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//===- FuzzerUtilFuchsia.cpp - Misc utils for Fuchsia. --------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// Misc utils implementation using Fuchsia/Zircon APIs.
#include "FuzzerPlatform.h"
#include "FuzzerInternal.h"
#include "FuzzerUtil.h"
#include <cassert>
#include <cerrno>
#include <cinttypes>
#include <cstdint>
#include <fcntl.h>
#include <lib/fdio/fdio.h>
#include <lib/fdio/spawn.h>
#include <string>
#include <sys/select.h>
#include <thread>
#include <unistd.h>
#include <zircon/errors.h>
#include <zircon/process.h>
#include <zircon/sanitizer.h>
#include <zircon/status.h>
#include <zircon/syscalls.h>
#include <zircon/syscalls/debug.h>
#include <zircon/syscalls/exception.h>
#include <zircon/syscalls/object.h>
#include <zircon/types.h>
#include <vector>
namespace fuzzer {
// Given that Fuchsia doesn't have the POSIX signals that libFuzzer was written
// around, the general approach is to spin up dedicated threads to watch for
// each requested condition (alarm, interrupt, crash). Of these, the crash
// handler is the most involved, as it requires resuming the crashed thread in
// order to invoke the sanitizers to get the needed state.
// Forward declaration of assembly trampoline needed to resume crashed threads.
// This appears to have external linkage to C++, which is why it's not in the
// anonymous namespace. The assembly definition inside MakeTrampoline()
// actually defines the symbol with internal linkage only.
void CrashTrampolineAsm() __asm__("CrashTrampolineAsm");
namespace {
// The signal handler thread uses Zircon exceptions to resume crashed threads
// into libFuzzer's POSIX signal handlers. The associated event is used to
// signal when the thread is running, and when it should stop.
std::thread SignalHandler;
zx_handle_t SignalHandlerEvent = ZX_HANDLE_INVALID;
// Helper function to handle Zircon syscall failures.
void ExitOnErr(zx_status_t Status, const char *Syscall) {
if (Status != ZX_OK) {
Printf("libFuzzer: %s failed: %s\n", Syscall,
void AlarmHandler(int Seconds) {
while (true) {
// For the crash handler, we need to call Fuzzer::StaticCrashSignalCallback
// without POSIX signal handlers. To achieve this, we use an assembly function
// to add the necessary CFI unwinding information and a C function to bridge
// from that back into C++.
// FIXME: This works as a short-term solution, but this code really shouldn't be
// architecture dependent. A better long term solution is to implement remote
// unwinding and expose the necessary APIs through sanitizer_common and/or ASAN
// to allow the exception handling thread to gather the crash state directly.
// Alternatively, Fuchsia may in future actually implement basic signal
// handling for the machine trap signals.
#if defined(__x86_64__)
OP_REG(rax) \
OP_REG(rbx) \
OP_REG(rcx) \
OP_REG(rdx) \
OP_REG(rsi) \
OP_REG(rdi) \
OP_REG(rbp) \
OP_REG(rsp) \
OP_REG(r8) \
OP_REG(r9) \
OP_REG(r10) \
OP_REG(r11) \
OP_REG(r12) \
OP_REG(r13) \
OP_REG(r14) \
OP_REG(r15) \
#elif defined(__aarch64__)
OP_NUM(0) \
OP_NUM(1) \
OP_NUM(2) \
OP_NUM(3) \
OP_NUM(4) \
OP_NUM(5) \
OP_NUM(6) \
OP_NUM(7) \
OP_NUM(8) \
OP_NUM(9) \
OP_NUM(10) \
OP_NUM(11) \
OP_NUM(12) \
OP_NUM(13) \
OP_NUM(14) \
OP_NUM(15) \
OP_NUM(16) \
OP_NUM(17) \
OP_NUM(18) \
OP_NUM(19) \
OP_NUM(20) \
OP_NUM(21) \
OP_NUM(22) \
OP_NUM(23) \
OP_NUM(24) \
OP_NUM(25) \
OP_NUM(26) \
OP_NUM(27) \
OP_NUM(28) \
OP_NUM(29) \
#error "Unsupported architecture for fuzzing on Fuchsia"
// Produces a CFI directive for the named or numbered register.
// The value used refers to an assembler immediate operand with the same name
// as the register (see ASM_OPERAND_REG).
#define CFI_OFFSET_REG(reg) ".cfi_offset " #reg ", %c[" #reg "]\n"
#define CFI_OFFSET_NUM(num) CFI_OFFSET_REG(x##num)
// Produces an assembler immediate operand for the named or numbered register.
// This operand contains the offset of the register relative to the CFA.
#define ASM_OPERAND_REG(reg) \
[reg] "i"(offsetof(zx_thread_state_general_regs_t, reg)),
#define ASM_OPERAND_NUM(num) \
[x##num] "i"(offsetof(zx_thread_state_general_regs_t, r[num])),
// Trampoline to bridge from the assembly below to the static C++ crash
// callback.
static void StaticCrashHandler() {
for (;;) {
// This trampoline function has the necessary CFI information to unwind
// and get a backtrace:
// * The stack contains a copy of all the registers at the point of crash,
// the code has CFI directives specifying how to restore them.
// * A call to StaticCrashHandler, which will print the stacktrace and exit
// the fuzzer, generating a crash artifact.
// The __attribute__((used)) is necessary because the function
// is never called; it's just a container around the assembly to allow it to
// use operands for compile-time computed constants.
void MakeTrampoline() {
".pushsection .text.CrashTrampolineAsm\n"
".type CrashTrampolineAsm,STT_FUNC\n"
".cfi_startproc simple\n"
#if defined(__x86_64__)
".cfi_return_column rip\n"
".cfi_def_cfa rsp, 0\n"
"call %c[StaticCrashHandler]\n"
#elif defined(__aarch64__)
".cfi_return_column 33\n"
".cfi_def_cfa sp, 0\n"
".cfi_offset 33, %c[pc]\n"
".cfi_offset 30, %c[lr]\n"
"bl %c[StaticCrashHandler]\n"
"brk 1\n"
#error "Unsupported architecture for fuzzing on Fuchsia"
".size CrashTrampolineAsm, . - CrashTrampolineAsm\n"
: // No outputs
#if defined(__aarch64__)
[StaticCrashHandler] "i"(StaticCrashHandler));
void CrashHandler() {
assert(SignalHandlerEvent != ZX_HANDLE_INVALID);
// This structure is used to ensure we close handles to objects we create in
// this handler.
struct ScopedHandle {
~ScopedHandle() { _zx_handle_close(Handle); }
zx_handle_t Handle = ZX_HANDLE_INVALID;
// Create the exception channel. We need to claim to be a "debugger" so the
// kernel will allow us to modify and resume dying threads (see below). Once
// the channel is set, we can signal the main thread to continue and wait
// for the exception to arrive.
ScopedHandle Channel;
zx_handle_t Self = _zx_process_self();
ExitOnErr(_zx_object_signal(SignalHandlerEvent, 0, ZX_USER_SIGNAL_0),
// This thread lives as long as the process in order to keep handling
// crashes. In practice, the first crashed thread to reach the end of the
// StaticCrashHandler will end the process.
while (true) {
zx_wait_item_t WaitItems[] = {
.handle = SignalHandlerEvent,
.pending = 0,
.handle = Channel.Handle,
.pending = 0,
auto Status = _zx_object_wait_many(
WaitItems, sizeof(WaitItems) / sizeof(WaitItems[0]), ZX_TIME_INFINITE);
if (Status != ZX_OK || (WaitItems[1].pending & ZX_CHANNEL_READABLE) == 0) {
zx_exception_info_t ExceptionInfo;
ScopedHandle Exception;
ExitOnErr(_zx_channel_read(Channel.Handle, 0, &ExceptionInfo,
&Exception.Handle, sizeof(ExceptionInfo), 1,
nullptr, nullptr),
// Ignore informational synthetic exceptions.
if (ZX_EXCP_THREAD_STARTING == ExceptionInfo.type ||
ZX_EXCP_THREAD_EXITING == ExceptionInfo.type ||
ZX_EXCP_PROCESS_STARTING == ExceptionInfo.type) {
// At this point, we want to get the state of the crashing thread, but
// libFuzzer and the sanitizers assume this will happen from that same
// thread via a POSIX signal handler. "Resurrecting" the thread in the
// middle of the appropriate callback is as simple as forcibly setting the
// instruction pointer/program counter, provided we NEVER EVER return from
// that function (since otherwise our stack will not be valid).
ScopedHandle Thread;
ExitOnErr(_zx_exception_get_thread(Exception.Handle, &Thread.Handle),
zx_thread_state_general_regs_t GeneralRegisters;
ExitOnErr(_zx_thread_read_state(Thread.Handle, ZX_THREAD_STATE_GENERAL_REGS,
// To unwind properly, we need to push the crashing thread's register state
// onto the stack and jump into a trampoline with CFI instructions on how
// to restore it.
#if defined(__x86_64__)
uintptr_t StackPtr =
(GeneralRegisters.rsp - (128 + sizeof(GeneralRegisters))) &
__unsanitized_memcpy(reinterpret_cast<void *>(StackPtr), &GeneralRegisters,
GeneralRegisters.rsp = StackPtr; = reinterpret_cast<zx_vaddr_t>(CrashTrampolineAsm);
#elif defined(__aarch64__)
uintptr_t StackPtr =
(GeneralRegisters.sp - sizeof(GeneralRegisters)) & -(uintptr_t)16;
__unsanitized_memcpy(reinterpret_cast<void *>(StackPtr), &GeneralRegisters,
GeneralRegisters.sp = StackPtr;
GeneralRegisters.pc = reinterpret_cast<zx_vaddr_t>(CrashTrampolineAsm);
#error "Unsupported architecture for fuzzing on Fuchsia"
// Now force the crashing thread's state.
_zx_thread_write_state(Thread.Handle, ZX_THREAD_STATE_GENERAL_REGS,
&GeneralRegisters, sizeof(GeneralRegisters)),
// Set the exception to HANDLED so it resumes the thread on close.
uint32_t ExceptionState = ZX_EXCEPTION_STATE_HANDLED;
ExitOnErr(_zx_object_set_property(Exception.Handle, ZX_PROP_EXCEPTION_STATE,
&ExceptionState, sizeof(ExceptionState)),
void StopSignalHandler() {
if (SignalHandler.joinable()) {
} // namespace
// Platform specific functions.
void SetSignalHandler(const FuzzingOptions &Options) {
// Make sure information from libFuzzer and the sanitizers are easy to
// reassemble. `__sanitizer_log_write` has the added benefit of ensuring the
// DSO map is always available for the symbolizer.
// A uint64_t fits in 20 chars, so 64 is plenty.
char Buf[64];
memset(Buf, 0, sizeof(Buf));
snprintf(Buf, sizeof(Buf), "==%lu== INFO: libFuzzer starting.\n", GetPid());
if (EF->__sanitizer_log_write)
__sanitizer_log_write(Buf, sizeof(Buf));
Printf("%s", Buf);
// Set up alarm handler if needed.
if (Options.HandleAlrm && Options.UnitTimeoutSec > 0) {
std::thread T(AlarmHandler, Options.UnitTimeoutSec / 2 + 1);
// Options.HandleInt and Options.HandleTerm are not supported on Fuchsia
// Early exit if no crash handler needed.
if (!Options.HandleSegv && !Options.HandleBus && !Options.HandleIll &&
!Options.HandleFpe && !Options.HandleAbrt)
// Set up the crash handler and wait until it is ready before proceeding.
ExitOnErr(_zx_event_create(0, &SignalHandlerEvent), "_zx_event_create");
SignalHandler = std::thread(CrashHandler);
zx_status_t Status = _zx_object_wait_one(SignalHandlerEvent, ZX_USER_SIGNAL_0,
ExitOnErr(Status, "_zx_object_wait_one");
void SleepSeconds(int Seconds) {
unsigned long GetPid() {
zx_status_t rc;
zx_info_handle_basic_t Info;
if ((rc = _zx_object_get_info(_zx_process_self(), ZX_INFO_HANDLE_BASIC, &Info,
sizeof(Info), NULL, NULL)) != ZX_OK) {
Printf("libFuzzer: unable to get info about self: %s\n",
return Info.koid;
size_t GetPeakRSSMb() {
zx_status_t rc;
zx_info_task_stats_t Info;
if ((rc = _zx_object_get_info(_zx_process_self(), ZX_INFO_TASK_STATS, &Info,
sizeof(Info), NULL, NULL)) != ZX_OK) {
Printf("libFuzzer: unable to get info about self: %s\n",
return (Info.mem_private_bytes + Info.mem_shared_bytes) >> 20;
template <typename Fn>
class RunOnDestruction {
explicit RunOnDestruction(Fn fn) : fn_(fn) {}
~RunOnDestruction() { fn_(); }
Fn fn_;
template <typename Fn>
RunOnDestruction<Fn> at_scope_exit(Fn fn) {
return RunOnDestruction<Fn>(fn);
static fdio_spawn_action_t clone_fd_action(int localFd, int targetFd) {
return {
.fd =
.local_fd = localFd,
.target_fd = targetFd,
int ExecuteCommand(const Command &Cmd) {
zx_status_t rc;
// Convert arguments to C array
auto Args = Cmd.getArguments();
size_t Argc = Args.size();
assert(Argc != 0);
std::unique_ptr<const char *[]> Argv(new const char *[Argc + 1]);
for (size_t i = 0; i < Argc; ++i)
Argv[i] = Args[i].c_str();
Argv[Argc] = nullptr;
// Determine output. On Fuchsia, the fuzzer is typically run as a component
// that lacks a mutable working directory. Fortunately, when this is the case
// a mutable output directory must be specified using "-artifact_prefix=...",
// so write the log file(s) there.
// However, we don't want to apply this logic for absolute paths.
bool discardStdout = false;
bool discardStderr = false;
if (Cmd.hasOutputFile()) {
std::string Path = Cmd.getOutputFile();
if (Path == getDevNull()) {
// On Fuchsia, there's no "/dev/null" like-file, so we
// just don't copy the FDs into the spawned process.
discardStdout = true;
} else {
bool IsAbsolutePath = Path.length() > 1 && Path[0] == '/';
if (!IsAbsolutePath && Cmd.hasFlag("artifact_prefix"))
Path = Cmd.getFlagValue("artifact_prefix") + "/" + Path;
FdOut = open(Path.c_str(), O_WRONLY | O_CREAT | O_TRUNC, 0);
if (FdOut == -1) {
Printf("libFuzzer: failed to open %s: %s\n", Path.c_str(),
return ZX_ERR_IO;
auto CloseFdOut = at_scope_exit([FdOut]() {
// Determine stderr
if (Cmd.isOutAndErrCombined()) {
FdErr = FdOut;
if (discardStdout)
discardStderr = true;
// Clone the file descriptors into the new process
std::vector<fdio_spawn_action_t> SpawnActions;
SpawnActions.push_back(clone_fd_action(STDIN_FILENO, STDIN_FILENO));
if (!discardStdout)
SpawnActions.push_back(clone_fd_action(FdOut, STDOUT_FILENO));
if (!discardStderr)
SpawnActions.push_back(clone_fd_action(FdErr, STDERR_FILENO));
// Start the process.
zx_handle_t ProcessHandle = ZX_HANDLE_INVALID;
rc = fdio_spawn_etc(ZX_HANDLE_INVALID,
Argv.get(), nullptr, SpawnActions.size(),, &ProcessHandle, ErrorMsg);
if (rc != ZX_OK) {
Printf("libFuzzer: failed to launch '%s': %s, %s\n", Argv[0], ErrorMsg,
return rc;
auto CloseHandle = at_scope_exit([&]() { _zx_handle_close(ProcessHandle); });
// Now join the process and return the exit status.
if ((rc = _zx_object_wait_one(ProcessHandle, ZX_PROCESS_TERMINATED,
ZX_TIME_INFINITE, nullptr)) != ZX_OK) {
Printf("libFuzzer: failed to join '%s': %s\n", Argv[0],
return rc;
zx_info_process_t Info;
if ((rc = _zx_object_get_info(ProcessHandle, ZX_INFO_PROCESS, &Info,
sizeof(Info), nullptr, nullptr)) != ZX_OK) {
Printf("libFuzzer: unable to get return code from '%s': %s\n", Argv[0],
return rc;
return static_cast<int>(Info.return_code);
bool ExecuteCommand(const Command &BaseCmd, std::string *CmdOutput) {
auto LogFilePath = TempPath("SimPopenOut", ".txt");
Command Cmd(BaseCmd);
int Ret = ExecuteCommand(Cmd);
*CmdOutput = FileToString(LogFilePath);
return Ret == 0;
const void *SearchMemory(const void *Data, size_t DataLen, const void *Patt,
size_t PattLen) {
return memmem(Data, DataLen, Patt, PattLen);
// In fuchsia, accessing /dev/null is not supported. There's nothing
// similar to a file that discards everything that is written to it.
// The way of doing something similar in fuchsia is by using
// fdio_null_create and binding that to a file descriptor.
void DiscardOutput(int Fd) {
fdio_t *fdio_null = fdio_null_create();
if (fdio_null == nullptr) return;
int nullfd = fdio_bind_to_fd(fdio_null, -1, 0);
if (nullfd < 0) return;
dup2(nullfd, Fd);
} // namespace fuzzer