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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LEARN ABOUT NETBSD

stack_shrink

STACK(9)						   BSD Kernel Developer's Manual						  STACK(9)

NAME

     STACK -- stack macros

SYNOPSIS

     #include <sys/param.h>

     type
     STACK_ALLOC(sp, size);

     type
     STACK_MAX(sp, size);

     type
     STACK_ALIGN(sp, bytes);

     type
     STACK_GROW(sp, size);

     type
     STACK_SHRINK(sp, size);

DESCRIPTION

     A stack is an area of memory with a fixed origin but with a variable size.  A stack pointer points to the most recently referenced location
     on the stack.  Initially, when the stack has a size of zero, the stack pointer points to the origin of the stack.	When data items are added
     to the stack, the stack pointer moves away from the origin.

     The STACK_ALLOC() macro returns a pointer to allocated stack space of some size.  Given the returned pointer sp and size, STACK_MAX() returns
     the maximum stack address of the allocated stack space.  The STACK_ALIGN() macro can be used to align the stack pointer sp by the specified
     amount of bytes.

     Two basic operations are common to all stacks: a data item is added (``push'') to the location pointed by sp or a data item is removed
     (``pop'') from the stack.	The stack pointer must be subsequently adjusted by the size of the data item.  The STACK_GROW() and STACK_SHRINK()
     macros adjust the stack pointer sp by given size.

     A stack may grow either up or down.  The described macros take this into account by using the __MACHINE_STACK_GROWS_UP preprocessor define.

SEE ALSO

     param(3), queue(3)

BSD
								   April 8, 2011							       BSD