llvm-project/lldb/source/Plugins/Process/Linux/SingleStepCheck.cpp

//===-- SingleStepCheck.cpp ----------------------------------- -*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "SingleStepCheck.h"

#include <sched.h>
#include <signal.h>
#include <sys/wait.h>
#include <unistd.h>

#include "NativeProcessLinux.h"

#include "llvm/Support/Compiler.h"

#include "lldb/Core/Error.h"
#include "lldb/Core/Log.h"
#include "lldb/Host/linux/Ptrace.h"

using namespace lldb_private::process_linux;

#if defined(__arm64__) || defined(__aarch64__)
namespace
{

void LLVM_ATTRIBUTE_NORETURN
Child()
{
    if (ptrace(PTRACE_TRACEME, 0, nullptr, nullptr) == -1)
        _exit(1);

    // We just do an endless loop SIGSTOPPING ourselves until killed. The tracer will fiddle with our cpu
    // affinities and monitor the behaviour.
    for (;;)
    {
        raise(SIGSTOP);

        // Generate a bunch of instructions here, so that a single-step does not land in the
        // raise() accidentally. If single-stepping works, we will be spinning in this loop. If
        // it doesn't, we'll land in the raise() call above.
        for (volatile unsigned i = 0; i < CPU_SETSIZE; ++i)
            ;
    }
}

struct ChildDeleter
{
    ::pid_t pid;

    ~ChildDeleter()
    {
        int status;
        kill(pid, SIGKILL);            // Kill the child.
        waitpid(pid, &status, __WALL); // Pick up the remains.
    }
};

} // end anonymous namespace

bool
impl::SingleStepWorkaroundNeeded()
{
    // We shall spawn a child, and use it to verify the debug capabilities of the cpu. We shall
    // iterate through the cpus, bind the child to each one in turn, and verify that
    // single-stepping works on that cpu. A workaround is needed if we find at least one broken
    // cpu.

    Log *log(GetLogIfAllCategoriesSet(LIBLLDB_LOG_THREAD));
    Error error;
    ::pid_t child_pid = fork();
    if (child_pid == -1)
    {
        if (log)
        {
            error.SetErrorToErrno();
            log->Printf("%s failed to fork(): %s", __FUNCTION__, error.AsCString());
        }
        return false;
    }
    if (child_pid == 0)
        Child();

    ChildDeleter child_deleter{child_pid};
    cpu_set_t available_cpus;
    if (sched_getaffinity(child_pid, sizeof available_cpus, &available_cpus) == -1)
    {
        if (log)
        {
            error.SetErrorToErrno();
            log->Printf("%s failed to get available cpus: %s", __FUNCTION__, error.AsCString());
        }
        return false;
    }

    int status;
    ::pid_t wpid = waitpid(child_pid, &status, __WALL);
    if (wpid != child_pid || !WIFSTOPPED(status))
    {
        if (log)
        {
            error.SetErrorToErrno();
            log->Printf("%s waitpid() failed (status = %x): %s", __FUNCTION__, status, error.AsCString());
        }
        return false;
    }

    unsigned cpu;
    for (cpu = 0; cpu < CPU_SETSIZE; ++cpu)
    {
        if (!CPU_ISSET(cpu, &available_cpus))
            continue;

        cpu_set_t cpus;
        CPU_ZERO(&cpus);
        CPU_SET(cpu, &cpus);
        if (sched_setaffinity(child_pid, sizeof cpus, &cpus) == -1)
        {
            if (log)
            {
                error.SetErrorToErrno();
                log->Printf("%s failed to switch to cpu %u: %s", __FUNCTION__, cpu, error.AsCString());
            }
            continue;
        }

        int status;
        error = NativeProcessLinux::PtraceWrapper(PTRACE_SINGLESTEP, child_pid);
        if (error.Fail())
        {
            if (log)
                log->Printf("%s single step failed: %s", __FUNCTION__, error.AsCString());
            break;
        }

        wpid = waitpid(child_pid, &status, __WALL);
        if (wpid != child_pid || !WIFSTOPPED(status))
        {
            if (log)
            {
                error.SetErrorToErrno();
                log->Printf("%s waitpid() failed (status = %x): %s", __FUNCTION__, status, error.AsCString());
            }
            break;
        }
        if (WSTOPSIG(status) != SIGTRAP)
        {
            if (log)
                log->Printf("%s single stepping on cpu %d failed with status %x", __FUNCTION__, cpu, status);
            break;
        }
    }

    // cpu is either the index of the first broken cpu, or CPU_SETSIZE.
    if (cpu == 0)
    {
        if (log)
            log->Printf("%s SINGLE STEPPING ON FIRST CPU IS NOT WORKING. DEBUGGING LIKELY TO BE UNRELIABLE.",
                        __FUNCTION__);
        // No point in trying to fiddle with the affinities, just give it our best shot and see how it goes.
        return false;
    }

    return cpu != CPU_SETSIZE;
}

#else // !arm64
bool
impl::SingleStepWorkaroundNeeded()
{
    return false;
}
#endif