Repairing a SIGBUS from an OOB read of an mmap'd file

mmap_sigbus.c

#include <assert.h>
#include <stdio.h>
#include <stdint.h>
#include <stddef.h>
#include <stdlib.h>

#include <fcntl.h>
#include <signal.h>
#include <unistd.h>

#include <sys/mman.h>
#include <sys/stat.h>

char *ptr;
int fd;
const size_t range = 1024*1024*1024;

volatile sig_atomic_t resized = 0;

static void handler(int sig, siginfo_t *si, void *unused) {
    char *read_addr = si->si_addr;
    ptrdiff_t diff = read_addr - ptr;

    // if it's within the bounds of our mapped range..
    if (diff >= 0 && diff <= range) {
        // if the file is already big enough, panic!
        off_t size = lseek(fd, 0, SEEK_END);
        if (diff <= size || -1 == size) {
            exit(67);
        }

        // enlarge the file so the read can succeed
        if (ftruncate(fd, diff)) {
            exit(68);
        }

        // report error
        resized = 1 + size;
    } else {
        exit(69);
    }
}

int main() {
    fd = open("data", O_CREAT | O_RDWR, S_IRUSR | S_IWUSR);
    if (fd < 0) {
        perror("open");
        return 3;
    }

    ptr = mmap(NULL, range, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
    if (-1 == ptr) {
        perror("mmap");
        return 3;
    }

    struct sigaction sa = {};
    sa.sa_flags = SA_SIGINFO;
    sigemptyset(&sa.sa_mask);
    sa.sa_sigaction = &handler;
    assert(-1 != sigaction(SIGBUS, &sa, NULL));


    char found = ptr[9005];
    if (resized) {
        printf("first reading failed, fixing file back to original observed length: %d\n", resized - 1);
        if (ftruncate(fd, resized - 1)) {
            perror("couldn't retruncate");
        }
        return 5;
    } else {
        printf("first read success: %d\n", (int)found);
    }

    char found2 = ptr[90005];
    if (resized) {
        printf("second reading failed, fixing file back to original observed length: %d\n", resized - 1);
        if (ftruncate(fd, resized - 1)) {
            perror("couldn't retruncate");
        }

        return 6;
    } else {
        printf("second read success: %d\n", (int)found2);
    }
}