Handling interrupts with signals on Linux

linux_handling_interrupts_with_signals.c

#include <stdio.h>
#include <stdlib.h>     // exit
#include <string.h>     // memset
#include <unistd.h>     // sleep
#include <signal.h>

// Enum for when the program is 32 bits
// Source: https://sites.uclouvain.be/SystInfo/usr/include/sys/ucontext.h.html
enum
{
    REG_GS = 0,
    REG_FS,
    REG_ES,
    REG_DS,
    REG_EDI,
    REG_ESI,
    REG_EBP,
    REG_ESP,
    REG_EBX,
    REG_EDX,
    REG_ECX,
    REG_EAX,
    REG_TRAPNO,
    REG_ERR,
    REG_EIP,
    REG_CS,
    REG_EFL,
    REG_UESP,
    REG_SS
};

/*
// Enum for when the program is 64 bits
// Source: https://code.woboq.org/userspace/glibc/sysdeps/unix/sysv/linux/x86/sys/ucontext.h.html
enum
{
    REG_R8 = 0,
    REG_R9,
    REG_R10,
    REG_R11,
    REG_R12,
    REG_R13,
    REG_R14,
    REG_R15,
    REG_RDI,
    REG_RSI,
    REG_RBP,
    REG_RBX,
    REG_RDX,
    REG_RAX,
    REG_RCX,
    REG_RSP,
    REG_RIP,
    REG_EFL,
    REG_CSGSFS,        // Actually short cs, gs, fs, __pad0.
    REG_ERR,
    REG_TRAPNO,
    REG_OLDMASK,
    REG_CR2
};*/

int count = 0;

void dumpbytes(unsigned char *data, int count)
{
    int i;

    printf("\n");

    for (i = 0; i < count; i++)
    {
        if (i % 16 == 0)
        {
            printf("\n");
        }

        printf("%02X ", data[i]);
    }

    printf("\n\n");
}

// http://courses.cms.caltech.edu/cs124/lectures-wi2016/CS124Lec15.pdf
void singal_handler(int sig, siginfo_t *sinfo, void *ctxt)
{
    count++;

    ucontext_t *context = (ucontext_t *)ctxt;

    // https://github.molgen.mpg.de/git-mirror/glibc/blob/master/sysdeps/unix/sysv/linux/x86/sys/ucontext.h#L152

    printf("EAX: %X\n", context->uc_mcontext.gregs[REG_EAX]);
    printf("EBX: %X\n", context->uc_mcontext.gregs[REG_EBX]);
    printf("ECX: %X\n", context->uc_mcontext.gregs[REG_ECX]);
    printf("EDX: %X\n", context->uc_mcontext.gregs[REG_EDX]);

    printf("ESI: %X\n", context->uc_mcontext.gregs[REG_ESI]);
    printf("EDI: %X\n", context->uc_mcontext.gregs[REG_EDI]);

    printf("EBP: %X\n", context->uc_mcontext.gregs[REG_EBP]);
    printf("ESP: %X\n", context->uc_mcontext.gregs[REG_ESP]);

    // address of instruction that triggered the signal
    printf("EIP: %X\n", context->uc_mcontext.gregs[REG_EIP]);

    sleep(1);

    printf("±±±±±±±±±±±±±±±±±±±±±±±±±±±±\n");

    if (count >= 3) {
        // Dump a few bytes
        dumpbytes(context->uc_mcontext.gregs[REG_EIP], 32);

        // Skip over the 2 bytes that caused the interrupt
        context->uc_mcontext.gregs[REG_EIP] += 2;
    }
}

int main()
{
    struct sigaction act;
    memset(&act, 0, sizeof(struct sigaction));
    sigemptyset(&act.sa_mask);

    act.sa_sigaction = singal_handler;
    act.sa_flags = SA_SIGINFO;

    if (sigaction(SIGSEGV, &act, NULL) < 0)
    {
        printf("sigaction() error! couldn't set handler.");
        exit(1);
    }

    printf("hello world\n");

    // interrupt, causes a segmentation fault
    asm(".byte 0xCD");
    asm(".byte 0xFF");

    printf("skipped the bytes\n");

    sleep(1);

    return 0;
}