/*
 *  (c) 2004 National Research Council Canada
 *      Written by: <Serguei.Patchkovskii@nrc.ca>
 *
 *  This program is free software; you can redistribute it and/or
 *  modify it under the terms of the GNU General Public License
 *  as published by the Free Software Foundation; either version 2
 *  of the License, or (at your option) any later version.
 * 
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  This program is a set-uid wrapper, which will set
 *  mapped_base of a task to a suitably low value, and
 *  exec the production task. For many programs, this
 *  will increase the amount of available memory visible
 *  with malloc or ALLOCATE by about 800Mbytes.
 *
 *  CAUTION: Changing task's mapped base may cause codes
 *  using sbrk() for memory allocation to malfunction!
 *
 *  If is intended for reasonably recent Linux kernels 
 *  (e.g. 2.4.21 works, but 2.4.19 does not).
 */
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>

static char	*new_task_base = "0x10000000" ;

int
main( int argc, char *argv[] )
{
	uid_t	euid, uid, gid ;
	int	len, wrlen ;
	int	fd, err ;
	char	*shell ;

/*
 *  Warn if not root
 */	
euid = geteuid() ;
if (euid!=0) {
  fprintf(stderr,"Changing task mapped base required special privileges.\n") ;
  fprintf(stderr,"Since I am running as euid = %d, change will likely fail.\n",euid) ;
  }
/*
 *  Change mapped_base
 */
fd = open("/proc/self/mapped_base",O_WRONLY) ;
if (fd<0) {
  err = errno ;
  fprintf(stderr,"Unable to open /proc/self/mapped_base for write access.\n") ;
  fprintf(stderr,"System error code was %d (%s)\n",err,strerror(err)) ;
  fprintf(stderr,"Task mapped base will not be changed.\n") ;
  }
if (fd>=0) {
  len = strlen(new_task_base) ;
  wrlen = write(fd,new_task_base,len) ;
  err = (wrlen<0)?errno:0 ;
  if (len!=wrlen) {
    fprintf(stderr,"Error writing new task base to /proc/self/mapped_base.\n") ;
    fprintf(stderr,"Requested transfer length = %d, actual transfer length = %d\n",
            len, wrlen) ;
    fprintf(stderr,"System error code was %d (%s)\n",err,strerror(err)) ;
    }
  close(fd) ;
  }
/*
 *  Drop special privileges
 */
uid = getuid() ; 
gid = getgid() ;
if ( setresgid(gid,gid,gid) != 0 ) {
  err = errno ;
  fprintf(stderr,"FATAL: Unable to clear effective/saved group id.\n") ;
  fprintf(stderr,"System error code was %d (%s)\n",err,strerror(err)) ;
  exit(EXIT_FAILURE) ;
  }
if ( setresuid(uid,uid,uid) != 0 ) {
  err = errno ;
  fprintf(stderr,"FATAL: Unable to clear effective/saved user id.\n") ;
  fprintf(stderr,"System error code was %d (%s)\n",err,strerror(err)) ;
  exit(EXIT_FAILURE) ;
  }
/*
 *  Exec the actual program, or shell if there were no arguments
 */
if (argc<2) {
  shell = getenv("SHELL") ;
  if (!shell) shell = "/bin/sh" ;
  fprintf(stderr,"Starting interactive shell %s\n",shell) ;
  fprintf(stderr,"Type 'exit' to return\n") ;
  execl (shell,shell,NULL) ;
  }
else {
  shell = argv[1] ;
  execv( argv[1], argv + 1 ) ;
  }
/* 
 *  If we end up here, exec() failed
 */
err = errno ;
fprintf(stderr,"FATAL: Unable to run %s\n",shell) ;
fprintf(stderr,"System error code was %d (%s)\n",err,strerror(err)) ;
exit(EXIT_FAILURE) ;
}