavior to default, so that the program actually
319  // crashes when we return and the signal reissues.  This also ensures that if
320  // we crash in our signal handler that the program will terminate immediately
321  // instead of recursing in the signal handler.
322  UnregisterHandlers();
323
324  // Unmask all potentially blocked kill signals.
325  sigset_t SigMask;
326  sigfillset(&SigMask);
327  sigprocmask(SIG_UNBLOCK, &SigMask, nullptr);
328
329  {
330    RemoveFilesToRemove();
331
332    if (std::find(std::begin(IntSigs), std::end(IntSigs), Sig)
333        != std::end(IntSigs)) {
334      if (auto OldInterruptFunction = InterruptFunction.exchange(nullptr))
335        return OldInterruptFunction();
336
337      // Send a special return code that drivers can check for, from sysexits.h.
338      if (Sig == SIGPIPE)
339        exit(EX_IOERR);
340
341      raise(Sig);   // Execute the default handler.
342      return;
343   }
344  }
345
346  // Otherwise if it is a fault (like SEGV) run any handler.
347  llvm::sys::RunSignalHandlers();
348
349#ifdef __s390__
350  // On S/390, certain signals are delivered with PSW Address pointing to
351  // *after* the faulting instruction.  Simply returning from the signal
352  // handler would continue execution after that point, instead of
353  // re-raising the signal.  Raise the signal manually in those cases.
354  if (Sig == SIGILL || Sig == SIGFPE || Sig == SIGTRAP)
355    raise(Sig);
356#endif
357}
358
359void llvm::sys::RunInterruptHandlers() {
360  RemoveFilesToRemove();
361}
362
363void llvm::sys::SetInterruptFunction(void (*IF)()) {
364  InterruptFunction.exchange(IF);
365  RegisterHandlers();
366}
367
368// The public API
369bool llvm::sys::RemoveFileOnSignal(StringRef Filename,
370                                   std::string* ErrMsg) {
371  // Ensure that cleanup will occur as soon as one file is added.
372  static ManagedStatic<FilesToRemoveCleanup> FilesToRemoveCleanup;
373  *FilesToRemoveCleanup;
374  FileToRemoveList::insert(FilesToRemove, Filename.str());
375  RegisterHandlers();
376  return false;
377}
378
379// The public API
380void llvm::sys::DontRemoveFileOnSignal(StringRef Filename) {
381  FileToRemoveList::erase(FilesToRemove, Filename.str());
382}
383
384/// Add a function to be called when a signal is delivered to the process. The
385/// handler can have a cookie passed to it to identify what instance of the
386/// handler it is.
387void llvm::sys::AddSignalHandler(sys::SignalHandlerCallback FnPtr,
388                                 void *Cookie) { // Signal-safe.
389  insertSignalHandler(FnPtr, Cookie);
390  RegisterHandlers();
391}
392
393#if defined(HAVE_BACKTRACE) && ENABLE_BACKTRACES && HAVE_LINK_H &&    \
394    (defined(__linux__) || defined(__FreeBSD__) ||                             \
395     defined(__FreeBSD_kernel__) || defined(__NetBSD__))
396struct DlIteratePhdrData {
397  void **StackTrace;
398  int depth;
399  bool first;
400  const char **modules;
401  intptr_t *offsets;
402  const char *main_exec_name;
403};
404
405static int dl_iterate_phdr_cb(dl_phdr_info *info, size_t size, void *arg) {
406  DlIteratePhdrData *data = (DlIteratePhdrData*)arg;
407  const char *name = data->first ? data->main_exec_name : info->dlpi_name;
408  data->first = false;
409  for (int i = 0; i < info->dlpi_phnum; i++) {
410    const auto *phdr = &info->dlpi_phdr[i];
411    if (phdr->p_type != PT_LOAD)
412      continue;
413    intptr_t beg = info->dlpi_addr + phdr->p_vaddr;
414    intptr_t end = beg + phdr->p_memsz;
415    for (int j = 0; j < data->depth; j++) {
416      if (data->modules[j])
417        continue;
418      intptr_t addr = (intptr_t)data->StackTrace[j];
419      if (beg <= addr && addr < end) {
420        data->modules[j] = name;
421        data->offsets[j] = addr - info->dlpi_addr;
422      }
423    }
424  }
425  return 0;
426}
427
428/// If this is an ELF platform, we can find all loaded modules and their virtual
429/// addresses with dl_iterate_phdr.
430static bool findModulesAndOffsets(void **StackTrace, int Depth,
431                                  const char **Modules, intptr_t *Offsets,
432                                  const char *MainExecutableName,
433                                  StringSaver &StrPool) {
434  DlIteratePhdrData data = {StackTrace, Depth,   true,
435                            Modules,    Offsets, MainExecutableName};
436  dl_iterate_phdr(dl_iterate_phdr_cb, &data);
437  return true;
438}
439#else
440/// This platform does not have dl_iterate_phdr, so we do not yet know how to
441/// find all loaded DSOs.
442static bool findModulesAndOffsets(void **StackTrace, int Depth,
443                                  const char **Modules, intptr_t *Offsets,
444                                  const char *MainExecutableName,
445                                  StringSaver &StrPool) {
446  return false;
447}
448#endif // defined(HAVE_BACKTRACE) && ENABLE_BACKTRACES && ...
449
450#if ENABLE_BACKTRACES && defined(HAVE__UNWIND_BACKTRACE)
451static int unwindBacktrace(void **StackTrace, int MaxEntries) {
452  if (MaxEntries < 0)
453    return 0;
454
455  // Skip the first frame ('unwindBacktrace' itself).
456  int Entries = -1;
457
458  auto HandleFrame = [&](_Unwind_Context *Context) -> _Unwind_Reason_Code {
459    // Apparently we need to detect reaching the end of the stack ourselves.
460    void *IP = (void *)_Unwind_GetIP(Context);
461    if (!IP)
462      return _URC_END_OF_STACK;
463
464    assert(Entries < MaxEntries && "recursively called after END_OF_STACK?");
465    if (Entries >= 0)
466      StackTrace[Entries] = IP;
467
468    if (++Entries == MaxEntries)
469      return _URC_END_OF_STACK;
470    return _URC_NO_REASON;
471  };
472
473  _Unwind_Backtrace(
474      [](_Unwind_Context *Context, void *Handler) {
475        return (*static_cast<decltype(HandleFrame) *>(Handler))(Context);
476      },
477      static_cast<void *>(&HandleFrame));
478  return std::max(Entries, 0);
479}
480#endif
481
482// In the case of a program crash or fault, print out a stack trace so that the
483// user has an indication of why and where we died.
484//
485// On glibc systems we have the 'backtrace' function, which works nicely, but
486// doesn't demangle symbols.
487void llvm::sys::PrintStackTrace(raw_ostream &OS) {
488#if ENABLE_BACKTRACES
489  static void *StackTrace[256];
490  int depth = 0;
491#if defined(HAVE_BACKTRACE)
492  // Use backtrace() to output a backtrace on Linux systems with glibc.
493  if (!depth)
494    depth = backtrace(StackTrace, static_cast<int>(array_lengthof(StackTrace)));
495#endif
496#if defined(HAVE__UNWIND_BACKTRACE)
497  // Try _Unwind_Backtrace() if backtrace() failed.
498  if (!depth)
499    depth = unwindBacktrace(StackTrace,
500                        static_cast<int>(array_lengthof(StackTrace)));
501#endif
502  if (!depth)
503    return;
504
505  if (printSymbolizedStackTrace(Argv0, StackTrace, depth, OS))
506    return;
507#if HAVE_DLFCN_H && HAVE_DLADDR
508  int width = 0;
509  for (int i = 0; i < depth; ++i) {
510    Dl_info dlinfo;
511    dladdr(StackTrace[i], &dlinfo);
512    const char* name = strrchr(dlinfo.dli_fname, '/');
513
514    int nwidth;
515    if (!name) nwidth = strlen(dlinfo.dli_fname);
516    else       nwidth = strlen(name) - 1;
517
518    if (nwidth > width) width = nwidth;
519  }
520
521  for (int i = 0; i < depth; ++i) {
522    Dl_info dlinfo;
523    dladdr(StackTrace[i], &dlinfo);
524
525    OS << format("%-2d", i);
526
527    const char* name = strrchr(dlinfo.dli_fname, '/');
528    if (!name) OS << format(" %-*s", width, dlinfo.dli_fname);
529    else       OS << format(" %-*s", width, name+1);
530
531    OS << format(" %#0*lx", (int)(sizeof(void*) * 2) + 2,
532                 (unsigned long)StackTrace[i]);
533
534    if (dlinfo.dli_sname != nullptr) {
535      OS << ' ';
536      int res;
537      char* d = itaniumDemangle(dlinfo.dli_sname, nullptr, nullptr, &res);
538      if (!d) OS << dlinfo.dli_sname;
539      else    OS << d;
540      free(d);
541
542      OS << format(" + %tu", (static_cast<const char*>(StackTrace[i])-
543                              static_cast<const char*>(dlinfo.dli_saddr)));
544    }
545    OS << '\n';
546  }
547#elif defined(HAVE_BACKTRACE)
548  backtrace_symbols_fd(StackTrace, depth, STDERR_FILENO);
549#endif
550#endif
551}
552
553static void PrintStackTraceSignalHandler(void *) {
554  sys::PrintStackTrace(llvm::errs());
555}
556
557void llvm::sys::DisableSystemDialogsOnCrash() {}
558
559/// When an error signal (such as SIGABRT or SIGSEGV) is delivered to the
560/// process, print a stack trace and then exit.
561void llvm::sys::PrintStackTraceOnErrorSignal(StringRef Argv0,
562                                             bool DisableCrashReporting) {
563  ::Argv0 = Argv0;
564
565  AddSignalHandler(PrintStackTraceSignalHandler, nullptr);
566
567#if defined(__APPLE__) && ENABLE_CRASH_OVERRIDES
568  // Environment variable to disable any kind of crash dialog.
569  if (DisableCrashReporting || getenv("LLVM_DISABLE_CRASH_REPORT")) {
570    mach_port_t self = mach_task_self();
571
572    exception_mask_t mask = EXC_MASK_CRASH;
573
574    kern_return_t ret = task_set_exception_ports(self,
575                             mask,
576                             MACH_PORT_NULL,
577                             EXCEPTION_STATE_IDENTITY | MACH_EXCEPTION_CODES,
578                             THREAD_STATE_NONE);
579    (void)ret;
580  }
581#endif
582}
583