Unix-Linux 2. Unix is supposed to leave room in the process table for a superuser process that could be used to kill errant processes.

Transcription

1 Unix-Linux 2 fork( ) system call is successful parent suspended child created fork( ) returns child pid to parent fork( ) returns zero value to child; zero is the pid of the swapper/scheduler process both processes placed on ready queue another process selected by scheduler fork( ) system call fails fork( ) returns -1 to parent sets the global variable errno o number of processes created exceeds limit of the process table o lack of available swap space to create new process Unix is supposed to leave room in the process table for a superuser process that could be used to kill errant processes. On many systems, this feature fails, hence it is possible to create processes to fill the table, precluding the ability of a superuser process that can kill the errant process. write( ) not buffered printf( ) buffered sleep( ) specified number of seconds daemon processes lpd inetd routed line printer internet services network routing exec text, data & stack segments are overlaid by a new process u-area is not changed if successful o exec( ) call does not return a value o program code begins execution at main( ) signals caught by the process before the exec( ) call are reset to their default actions profiling is turned off SUID is set EUID & EGID are set accordingly

2 shell level (system) command $cat file1.txt > file2.txt shell parses command line, i.e., segments the line into valid tokens shell calls fork process & generates child process shell closes stdout & opens file2.txt; mapping it to the stdout of the child shell issues execve( ) system call which overlays the child code with the cat code shell issues wait( ) system call child code executes child code returns a status value via the wait system call issued by the parent shell resumes execution, i.e., displays prompt the command $echo $status will return the status value parent resume execution parent can process the status value to determine subsequent actions shell level (system) command $exec command [arguments] shell code is replaced by code in the command program exec system call <unistd.h> int execl arguments are in a list format int ececv -- arguments are in a vector, i.e., array, format int exec?e int execle int execve int exec?p int execlp int execvp an environment variable list is provided in the char *const envp[ ] arguement current PATH string s to be used int execlp ( const char *file, const char *arg0,, const char *argn, (char *) NULL ); number of arguments are known in advance const char *file pointer to the file which contains the overlay code the indicated file must be marked as executable PATH variable must be pre-appended with the relevant file names to preclude the execution of the wrong code const char *arg0,, const char *argn are pointers to the variables that are to be passes to the called program const char *arg0 points to the name of the program to be executed (without a path) (char *) NULL call to NULL cast as a character pointer #include <stdio.h> #include <unistd.h> #include <stdlib.h> int main(int argc, char *argv[ ]) { if(argc>1) { execlp( /bin/cat, cat, argv[1], (char *) NULL); perror( exec failure: ); fprintf(stderr, Usage: %s text_file\n, *argv);

3 int execvp ( const char *file, char *const argv[ ]); char *const argv[ ] pointer to argv[ ] argv[ ] may o contain the values of the process before the exec call o may be provided with new values, with the last value being a NULL pointer #include <stdio.h> #include <unistd.h> #include <stdlib.h> int main(int argc, char *argv[ ]) { if(argc>1) { execvp(argv[1], *argv[1]); perror( exec failure: ); compiled as service_proxy test.txt Now is the time for all good UNIX programmers to pay close attention as magic is about to happen (or not)! #include <stdio.h> #include <unistd.h> #include <stdlib.h> int main( void ) { static char *new_argv[ ] = { cat, test.txt, (char *) 0 ; execvp( /bin/cat, new_argv ); perror( exec failure: ); compiled as service_proxy2 $service_proxy cat test.txt Now is the time for all good UNIX programmers to pay close attention as magic is about to happen (or not)! $service_proxy cat n test.txt 1 Now is the time for all good 2 UNIX programmers to pay 3 close attention as magic is 4 about to happen (or not)! $service_proxy2 Now is the time for all good UNIX programmers to pay close attention as magic is about to happen (or not)! spawn( ) system call fork( ) exec( ) bundled into one system call limited shell program Unix page 66 Linux page 95-96

4 Process Termination process issues exit or _exit call process issues a return in the main function process executes the last instruction in main and the main function is closed <stdlib> void exit ( int status ) status values o normal termination 0, otherwise non-zero o EXIT_SUCCESS o EXIT_FAILURE exit( ) atexit( ) function registered no yes execute the registered functions in reverse order of registration int atexit ( void (*func) (void)); each invocation of atexit registers one function returns 0 if successful otherwise returns 1 _cleanup( ) _exit( ) <unistd.h> void _exit( int status ) // status value is made available to the parent process via the wait( ) command Housekeeping Operations open file descriptors are flushed and closed parent process is notified via SIGCHLD signal status information is o returned to the parent process if it has executed a wait( ) command o held by the kernel until the parent process has executed a wait( ) command o available via the $echo $status command all child processes of the terminating process have their PPID s set to the PID of init, i.e., 1 terminating process is a group leader group members are sent SIGHUP/SIGCONT signals shared memory segments & semaphore references are readjusted accounting records are written to the accounting file

5 Waiting on Processes <sys/types.h> <sys/wait.h> pid_t wait(int *stat_loc); wait ( ) block parent process no child process exists no return -1 set errno yes child terminated yes return status & child s PID Status Information pointer int *stat_loc successful termination byte 0 contains 0 byte 1 contains exit code terminated due to uncaught signal byte 0 contains signal # leftmost bit set to 1 core file produced byte 1 contains 0 Zombie Processes child process that terminates but a wait( ) is never issued by the parent process occupy a slot in the process table process status command, e.g., $ps alx or $ps el returns a Z mark cannot be terminated with $kill -9 <pid> normally terminates when the parent process terminates, i.e., resources are recovered orphan processes are inherited by init which issues a wait( ) for the process on rare occasions, a system reboot is necessary to clear the process table wstat MACROS used to extract status information

6 Waiting on Processes (again) <sys/types.h> <sys/wait.h> pid_t waitpid( pid_t pid, int *loc_stat, int options ); pid_t pid o pid < -1 any child process whose GPID == abs(pid) o pid == -1 similar to wait( ) o pid == 0 any child process whose GPID == process caller s pid o pid > 0 child process with PID == pid int options o options == 0 don t care o options == bitwise or of flag values (see Gray Unix page 78, Linux page 113 BSD Compatibility Library Functions wait3( ) wait4( )