How to prevent VI from getting a shell?

[Tom Armistead]

Boy - am I brave posting this - or was it stupid? I forget...

Here is a real small C program that will batch the /usr/bin/vi, creating
an output file ./vi.new that will not have the ':set shell' command.
What it does is look for 'shell' and replace it with 5 spaces. So,
':set shell' gives an error...

Anyway, you can move vi to vi.orig and remove execution priv's from it
and move vi.new to /usr/bin/vi.

p.s. This works on System V machines (not tested on anything else).

-----------------------------CUT HERE----------------------------------
/*************************************************************************
** fixvi.c
** Description:
**	This program will create the executable file specified by VI_OUT
**	and remove the ':set shell' command from it.
** Disclaimer:
**	This program is hereby released as PUBLIC DOMAIN.
**	It comes with ABSOLUTELY NO warranty...
**************************************************************************/

#include <sys/types.h>
#include <sys/stat.h>
#include <malloc.h>
#include <string.h>
#include <fcntl.h>

#define	VI_IN	"/usr/bin/vi"		/* original version of vi */
#define	VI_OUT	"./vi.new"		/* new version (with shell) */

main()
{
    struct stat sbuf;			/* to get size of file VI_IN */
    char *read_buf=(char *)0;		/* read VI_IN into here */
    register char *bufptr;		/* pointer into read_buf */
    register rdlen;			/* read return value */
    register int i;			/* You know? */
    int fdin=(-1), fdout=(-1);		/* file descriptors for read/write */

    /***********************************************************************
    ** Stat VI_IN to get it's size, the open it for reading.
    ** Create VI_OUT (will contain modified version of VI_IN
    ************************************************************************/

    if( stat( VI_IN, &sbuf ) != -1 &&
	(fdin=open( VI_IN, O_RDONLY )) != -1 &&
	(fdout=open( VI_OUT, O_WRONLY|O_CREAT|O_TRUNC )) != -1 )
    {
	/********************************************************************
	** Malloc area large enough to hold entire file VI_IN.
	** Read entire file VI_IN into the malloc'd buffer.
	*********************************************************************/

	if( (read_buf=malloc( (unsigned)sbuf.st_size+1 )) != (char *)0 &&
	    (rdlen=read( fdin, read_buf, (unsigned)sbuf.st_size )) ==
								sbuf.st_size )
	{
	    /*****************************************************************
	    ** Look through buffer for all occurrences of the string 'shell'
	    ** and replace each one with 5 spaces.
	    ******************************************************************/

	    for( bufptr=read_buf; bufptr < read_buf+rdlen; bufptr++ )
		if( *bufptr == 's' && !strncmp( bufptr, "shell", 5 ) )
		    for( i=0; i<5; i++ )
			*(bufptr++) = ' ';

	    /*****************************************************************
	    ** Write out modified version of VI_IN to VI_OUT. This will be
	    ** the vi that has no 'set shell' command.
	    ******************************************************************/

	    if( write( fdout, read_buf, rdlen ) != rdlen )
		perror( "write" );
	}/*end if malloc*/
    }/*end if open()*/
    else
	perror( "open" );

    if( read_buf != (char *)0 )	free( read_buf );
    if( fdin  != -1 )		close( fdin );
    if( fdout != -1 )		close( fdout );

    chmod( VI_OUT, 0555 );	/* chmod +rx-w VI_OUT */

}/*end main*/

/*end fixvi.c*/

-- 
-------------------------------
{uunet,smu,ames}!sulaco!ozdaltx!toma      (Tom Armistead @ Garland, Texas)
{mic,void,egsner}!ozdaltx!toma