//-------------------------------------------------------------------
//	visual.c
//
//	This module constitutes a simple 'scientific visualization' 
//	exercise: it creates a '/proc' pseudo-file which lets users
//	display an image for the 'task_union' object, showing sizes
//	and relative locations for the process-descriptor and stack
//	regions, and an indication of the 'garbage' area separating
//	these data-structures.  It was prompted by a question about
//	the possibility, and liklihood, that the kernel stack might
//	expand downward far enough to overwrite some portion of the 
//	process descriptor, thereby corrupting and perhaps crashing
//	the operating system.   
//	
//	programmer: ALLAN CRUSE
//	written on: 12 FEB 2003
//-------------------------------------------------------------------

#define	__KERNEL__
#define	  MODULE  

#include <linux/module.h>	// for init_module() 
#include <linux/proc_fs.h>	// for create_proc_read_entry() 

#define	SUCCESS	0
#define GRANULARITY 8
#define CELLS ( sizeof( union task_union)/GRANULARITY )

static char modname[] = "visual";
static unsigned char glyph[ CELLS ];

static int my_proc_read( char *buf, char **start, off_t off,
					int count, int *eof, void *data );


int init_module( void )
{
	printk( "<1>\nInstalling \'%s\' module\n", modname );

	create_proc_read_entry( modname, 0, NULL, my_proc_read, NULL );

	return	SUCCESS;
}


void cleanup_module( void )
{
	remove_proc_entry( modname, NULL );

	printk( "<1>Removing \'%s\' module\n", modname );
}


MODULE_LICENSE("GPL"); 


static int my_proc_read( char *buf, char **start, off_t off,
					int count, int *eof, void *data )
{
	unsigned char	*cp = (unsigned char *)current;
	int	d_top = sizeof( struct task_struct )/GRANULARITY;
	int	s_top, i, j, len;
	unsigned long	tos;
	
	// use inline assembly-language to get esp-value
	asm(" movl %%esp, %0 " : "=m"  (tos) );
	s_top = (tos & 0x00001FFF)/GRANULARITY;

	// assign character-glyphs indicating zero or nonzero
	for (i = 0; i < CELLS; i++)
		{
		int	k = GRANULARITY*i;	
		unsigned char	total = 0;	
		for (j = 0; j < GRANULARITY; j++) total |= cp[ k + j ];
		glyph[ i ] = ( total ) ? '+' : '-';
		}
	
	// assign character-glyphs for descriptor and stack regions
	for (i = 0; i < CELLS; i++) 
		{
		if ( i < d_top ) glyph[i] = 'D';
		if ( i >= s_top ) glyph[i] = 'S';
		}
	
	// display title, assigned character-glyphs, and the legend	
	len = 0;
	len += sprintf( buf+len, "\n\n\n    A scientific visualization " );
	len += sprintf( buf+len, "of the Linux \'task_union\' object " );
	len += sprintf( buf+len, "(pid=%d)\n", current->pid );
	for (i = 0; i < CELLS; i++)
		{
		int	u = CELLS - (i+1);	
		int	k = GRANULARITY*u;	
		if ( ( i % 64 ) == 0 ) len += sprintf( buf+len, "\n  " );
		len += sprintf( buf+len, "%c", glyph[u] );
		if ( ( i % 64 ) == 63 ) 
			len += sprintf( buf+len, "  %04X", cp+k );	
		}
	len += sprintf( buf+len, "\n\n\n" );

	len += sprintf( buf+len, "\tLegend: " );
	len += sprintf( buf+len, "  \'S\'=stack  " );
	len += sprintf( buf+len, "  \'D\'=descriptor  " );
	len += sprintf( buf+len, "  \'-\'=zero  " );
	len += sprintf( buf+len, "  \'+\'=nonzero  " );
	len += sprintf( buf+len, "\n\n\n" );

	*eof = 1;
	return	len;
}