example2.c

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

#include <sys/ptrace.h>
#include <sys/reg.h>
#include <sys/syscall.h>
#include <sys/wait.h>
#include <unistd.h>

void wait_sigtrap() {
    int status;
    wait(&status);
    if (WIFEXITED(status)) exit(0);
}

int main() {
    pid_t child = fork();
    if (child == 0) {
        ptrace(PTRACE_TRACEME, 0, NULL, NULL);
        const char* file = "./pi.py";
        if (execl(file, file, "1000", NULL) < 0) {
            perror("execl");
            return 1;
        };
    } else {
        pid_t pid = child;
        wait_sigtrap(); // there will be an initial stop after traceme, ignore
                        // it

        ptrace(PTRACE_SYSCALL, pid, 0, 0); // wait for another
        for (;;) {
            // detect enter, get syscall no
            wait_sigtrap();
            long no = ptrace(PTRACE_PEEKUSER, pid, 8 * ORIG_RAX, 0);

            if (no != SYS_getrandom) {
                ptrace(PTRACE_SYSCALL, pid, 0, 0); // wait for another
                wait_sigtrap();                    // wait for exit
            } else {
                // getrandom
                long bufptr = ptrace(PTRACE_PEEKUSER, pid, 8 * RDI, 0);
                long buflen = ptrace(PTRACE_PEEKUSER, pid, 8 * RSI, 0);

                printf("getrandom request: 0x%016lx 0x%016lx\n", bufptr,
                       buflen);
                fflush(stdout);

                ptrace(PTRACE_SYSCALL, pid, 0, 0); // wait for another
                wait_sigtrap();                    // wait for exit

                long ret = ptrace(PTRACE_PEEKUSER, pid, 8 * ORIG_RAX, 0);
                if (ret < 0) {
                    printf("Syscall %ld exited with an error code %ld, not touching it",
                           no, ret);
                    fflush(stdout);
                } else {
                    long ind = 0;
                    while (ind < buflen) {
                        ptrace(PTRACE_POKEDATA, pid, bufptr + ind, 0);
                        ind += sizeof(long);
                    }
                }
            }

            ptrace(PTRACE_SYSCALL, pid, 0, 0); // wait for another
        }
    }
    return 0;
}