[Mono-list] help needed

[Soeren Sandmann]

Dietmar Maurer <dietmar@ximian.com> writes:

> One theory is that the handler returns to the same instruction how
> caused the signal, so the same instruction raises the same signal again
> and again?
> 
> If so, how can I modify the IP to point to a new location?

I don't think you can do that. 

You'll need to make sure that the signal won't be raised again, for
example by changing the value of b so that it won't divide by zero
again.  

If the code at IP is writable, you could also modify the
instruction to be something harmless, a jump for instance.

Here is an example that will raise the signal once and modify the
value of b.


#include <stdio.h>
#include <signal.h>

static int *b_addr;

static void
fp_signal_handler (int sig, siginfo_t *si, struct sigcontext *ctx)
{
    printf ("TEST %d %p %p\n", si->si_code, si->si_addr, ctx->eip);

    *b_addr = 2;
    
    return;
}

int
main ()
{
    int a = 5, b = 0, c;
    struct sigaction sa, sao;

    b_addr = &b;
    
    sa.sa_sigaction = fp_signal_handler;
    sigemptyset (&sa.sa_mask);
    sa.sa_flags = SA_SIGINFO;
    sigaction (SIGFPE, &sa, NULL);
    
    c = a/b;

    printf ("c is %d\n", c);
    return 0;
}