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From:	Charles Sandmann <sandmann AT clio DOT rice DOT edu>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: protecting program memory
Date:	Sun, 25 Mar 2007 23:27:27 CDT
Organization:	Rice University, Houston, TX
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> # Is there an easy way to protect the memory used to store the
> # program op codes from a rogue memory write from within the
> # application? I have a large (100,000+ line) DJGPP
> # application which sometimes crashes with SIGILL - It would
> # seem the program is cannibalizing itself! In order
> # to find how this is happening, I would like to protect the
> # entire block of memory that contains the application code
> # so that an exception occurs at the point the corruption occurs
> # rather than the point that the corrupted code is executed.

> # I guest I need to make the memory block read only, but I am
> # not sure how to do that.

> Not without DPMI 1.0, you can't.

Correct.  But CWSDPMI implements enough of the DPMI 1.0 specification
that you can make pages readonly.  This is the same API which is used
to make the "null" page non-mapped to catch all references to null pointers.

> You can get a pointer to the beginning of code and its size
> by applying some mild abuse of GCC and the linker map:

>    extern char*               _text  asm(".text");
>    extern char*               _etext asm("etext");
>    static char*               __my_progstart   = NULL;
>    static size_t              __my_progsize    = 0;
>    __my_progstart = (char*) &_text;
>    __my_progsize = (&_etext - &_text) - sizeof(void*);

There are some non-clean parts of the libc which have writeable code
sections, so you will need to identify those and keep them as writeable.
If I remember correctly they are in the exception handling code and
maybe the stub itself.

I don't remember completely, but you should check <sys/mman.h> for the
mprotect() function.

It's been a long time, but I remember supporting a memory allocation
package which protected it's control structures and fencing it's allocated
memory using these routines.

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