How do I attach gdbserver to a process at startup? - c++

Question

I am using gdbserver to debug a remote process. I am able to attach gdbserver to the process after it has launched and is waiting for input.
However, I want to attach gdbserver to the process while it is being launched. The process is launched via a shell script and I cannot change how this process is being launched i.e. I cannot modify the shell script to launch the process via a call to gdbserver.
How do I attach gdbserver to this process as soon as it launches?
Edit: I am able to create a wait loop at the start of main(). For example a loop which waits till it finds a file at a predetermined location :
#include <unistd.h>
int main() {
while( access("/home/username/CONTINUE", F_OK) == -1)
sleep(1);
/*
...all the rest of main()
*/
return 0;
}
We can attach gdbserver while the process is busy with this loop, set breakpoints as required and say touch /home/username/CONTINUE to exit the loop. But this requires us to be able to access the source code, compile the binary and place it on the target machine. I am looking for a better, easier way than this.

Answer 1

But this requires us to be able to access the source code1, compile the binary and place it on the target machine. I am looking for a better, easier way than this.
It looks like you are working on a linux / unix like remote operating system.
If you have admin access to the remote system, the simplest way I could think of is to rename the original executable, and replace that with a shell script named like the original executable, that starts the now renamed under control of gdbserver.
Something like (assumed executable /usr/bin/foo) at the target machine:
root:# cd /usr/bin
root:# mv foo foo_
root:# echo "#!/bin/sh\ngdbserver /dev/com1 foo_ foo.txt" > foo
root:# chmod a+x foo
As it says in the gdbserver man(1) page:
This tells gdbserver to debug foo_ with an argument of foo.txt, and
to communicate with GDB via /dev/com1. gdbserver now waits patiently
for the host GDB to communicate with it.
Another way I could think of without modifying the original process at all, could be a little program that monitors changes in the /dev/proc directory(ies), and attaches gdbserver at such event with the associated pid.
Though it's kind of luck then, if gdbserver is attached, before that process already schedules to main().
1You should have acces to the source code anyways for reasonable debugging with gdb. Though there are cases where you can just get with the (dis-)assembly code as well.