memory stack problem Post: 80773

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Top Forums Programming memory stack problem Post 80773 by Driver on Thursday 11th of August 2005 12:39:43 PM

08-11-2005

Registered User

Here's what I use to catch accidental variable modifications from other functions:

Code:

#include <sys/types.h>
#include <sys/mman.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

void *
debug_alloc_pages(size_t nbytes) {
        long    psize = sysconf(_SC_PAGESIZE);
        size_t  npages = nbytes / psize;
        int             fd;
        void    *ret;

        if (npages * psize < nbytes) {
                ++npages;
        }

#ifdef MAP_ANON
        ret = mmap(0, npages * psize, PROT_READ|PROT_WRITE,
                MAP_ANON, -1, 0);
        if (ret == MAP_FAILED) {
                perror("mmap");
                exit(EXIT_FAILURE);
        }
#else
        /*
         * Use MAP_ANONYMOUS on HP-UX and mmap() with an 
         * fd for /dev/zero everywhere else
         */
        puts("debug_alloc_pages() does not work on this system");
        exit(EXIT_FAILURE);
#endif
        return ret;
}

Now, instead of writing

char buf[128];

... write

char *buf = debug_alloc_pages(128);

When you're done initializing ``buf'', do

(void) mprotect(buf, sysconf(_SC_PAGE_SIZE), PROT_READ);

In every function that is allowed to modify ``buf'', execute an

(void) mprotect(buf, sysconf(_SC_PAGE_SIZE), PROT_READ|PROT_WRITE);

... when you enter it and

(void) mprotect(buf, sysconf(_SC_PAGE_SIZE, PROT_READ);

... when you return from it.

An invalid write access should now yield a bus error or segmentation fault which will provide you with a core dump from which you can obtain a stack trace showing you which function attempted to modify the data.

Hope this helps

Driver

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sigstack

sigstack(2)							System Calls Manual						       sigstack(2)

Name
       sigstack - set or get signal stack context

Syntax
       #include <signal.h>

       struct sigstack {
       caddr_t	 ss_sp;
       int  ss_onstack;
       };

       sigstack(ss, oss)
       struct sigstack *ss, *oss;

Description
       The  system  call  allows users to define an alternate stack on which signals are to be processed.  If ss is nonzero, it specifies a signal
       stack on which to deliver signals and tells the system if the process is currently executing on that stack.  When a signal's  action  indi-
       cates  its handler should execute on the signal stack (specified with a call), the system checks to see if the process is currently execut-
       ing on that stack.  If the process is not currently executing on the signal stack, the system arranges a switch to the signal stack for the
       duration of the signal handler's execution.  If oss is nonzero, the current signal stack state is returned.

       Signal stacks are not grown'automatically, as is done for the normal stack.  If the stack overflows, unpredictable results may occur.

Return Values
       Upon successful completion, a value of zero (0) is returned.  Otherwise, a value of -1 is returned and errno is set to indicate the error.

Diagnostics
       The system call fails and the signal stack context remains unchanged, if one of the following occurs.

       [EFAULT]       Either ss or oss points to memory that is not a valid part of the process address space.

See Also
       sigvec(2), setjmp(3)

																       sigstack(2)