testdebugcrash.c

/*
* Sample code to test what happens when a debugger sets a breakpoint.
*
* Written for #WeekendDevPuzzle dated 2022-01-15
* https://twitter.com/amodm/status/1482206401103818756
*
* Steps to execute this:
*   # compile program
*     gcc -o testdebugcrash testdebugcrash.c
*   # run program
*     ./testdebugcrash
*   # (another terminal) attach debugger and set breakpoint (if gdb)
*     gdb -p $(ps aux | grep testdebugcrash | grep -v grep | awk '{ print $2 }')
*       # inside debugger
*         break crashtest
*         continue
*   # (another terminal) kill debugger without giving it a chance to clean up
*     kill -9 $(ps aux | grep gdb | grep -v grep | awk '{ print $2 }')
*   # observe if testdebugcrash ends cleanly, or crashes
*/
#include <stdio.h>
#include <stdint.h>
#include <unistd.h>

// this is the address where we set breakpoint
void crashtest() {
    printf("weird! we seemed to have survived the crash test\n");
}

// simple helper to print the 1st 4 bytes at breakpoint site
// in serial order
void print_bytecode() {
    uint8_t *bytecodes = (uint8_t*)*crashtest;
    printf("function bytecode 0x%02x, 0x%02x, 0x%02x, 0x%02x\n",
            bytecodes[0] & 0xff,
            bytecodes[1] & 0xff,
            bytecodes[2] & 0xff,
            bytecodes[3] & 0xff
    );
}

int main() {
    printf("starting. please attach & set a breakpoint\n");
    print_bytecode();

    // let's sleep for enough time to allow debugger to attach
    // and set a breakpoint
    sleep(30);

    // let's see if bytecode at the breakpoint site, changed
    printf("checking for bytecode again\n");
    print_bytecode();

    printf("invoking breakpoint-ed function\n");
    crashtest();

    return 0;
}