sigsegv.c

sigsegv.c

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <ucontext.h>
#include <unistd.h>

void sigsegv_handler(int signum, siginfo_t *info, void *context);

int main() {

    struct sigaction sa;

    sa.sa_flags  = SA_SIGINFO;
    sa.sa_sigaction  = sigsegv_handler;
    sigemptyset(&sa.sa_mask);

    if (sigaction(SIGSEGV, &sa, NULL) == -1) {
        perror("failed to install SIGSEGV handler");
        return 1;
    }

    int *ptr = NULL;
    *ptr = 42;

    return 0;
}

void sigsegv_handler(int signum, siginfo_t *info, void *context) {
    ucontext_t *uc = (ucontext_t *)context;
    unsigned char *fault_pc;

#if defined(__aarch64__)
    fault_pc = (unsigned char *)uc->uc_mcontext.pc;
#else
    fault_pc = (unsigned char *)uc->uc_mcontext.gregs[REG_RIP];
#endif

    const char msg[] = "SIGSEGV, pc = 0x";
    write(STDERR_FILENO, msg, sizeof(msg)-1);

    char hex[20];
    hex[0] = '0';
    hex[1] = 'x';
    for (int i = 0; i < 16; i++) {
        int nibble = ((unsigned long long)fault_pc >> (60 - 4*i)) & 0xF;
        hex[i+2] = nibble < 10 ? '0' + nibble : 'a' + nibble - 10;
    }
    hex[18] = '\n';
    hex[19] = '\0';
    write(STDERR_FILENO, hex, 19);

    for (int i = 0; i < 64; i += 1) {
        printf("%02x ", fault_pc[i]);
    }
    printf("\n");

    printf("offset from main: 0x%ld\n", fault_pc - (unsigned char*) main);

    signal(SIGSEGV, SIG_DFL);
    raise(SIGSEGV);
}