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//===-- esan_sideline_linux.cpp ---------------------------------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file is a part of EfficiencySanitizer, a family of performance tuners.
// Support for a separate or "sideline" tool thread on Linux.
#include "sanitizer_common/sanitizer_platform.h"
#include "esan_sideline.h"
#include "sanitizer_common/sanitizer_atomic.h"
#include "sanitizer_common/sanitizer_common.h"
#include "sanitizer_common/sanitizer_linux.h"
#include <errno.h>
#include <sched.h>
#include <sys/prctl.h>
#include <sys/signal.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
namespace __esan {
static const int SigAltStackSize = 4*1024;
static const int SidelineStackSize = 4*1024;
static const uptr SidelineIdUninitialized = 1;
// FIXME: we'll need some kind of TLS (can we trust that a pthread key will
// work in our non-POSIX thread?) to access our data in our signal handler
// with multiple sideline threads. For now we assume there is only one
// sideline thread and we use a dirty solution of a global var.
static SidelineThread *TheThread;
// We aren't passing SA_NODEFER so the same signal is blocked while here.
void SidelineThread::handleSidelineSignal(int SigNum,
__sanitizer_siginfo *SigInfo,
void *Ctx) {
VPrintf(3, "Sideline signal %d\n", SigNum);
// See above about needing TLS to avoid this global var.
SidelineThread *Thread = TheThread;
if (atomic_load(&Thread->SidelineExit, memory_order_relaxed) != 0)
void SidelineThread::registerSignal(int SigNum) {
__sanitizer_sigaction SigAct;
internal_memset(&SigAct, 0, sizeof(SigAct));
SigAct.sigaction = handleSidelineSignal;
// We do not pass SA_NODEFER as we want to block the same signal.
SigAct.sa_flags = SA_ONSTACK | SA_SIGINFO;
int Res = internal_sigaction(SigNum, &SigAct, nullptr);
CHECK_EQ(Res, 0);
int SidelineThread::runSideline(void *Arg) {
VPrintf(1, "Sideline thread starting\n");
SidelineThread *Thread = static_cast<SidelineThread*>(Arg);
// If the parent dies, we want to exit also.
internal_prctl(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0);
// Set up a signal handler on an alternate stack for safety.
InternalMmapVector<char> StackMap(SigAltStackSize);
stack_t SigAltStack;
SigAltStack.ss_sp =;
SigAltStack.ss_size = SigAltStackSize;
SigAltStack.ss_flags = 0;
internal_sigaltstack(&SigAltStack, nullptr);
// We inherit the signal mask from the app thread. In case
// we weren't created at init time, we ensure the mask is empty.
__sanitizer_sigset_t SigSet;
int Res = internal_sigprocmask(SIG_UNBLOCK, &SigSet, nullptr);
CHECK_EQ(Res, 0);
bool TimerSuccess = Thread->adjustTimer(Thread->Freq);
// We loop, doing nothing but handling itimer signals.
while (atomic_load(&TheThread->SidelineExit, memory_order_relaxed) == 0)
if (!Thread->adjustTimer(0))
VPrintf(1, "Failed to disable timer\n");
VPrintf(1, "Sideline thread exiting\n");
return 0;
bool SidelineThread::launchThread(SidelineFunc takeSample, void *Arg,
u32 FreqMilliSec) {
// This can only be called once. However, we can't clear a field in
// the constructor and check for that here as the constructor for
// a static instance is called *after* our module_ctor and thus after
// this routine! Thus we rely on the TheThread check below.
CHECK(TheThread == nullptr); // Only one sideline thread is supported.
TheThread = this;
sampleFunc = takeSample;
FuncArg = Arg;
Freq = FreqMilliSec;
atomic_store(&SidelineExit, 0, memory_order_relaxed);
// We do without a guard page.
Stack = static_cast<char*>(MmapOrDie(SidelineStackSize, "SidelineStack"));
// We need to handle the return value from internal_clone() not having been
// assigned yet (for our CHECK in adjustTimer()) so we ensure this has a
// sentinel value.
SidelineId = SidelineIdUninitialized;
// By omitting CLONE_THREAD, the child is in its own thread group and will not
// receive any of the application's signals.
SidelineId = internal_clone(
runSideline, Stack + SidelineStackSize,
this, nullptr /* parent_tidptr */,
nullptr /* newtls */, nullptr /* child_tidptr */);
int ErrCode;
if (internal_iserror(SidelineId, &ErrCode)) {
Printf("FATAL: EfficiencySanitizer failed to spawn a thread (code %d).\n",
return false; // Not reached.
return true;
bool SidelineThread::joinThread() {
VPrintf(1, "Joining sideline thread\n");
bool Res = true;
atomic_store(&SidelineExit, 1, memory_order_relaxed);
while (true) {
uptr Status = internal_waitpid(SidelineId, nullptr, __WALL);
int ErrCode;
if (!internal_iserror(Status, &ErrCode))
if (ErrCode == EINTR)
VPrintf(1, "Failed to join sideline thread (errno %d)\n", ErrCode);
Res = false;
UnmapOrDie(Stack, SidelineStackSize);
return Res;
// Must be called from the sideline thread itself.
bool SidelineThread::adjustTimer(u32 FreqMilliSec) {
// The return value of internal_clone() may not have been assigned yet:
CHECK(internal_getpid() == SidelineId ||
SidelineId == SidelineIdUninitialized);
Freq = FreqMilliSec;
struct itimerval TimerVal;
TimerVal.it_interval.tv_sec = (time_t) Freq / 1000;
TimerVal.it_interval.tv_usec = (time_t) (Freq % 1000) * 1000;
TimerVal.it_value.tv_sec = (time_t) Freq / 1000;
TimerVal.it_value.tv_usec = (time_t) (Freq % 1000) * 1000;
// As we're in a different thread group, we cannot use either
// ITIMER_PROF or ITIMER_VIRTUAL without taking up scheduled
// time ourselves: thus we must use real time.
int Res = setitimer(ITIMER_REAL, &TimerVal, nullptr);
return (Res == 0);
} // namespace __esan