libsanitizer/sanitizer_common/sanitizer_stoptheworld_linux_libcdep.cpp

   1 //===-- sanitizer_stoptheworld_linux_libcdep.cpp --------------------------===//
   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 // See sanitizer_stoptheworld.h for details.
  10 // This implementation was inspired by Markus Gutschke's linuxthreads.cc.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "sanitizer_platform.h"
  15
  16 #if SANITIZER_LINUX &&                                                   \
  17     (defined(__x86_64__) || defined(__mips__) || defined(__aarch64__) || \
  18      defined(__powerpc64__) || defined(__s390__) || defined(__i386__) || \
  19      defined(__arm__) || SANITIZER_RISCV64)
  20
  21 #include "sanitizer_stoptheworld.h"
  22
  23 #include "sanitizer_platform_limits_posix.h"
  24 #include "sanitizer_atomic.h"
  25
  26 #include <errno.h>
  27 #include <sched.h> // for CLONE_* definitions
  28 #include <stddef.h>
  29 #include <sys/prctl.h> // for PR_* definitions
  30 #include <sys/ptrace.h> // for PTRACE_* definitions
  31 #include <sys/types.h> // for pid_t
  32 #include <sys/uio.h> // for iovec
  33 #include <elf.h> // for NT_PRSTATUS
  34 #if (defined(__aarch64__) || SANITIZER_RISCV64) && !SANITIZER_ANDROID
  35 // GLIBC 2.20+ sys/user does not include asm/ptrace.h
  36 # include <asm/ptrace.h>
  37 #endif
  38 #include <sys/user.h>  // for user_regs_struct
  39 #if SANITIZER_ANDROID && SANITIZER_MIPS
  40 # include <asm/reg.h>  // for mips SP register in sys/user.h
  41 #endif
  42 #include <sys/wait.h> // for signal-related stuff
  43
  44 #ifdef sa_handler
  45 # undef sa_handler
  46 #endif
  47
  48 #ifdef sa_sigaction
  49 # undef sa_sigaction
  50 #endif
  51
  52 #include "sanitizer_common.h"
  53 #include "sanitizer_flags.h"
  54 #include "sanitizer_libc.h"
  55 #include "sanitizer_linux.h"
  56 #include "sanitizer_mutex.h"
  57 #include "sanitizer_placement_new.h"
  58
  59 // Sufficiently old kernel headers don't provide this value, but we can still
  60 // call prctl with it. If the runtime kernel is new enough, the prctl call will
  61 // have the desired effect; if the kernel is too old, the call will error and we
  62 // can ignore said error.
  63 #ifndef PR_SET_PTRACER
  64 #define PR_SET_PTRACER 0x59616d61
  65 #endif
  66
  67 // This module works by spawning a Linux task which then attaches to every
  68 // thread in the caller process with ptrace. This suspends the threads, and
  69 // PTRACE_GETREGS can then be used to obtain their register state. The callback
  70 // supplied to StopTheWorld() is run in the tracer task while the threads are
  71 // suspended.
  72 // The tracer task must be placed in a different thread group for ptrace to
  73 // work, so it cannot be spawned as a pthread. Instead, we use the low-level
  74 // clone() interface (we want to share the address space with the caller
  75 // process, so we prefer clone() over fork()).
  76 //
  77 // We don't use any libc functions, relying instead on direct syscalls. There
  78 // are two reasons for this:
  79 // 1. calling a library function while threads are suspended could cause a
  80 // deadlock, if one of the treads happens to be holding a libc lock;
  81 // 2. it's generally not safe to call libc functions from the tracer task,
  82 // because clone() does not set up a thread-local storage for it. Any
  83 // thread-local variables used by libc will be shared between the tracer task
  84 // and the thread which spawned it.
  85
  86 namespace __sanitizer {
  87
  88 class SuspendedThreadsListLinux final : public SuspendedThreadsList {
  89  public:
  90   SuspendedThreadsListLinux() { thread_ids_.reserve(1024); }
  91
  92   tid_t GetThreadID(uptr index) const override;
  93   uptr ThreadCount() const override;
  94   bool ContainsTid(tid_t thread_id) const;
  95   void Append(tid_t tid);
  96
  97   PtraceRegistersStatus GetRegistersAndSP(uptr index,
  98                                           InternalMmapVector<uptr> *buffer,
  99                                           uptr *sp) const override;
 100
 101  private:
 102   InternalMmapVector<tid_t> thread_ids_;
 103 };
 104
 105 // Structure for passing arguments into the tracer thread.
 106 struct TracerThreadArgument {
 107   StopTheWorldCallback callback;
 108   void *callback_argument;
 109   // The tracer thread waits on this mutex while the parent finishes its
 110   // preparations.
 111   BlockingMutex mutex;
 112   // Tracer thread signals its completion by setting done.
 113   atomic_uintptr_t done;
 114   uptr parent_pid;
 115 };
 116
 117 // This class handles thread suspending/unsuspending in the tracer thread.
 118 class ThreadSuspender {
 119  public:
 120   explicit ThreadSuspender(pid_t pid, TracerThreadArgument *arg)
 121     : arg(arg)
 122     , pid_(pid) {
 123       CHECK_GE(pid, 0);
 124     }
 125   bool SuspendAllThreads();
 126   void ResumeAllThreads();
 127   void KillAllThreads();
 128   SuspendedThreadsListLinux &suspended_threads_list() {
 129     return suspended_threads_list_;
 130   }
 131   TracerThreadArgument *arg;
 132  private:
 133   SuspendedThreadsListLinux suspended_threads_list_;
 134   pid_t pid_;
 135   bool SuspendThread(tid_t thread_id);
 136 };
 137
 138 bool ThreadSuspender::SuspendThread(tid_t tid) {
 139   // Are we already attached to this thread?
 140   // Currently this check takes linear time, however the number of threads is
 141   // usually small.
 142   if (suspended_threads_list_.ContainsTid(tid)) return false;
 143   int pterrno;
 144   if (internal_iserror(internal_ptrace(PTRACE_ATTACH, tid, nullptr, nullptr),
 145                        &pterrno)) {
 146     // Either the thread is dead, or something prevented us from attaching.
 147     // Log this event and move on.
 148     VReport(1, "Could not attach to thread %zu (errno %d).\n", (uptr)tid,
 149             pterrno);
 150     return false;
 151   } else {
 152     VReport(2, "Attached to thread %zu.\n", (uptr)tid);
 153     // The thread is not guaranteed to stop before ptrace returns, so we must
 154     // wait on it. Note: if the thread receives a signal concurrently,
 155     // we can get notification about the signal before notification about stop.
 156     // In such case we need to forward the signal to the thread, otherwise
 157     // the signal will be missed (as we do PTRACE_DETACH with arg=0) and
 158     // any logic relying on signals will break. After forwarding we need to
 159     // continue to wait for stopping, because the thread is not stopped yet.
 160     // We do ignore delivery of SIGSTOP, because we want to make stop-the-world
 161     // as invisible as possible.
 162     for (;;) {
 163       int status;
 164       uptr waitpid_status;
 165       HANDLE_EINTR(waitpid_status, internal_waitpid(tid, &status, __WALL));
 166       int wperrno;
 167       if (internal_iserror(waitpid_status, &wperrno)) {
 168         // Got a ECHILD error. I don't think this situation is possible, but it
 169         // doesn't hurt to report it.
 170         VReport(1, "Waiting on thread %zu failed, detaching (errno %d).\n",
 171                 (uptr)tid, wperrno);
 172         internal_ptrace(PTRACE_DETACH, tid, nullptr, nullptr);
 173         return false;
 174       }
 175       if (WIFSTOPPED(status) && WSTOPSIG(status) != SIGSTOP) {
 176         internal_ptrace(PTRACE_CONT, tid, nullptr,
 177                         (void*)(uptr)WSTOPSIG(status));
 178         continue;
 179       }
 180       break;
 181     }
 182     suspended_threads_list_.Append(tid);
 183     return true;
 184   }
 185 }
 186
 187 void ThreadSuspender::ResumeAllThreads() {
 188   for (uptr i = 0; i < suspended_threads_list_.ThreadCount(); i++) {
 189     pid_t tid = suspended_threads_list_.GetThreadID(i);
 190     int pterrno;
 191     if (!internal_iserror(internal_ptrace(PTRACE_DETACH, tid, nullptr, nullptr),
 192                           &pterrno)) {
 193       VReport(2, "Detached from thread %d.\n", tid);
 194     } else {
 195       // Either the thread is dead, or we are already detached.
 196       // The latter case is possible, for instance, if this function was called
 197       // from a signal handler.
 198       VReport(1, "Could not detach from thread %d (errno %d).\n", tid, pterrno);
 199     }
 200   }
 201 }
 202
 203 void ThreadSuspender::KillAllThreads() {
 204   for (uptr i = 0; i < suspended_threads_list_.ThreadCount(); i++)
 205     internal_ptrace(PTRACE_KILL, suspended_threads_list_.GetThreadID(i),
 206                     nullptr, nullptr);
 207 }
 208
 209 bool ThreadSuspender::SuspendAllThreads() {
 210   ThreadLister thread_lister(pid_);
 211   bool retry = true;
 212   InternalMmapVector<tid_t> threads;
 213   threads.reserve(128);
 214   for (int i = 0; i < 30 && retry; ++i) {
 215     retry = false;
 216     switch (thread_lister.ListThreads(&threads)) {
 217       case ThreadLister::Error:
 218         ResumeAllThreads();
 219         return false;
 220       case ThreadLister::Incomplete:
 221         retry = true;
 222         break;
 223       case ThreadLister::Ok:
 224         break;
 225     }
 226     for (tid_t tid : threads) {
 227       if (SuspendThread(tid))
 228         retry = true;
 229     }
 230   }
 231   return suspended_threads_list_.ThreadCount();
 232 }
 233
 234 // Pointer to the ThreadSuspender instance for use in signal handler.
 235 static ThreadSuspender *thread_suspender_instance = nullptr;
 236
 237 // Synchronous signals that should not be blocked.
 238 static const int kSyncSignals[] = { SIGABRT, SIGILL, SIGFPE, SIGSEGV, SIGBUS,
 239                                     SIGXCPU, SIGXFSZ };
 240
 241 static void TracerThreadDieCallback() {
 242   // Generally a call to Die() in the tracer thread should be fatal to the
 243   // parent process as well, because they share the address space.
 244   // This really only works correctly if all the threads are suspended at this
 245   // point. So we correctly handle calls to Die() from within the callback, but
 246   // not those that happen before or after the callback. Hopefully there aren't
 247   // a lot of opportunities for that to happen...
 248   ThreadSuspender *inst = thread_suspender_instance;
 249   if (inst && stoptheworld_tracer_pid == internal_getpid()) {
 250     inst->KillAllThreads();
 251     thread_suspender_instance = nullptr;
 252   }
 253 }
 254
 255 // Signal handler to wake up suspended threads when the tracer thread dies.
 256 static void TracerThreadSignalHandler(int signum, __sanitizer_siginfo *siginfo,
 257                                       void *uctx) {
 258   SignalContext ctx(siginfo, uctx);
 259   Printf("Tracer caught signal %d: addr=0x%zx pc=0x%zx sp=0x%zx\n", signum,
 260          ctx.addr, ctx.pc, ctx.sp);
 261   ThreadSuspender *inst = thread_suspender_instance;
 262   if (inst) {
 263     if (signum == SIGABRT)
 264       inst->KillAllThreads();
 265     else
 266       inst->ResumeAllThreads();
 267     RAW_CHECK(RemoveDieCallback(TracerThreadDieCallback));
 268     thread_suspender_instance = nullptr;
 269     atomic_store(&inst->arg->done, 1, memory_order_relaxed);
 270   }
 271   internal__exit((signum == SIGABRT) ? 1 : 2);
 272 }
 273
 274 // Size of alternative stack for signal handlers in the tracer thread.
 275 static const int kHandlerStackSize = 8192;
 276
 277 // This function will be run as a cloned task.
 278 static int TracerThread(void* argument) {
 279   TracerThreadArgument *tracer_thread_argument =
 280       (TracerThreadArgument *)argument;
 281
 282   internal_prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
 283   // Check if parent is already dead.
 284   if (internal_getppid() != tracer_thread_argument->parent_pid)
 285     internal__exit(4);
 286
 287   // Wait for the parent thread to finish preparations.
 288   tracer_thread_argument->mutex.Lock();
 289   tracer_thread_argument->mutex.Unlock();
 290
 291   RAW_CHECK(AddDieCallback(TracerThreadDieCallback));
 292
 293   ThreadSuspender thread_suspender(internal_getppid(), tracer_thread_argument);
 294   // Global pointer for the signal handler.
 295   thread_suspender_instance = &thread_suspender;
 296
 297   // Alternate stack for signal handling.
 298   InternalMmapVector<char> handler_stack_memory(kHandlerStackSize);
 299   stack_t handler_stack;
 300   internal_memset(&handler_stack, 0, sizeof(handler_stack));
 301   handler_stack.ss_sp = handler_stack_memory.data();
 302   handler_stack.ss_size = kHandlerStackSize;
 303   internal_sigaltstack(&handler_stack, nullptr);
 304
 305   // Install our handler for synchronous signals. Other signals should be
 306   // blocked by the mask we inherited from the parent thread.
 307   for (uptr i = 0; i < ARRAY_SIZE(kSyncSignals); i++) {
 308     __sanitizer_sigaction act;
 309     internal_memset(&act, 0, sizeof(act));
 310     act.sigaction = TracerThreadSignalHandler;
 311     act.sa_flags = SA_ONSTACK | SA_SIGINFO;
 312     internal_sigaction_norestorer(kSyncSignals[i], &act, 0);
 313   }
 314
 315   int exit_code = 0;
 316   if (!thread_suspender.SuspendAllThreads()) {
 317     VReport(1, "Failed suspending threads.\n");
 318     exit_code = 3;
 319   } else {
 320     tracer_thread_argument->callback(thread_suspender.suspended_threads_list(),
 321                                      tracer_thread_argument->callback_argument);
 322     thread_suspender.ResumeAllThreads();
 323     exit_code = 0;
 324   }
 325   RAW_CHECK(RemoveDieCallback(TracerThreadDieCallback));
 326   thread_suspender_instance = nullptr;
 327   atomic_store(&tracer_thread_argument->done, 1, memory_order_relaxed);
 328   return exit_code;
 329 }
 330
 331 class ScopedStackSpaceWithGuard {
 332  public:
 333   explicit ScopedStackSpaceWithGuard(uptr stack_size) {
 334     stack_size_ = stack_size;
 335     guard_size_ = GetPageSizeCached();
 336     // FIXME: Omitting MAP_STACK here works in current kernels but might break
 337     // in the future.
 338     guard_start_ = (uptr)MmapOrDie(stack_size_ + guard_size_,
 339                                    "ScopedStackWithGuard");
 340     CHECK(MprotectNoAccess((uptr)guard_start_, guard_size_));
 341   }
 342   ~ScopedStackSpaceWithGuard() {
 343     UnmapOrDie((void *)guard_start_, stack_size_ + guard_size_);
 344   }
 345   void *Bottom() const {
 346     return (void *)(guard_start_ + stack_size_ + guard_size_);
 347   }
 348
 349  private:
 350   uptr stack_size_;
 351   uptr guard_size_;
 352   uptr guard_start_;
 353 };
 354
 355 // We have a limitation on the stack frame size, so some stuff had to be moved
 356 // into globals.
 357 static __sanitizer_sigset_t blocked_sigset;
 358 static __sanitizer_sigset_t old_sigset;
 359
 360 class StopTheWorldScope {
 361  public:
 362   StopTheWorldScope() {
 363     // Make this process dumpable. Processes that are not dumpable cannot be
 364     // attached to.
 365     process_was_dumpable_ = internal_prctl(PR_GET_DUMPABLE, 0, 0, 0, 0);
 366     if (!process_was_dumpable_)
 367       internal_prctl(PR_SET_DUMPABLE, 1, 0, 0, 0);
 368   }
 369
 370   ~StopTheWorldScope() {
 371     // Restore the dumpable flag.
 372     if (!process_was_dumpable_)
 373       internal_prctl(PR_SET_DUMPABLE, 0, 0, 0, 0);
 374   }
 375
 376  private:
 377   int process_was_dumpable_;
 378 };
 379
 380 // When sanitizer output is being redirected to file (i.e. by using log_path),
 381 // the tracer should write to the parent's log instead of trying to open a new
 382 // file. Alert the logging code to the fact that we have a tracer.
 383 struct ScopedSetTracerPID {
 384   explicit ScopedSetTracerPID(uptr tracer_pid) {
 385     stoptheworld_tracer_pid = tracer_pid;
 386     stoptheworld_tracer_ppid = internal_getpid();
 387   }
 388   ~ScopedSetTracerPID() {
 389     stoptheworld_tracer_pid = 0;
 390     stoptheworld_tracer_ppid = 0;
 391   }
 392 };
 393
 394 void StopTheWorld(StopTheWorldCallback callback, void *argument) {
 395   StopTheWorldScope in_stoptheworld;
 396   // Prepare the arguments for TracerThread.
 397   struct TracerThreadArgument tracer_thread_argument;
 398   tracer_thread_argument.callback = callback;
 399   tracer_thread_argument.callback_argument = argument;
 400   tracer_thread_argument.parent_pid = internal_getpid();
 401   atomic_store(&tracer_thread_argument.done, 0, memory_order_relaxed);
 402   const uptr kTracerStackSize = 2 * 1024 * 1024;
 403   ScopedStackSpaceWithGuard tracer_stack(kTracerStackSize);
 404   // Block the execution of TracerThread until after we have set ptrace
 405   // permissions.
 406   tracer_thread_argument.mutex.Lock();
 407   // Signal handling story.
 408   // We don't want async signals to be delivered to the tracer thread,
 409   // so we block all async signals before creating the thread. An async signal
 410   // handler can temporary modify errno, which is shared with this thread.
 411   // We ought to use pthread_sigmask here, because sigprocmask has undefined
 412   // behavior in multithreaded programs. However, on linux sigprocmask is
 413   // equivalent to pthread_sigmask with the exception that pthread_sigmask
 414   // does not allow to block some signals used internally in pthread
 415   // implementation. We are fine with blocking them here, we are really not
 416   // going to pthread_cancel the thread.
 417   // The tracer thread should not raise any synchronous signals. But in case it
 418   // does, we setup a special handler for sync signals that properly kills the
 419   // parent as well. Note: we don't pass CLONE_SIGHAND to clone, so handlers
 420   // in the tracer thread won't interfere with user program. Double note: if a
 421   // user does something along the lines of 'kill -11 pid', that can kill the
 422   // process even if user setup own handler for SEGV.
 423   // Thing to watch out for: this code should not change behavior of user code
 424   // in any observable way. In particular it should not override user signal
 425   // handlers.
 426   internal_sigfillset(&blocked_sigset);
 427   for (uptr i = 0; i < ARRAY_SIZE(kSyncSignals); i++)
 428     internal_sigdelset(&blocked_sigset, kSyncSignals[i]);
 429   int rv = internal_sigprocmask(SIG_BLOCK, &blocked_sigset, &old_sigset);
 430   CHECK_EQ(rv, 0);
 431   uptr tracer_pid = internal_clone(
 432       TracerThread, tracer_stack.Bottom(),
 433       CLONE_VM | CLONE_FS | CLONE_FILES | CLONE_UNTRACED,
 434       &tracer_thread_argument, nullptr /* parent_tidptr */,
 435       nullptr /* newtls */, nullptr /* child_tidptr */);
 436   internal_sigprocmask(SIG_SETMASK, &old_sigset, 0);
 437   int local_errno = 0;
 438   if (internal_iserror(tracer_pid, &local_errno)) {
 439     VReport(1, "Failed spawning a tracer thread (errno %d).\n", local_errno);
 440     tracer_thread_argument.mutex.Unlock();
 441   } else {
 442     ScopedSetTracerPID scoped_set_tracer_pid(tracer_pid);
 443     // On some systems we have to explicitly declare that we want to be traced
 444     // by the tracer thread.
 445     internal_prctl(PR_SET_PTRACER, tracer_pid, 0, 0, 0);
 446     // Allow the tracer thread to start.
 447     tracer_thread_argument.mutex.Unlock();
 448     // NOTE: errno is shared between this thread and the tracer thread.
 449     // internal_waitpid() may call syscall() which can access/spoil errno,
 450     // so we can't call it now. Instead we for the tracer thread to finish using
 451     // the spin loop below. Man page for sched_yield() says "In the Linux
 452     // implementation, sched_yield() always succeeds", so let's hope it does not
 453     // spoil errno. Note that this spin loop runs only for brief periods before
 454     // the tracer thread has suspended us and when it starts unblocking threads.
 455     while (atomic_load(&tracer_thread_argument.done, memory_order_relaxed) == 0)
 456       sched_yield();
 457     // Now the tracer thread is about to exit and does not touch errno,
 458     // wait for it.
 459     for (;;) {
 460       uptr waitpid_status = internal_waitpid(tracer_pid, nullptr, __WALL);
 461       if (!internal_iserror(waitpid_status, &local_errno))
 462         break;
 463       if (local_errno == EINTR)
 464         continue;
 465       VReport(1, "Waiting on the tracer thread failed (errno %d).\n",
 466               local_errno);
 467       break;
 468     }
 469   }
 470 }
 471
 472 // Platform-specific methods from SuspendedThreadsList.
 473 #if SANITIZER_ANDROID && defined(__arm__)
 474 typedef pt_regs regs_struct;
 475 #define REG_SP ARM_sp
 476
 477 #elif SANITIZER_LINUX && defined(__arm__)
 478 typedef user_regs regs_struct;
 479 #define REG_SP uregs[13]
 480
 481 #elif defined(__i386__) || defined(__x86_64__)
 482 typedef user_regs_struct regs_struct;
 483 #if defined(__i386__)
 484 #define REG_SP esp
 485 #else
 486 #define REG_SP rsp
 487 #endif
 488 #define ARCH_IOVEC_FOR_GETREGSET
 489 // Support ptrace extensions even when compiled without required kernel support
 490 #ifndef NT_X86_XSTATE
 491 #define NT_X86_XSTATE 0x202
 492 #endif
 493 // Compiler may use FP registers to store pointers.
 494 static constexpr uptr kExtraRegs[] = {NT_X86_XSTATE, NT_FPREGSET};
 495
 496 #elif defined(__powerpc__) || defined(__powerpc64__)
 497 typedef pt_regs regs_struct;
 498 #define REG_SP gpr[PT_R1]
 499
 500 #elif defined(__mips__)
 501 typedef struct user regs_struct;
 502 # if SANITIZER_ANDROID
 503 #  define REG_SP regs[EF_R29]
 504 # else
 505 #  define REG_SP regs[EF_REG29]
 506 # endif
 507
 508 #elif defined(__aarch64__)
 509 typedef struct user_pt_regs regs_struct;
 510 #define REG_SP sp
 511 static constexpr uptr kExtraRegs[] = {0};
 512 #define ARCH_IOVEC_FOR_GETREGSET
 513
 514 #elif SANITIZER_RISCV64
 515 typedef struct user_regs_struct regs_struct;
 516 #define REG_SP sp
 517 static constexpr uptr kExtraRegs[] = {0};
 518 #define ARCH_IOVEC_FOR_GETREGSET
 519
 520 #elif defined(__s390__)
 521 typedef _user_regs_struct regs_struct;
 522 #define REG_SP gprs[15]
 523 static constexpr uptr kExtraRegs[] = {0};
 524 #define ARCH_IOVEC_FOR_GETREGSET
 525
 526 #else
 527 #error "Unsupported architecture"
 528 #endif // SANITIZER_ANDROID && defined(__arm__)
 529
 530 tid_t SuspendedThreadsListLinux::GetThreadID(uptr index) const {
 531   CHECK_LT(index, thread_ids_.size());
 532   return thread_ids_[index];
 533 }
 534
 535 uptr SuspendedThreadsListLinux::ThreadCount() const {
 536   return thread_ids_.size();
 537 }
 538
 539 bool SuspendedThreadsListLinux::ContainsTid(tid_t thread_id) const {
 540   for (uptr i = 0; i < thread_ids_.size(); i++) {
 541     if (thread_ids_[i] == thread_id) return true;
 542   }
 543   return false;
 544 }
 545
 546 void SuspendedThreadsListLinux::Append(tid_t tid) {
 547   thread_ids_.push_back(tid);
 548 }
 549
 550 PtraceRegistersStatus SuspendedThreadsListLinux::GetRegistersAndSP(
 551     uptr index, InternalMmapVector<uptr> *buffer, uptr *sp) const {
 552   pid_t tid = GetThreadID(index);
 553   constexpr uptr uptr_sz = sizeof(uptr);
 554   int pterrno;
 555 #ifdef ARCH_IOVEC_FOR_GETREGSET
 556   auto append = [&](uptr regset) {
 557     uptr size = buffer->size();
 558     // NT_X86_XSTATE requires 64bit alignment.
 559     uptr size_up = RoundUpTo(size, 8 / uptr_sz);
 560     buffer->reserve(Max<uptr>(1024, size_up));
 561     struct iovec regset_io;
 562     for (;; buffer->resize(buffer->capacity() * 2)) {
 563       buffer->resize(buffer->capacity());
 564       uptr available_bytes = (buffer->size() - size_up) * uptr_sz;
 565       regset_io.iov_base = buffer->data() + size_up;
 566       regset_io.iov_len = available_bytes;
 567       bool fail =
 568           internal_iserror(internal_ptrace(PTRACE_GETREGSET, tid,
 569                                            (void *)regset, (void *)&regset_io),
 570                            &pterrno);
 571       if (fail) {
 572         VReport(1, "Could not get regset %p from thread %d (errno %d).\n",
 573                 (void *)regset, tid, pterrno);
 574         buffer->resize(size);
 575         return false;
 576       }
 577
 578       // Far enough from the buffer size, no need to resize and repeat.
 579       if (regset_io.iov_len + 64 < available_bytes)
 580         break;
 581     }
 582     buffer->resize(size_up + RoundUpTo(regset_io.iov_len, uptr_sz) / uptr_sz);
 583     return true;
 584   };
 585
 586   buffer->clear();
 587   bool fail = !append(NT_PRSTATUS);
 588   if (!fail) {
 589     // Accept the first available and do not report errors.
 590     for (uptr regs : kExtraRegs)
 591       if (regs && append(regs))
 592         break;
 593   }
 594 #else
 595   buffer->resize(RoundUpTo(sizeof(regs_struct), uptr_sz) / uptr_sz);
 596   bool fail = internal_iserror(
 597       internal_ptrace(PTRACE_GETREGS, tid, nullptr, buffer->data()), &pterrno);
 598   if (fail)
 599     VReport(1, "Could not get registers from thread %d (errno %d).\n", tid,
 600             pterrno);
 601 #endif
 602   if (fail) {
 603     // ESRCH means that the given thread is not suspended or already dead.
 604     // Therefore it's unsafe to inspect its data (e.g. walk through stack) and
 605     // we should notify caller about this.
 606     return pterrno == ESRCH ? REGISTERS_UNAVAILABLE_FATAL
 607                             : REGISTERS_UNAVAILABLE;
 608   }
 609
 610   *sp = reinterpret_cast<regs_struct *>(buffer->data())[0].REG_SP;
 611   return REGISTERS_AVAILABLE;
 612 }
 613
 614 } // namespace __sanitizer
 615
 616 #endif  // SANITIZER_LINUX && (defined(__x86_64__) || defined(__mips__)
 617         // || defined(__aarch64__) || defined(__powerpc64__)
 618         // || defined(__s390__) || defined(__i386__) || defined(__arm__)