Re: Run multiple parallel instances of gdb

[Simon Marchi <simon dot marchi at polymtl dot ca>]

On 2018-03-11 16:15, LE GARREC Vincent wrote:
> 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

Hi Vincent,

Do you get a "suspended (tty output)" message?  If so, what you have is 
a background process trying to output on the terminal while the terminal 
has the "tostop" flag set.  A simpler case to reproduce it is:

$ gdb -batch -ex run --args /bin/echo salut &
[1] 28223
$
[1]  + 28223 suspended (tty output)  gdb -q -batch -ex run --args 
/bin/echo salut

I don't understand why though, because my terminal does not have 
"tostop" enabled:

$ stty
speed 38400 baud; line = 0;
-brkint -imaxbel iutf8

So something in the process probably sets that flag...  Anyway, one way 
to get around it is to change the terminal for the newly created 
inferiors, for example to /dev/null if you don't need them to do I/O on 
the terminal:

$ gdb -batch -ex "tty /dev/null" -ex run --args /bin/echo salut &
[1] 28276
$ [Inferior 1 (process 28285) exited normally]

[1]  + 28276 done       gdb -q -batch -ex "tty /dev/null" -ex run --args 
/bin/echo salut

One comment below:

> [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);

You are missing the "do" from your do/while, which results in an 
infinite loop here (if wait_pid is 0).

Simon