exam_1.cc

      // Solution to Exam 1, Question 6
      
      #include <stdio.h>
      #include <sys/stat.h>
      #include <sys/types.h>
      #include <unistd.h>
      int
      main( int argc, char *argv[], char *envp[] )
      {
        const char *args[] =
        {
          { "/bin/ls" },
          { "-l"      },
          { "-a"      },
          { "-F"      },
          { 0         },
        };
        const char *env[] =
        {
          { "SHELL=/bin/sh" },
          { "USER=vickery"  },
          { 0               },
        };
      
        struct stat sb;
        if ( -1 == stat( "/bin/ls", &sb ) )
        {
          perror( "stat" );
          exit(1);
        }
        uid_t user  = geteuid();
        gid_t group = getegid();
      
        if ( 
             ( (user  == sb.st_uid) && (sb.st_mode & 0100) == 0100 )  ||
             ( (group == sb.st_gid) && (sb.st_mode & 0010) == 0010 )  ||
             (                         (sb.st_mode & 0001) == 0001 )
           )
        {
          execve( args[0], (char*const*)args, (char*const*)env );
          perror( "exec" );
          exit( 1 );
        }
        fprintf( stderr, "permission denied\n" );
        exit( 1 );
      }