Send EOF with to process with subprocess python

Question

I am doing some CTF for fun and I got stuck, I run binary with python script with subprocessing, The problem is the script need to send to the read function in the binary empty input to not change the current value (if there is a way to make the read operation fail and return -1, it will be cool to know also how to do)(if there is a way to make the read operation fail and return -1, it will be cool to know also how to do), and continue the running without closing the pipes or something.

short example to illustrate the problem: python script

import subprocess
import signal
import time
import os

def read_all(pipe, timeout=0.5):
    start = time.time()
    content = b''
    while True:
        line = pipe.readline()
        if line != b'':
            content += line
        if time.time() - start >= timeout:
            return content.decode('utf-8')

ps = subprocess.Popen('./test', stdin=subprocess.PIPE, stdout=subprocess.PIPE)
os.set_blocking(ps.stdout.fileno(), False)

print(read_all(ps.stdout))
ps.stdin.write(b'')
ps.stdin.flush()
print(read_all(ps.stdout))

test.c:

#include<stdio.h>
#include<stdlib.h>

#define some_random_value 0

int main()
{
        int num = some_random_value;
        setvbuf(stdout, NULL, _IONBF, 0);

        printf("Communication with the python code\n");
        printf("Enter Something: \n");
        printf("%d bytes read\n", read(0, &num, 0x4));

        if (num == some_random_value)
                printf("SUCCESS\n");
        if (num != some_random_value)
                printf("FAILED\n");

        //continue chatting, input, output etc...
        return 0;
}

In this example the binary want continue the execution pass the read function so it's not good

Answer 1

You send EOF by closing the pipe:

ps.stdin.close()

I don't think there's any way to trigger an error when reading from the pipe. Errors from read() are usually due to problems in the parameters. The only situation I can think of where a sender can trigger read errors is with TCP sockets, if the sender sends a RST segment before the reader tries to read (I think this will cause an ENOTCONN error).