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From: LE GARREC Vincent <legarrec.vincent@gmail.com>
Date: Sun, 11 Mar 2018 20:15:00 -0000
Message-ID: <CAH0JWTvJO7opYb=3ANZOiJG9KnJ+pDryMHP6mS5xoB2_V-BWEw@mail.gmail.com>
Subject: Run multiple parallel instances of gdb
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Hi everybody,

I would like to sort crashes found by fuzzing. So I have around 1000 files
that make my application crashes. I made a small program to run gdb and to
extract backtraces to file. To increase speed, I run parallel instances.

Problem, with parallel instances, my program stopped. It doesn't crashes,
it stops. I have to run "fg" from terminal to continue and it's happening
very often. So actually, I'm running with single thread.

Is it normal ? Did I do something wrong ? If you need more information, I
can give you.

Please find after simple steps to reproduce the case,

Thanks for you advices,

Vincent Le Garrec

[1] : crash program
main.c (in /tmp folder)

int main()
{
  int *t = 0xDEADBEEF;
  *t = 1;
}

Run it and it should crash.

[2] : multiple execution of gdb
loopgdb.cpp

#include <thread>
#include <future>
#include <vector>
#include <functional>
#include <unistd.h>
#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>

void run_gdb()
{
  pid_t child_pid = fork();
  std::cout << "run" << std::endl;
  if (child_pid != 0)
  {
    pid_t wait_pid;
    {
      wait_pid = waitpid(child_pid, nullptr, WNOHANG);
      sleep(1);
    }
    while(wait_pid == 0);
  }
  else
  {
    execlp("/usr/bin/gdb", "-batch-silent", "-ex", "run", "-ex", "set
logging overwrite on", "-ex", "set logging on", "-ex", "set pagination
off", "-ex", "handle SIG33 pass nostop noprint", "-ex", "backtrace full",
"-ex", "set logging off", "-ex", "quit", "--args", "/tmp/main", nullptr);
  }
}

int main()
{
  int nthreads = std::thread::hardware_concurrency();
  std::vector<std::future<void>> threads(nthreads);
  for (size_t t = 0; t < nthreads; t++)
  {
    threads[t] = std::async(std::launch::async, std::bind(
        []() {
          while(true)
          {
            run_gdb();
          }
        }));
  }
  for (std::future<void> & t : threads)
  {
    t.get();
  }

}

Run it (g++ loopgdb.cpp -o loopgdb -lpthread) and you will see that the
program in command-line will stopped.

[3] gdb --version
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