Signal Handling

From: Sontu (abhaywit_at_gmail.com)
Date: 01/31/05 

Date: 31 Jan 2005 08:40:23 -0800

Hi all,

Consider the following code:

#include<signal.h>
#include<stdio.h>
#include<sys/mman.h>

void handler(int sig)
{
printf("abhay: caught SIGSEGV\n\n");
}

void func(char *buffer)
{

unsigned int start=0;
//to make the starting address in mprotect as page aligned
asm("andl $-4096, %esp");
asm("movl %%esp, %0":"=r"(start));

//raise(SIGSEGV);

printf("Mprotect worked:
%d\n\n",mprotect((void*)start,4096,PROT_READ));

buffer[3]='c';

printf("Mprotect worked:
%d\n\n",mprotect((void*)start,4096,PROT_WRITE|PROT_READ|PROT_EXEC));
}

int main(void)
{
char buffer[10];

if( signal(SIGSEGV, handler)== SIG_ERR )
printf("problem installing new signal handler\n\n");

func(buffer);

printf("into main\n\n");

return 0;
}

My program makes the previous frame as write protected, thus when i am
in func() and i will try to write into the buffer that is allocated in
main(), it will generate SIGSEGV signal that is handled by my handler()

There is something wierd going on that i am not able to understand:

1. If both the mprotect functions are uncommented and i try to write
into the buffer[3]='c', SIGSEGV is generated
handler is called and it starts printing "abhay: caught SIGSEGV"
continuously on the screen until stack overflows. But it should have
printed it once and should have returned back into the func()

2. But if i comment both the mprotect and uncomment "raise(SIGSEGV)" to
generate the SIGSEGV signal explicitly, then this doesn't happen.

I am running the program on RedHat linux 8.0 and using GCC compiler.
Can anyone help me out?

Thanks.

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