Process Control - calab.hanyang.ac.krcalab.hanyang.ac.kr/courses/SP_taesoo/08_process.pdf · Carnegie Mellon Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective,

Process Control

1

Carnegie Mellon

2Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective, Third Edition

Definition: A process is an instance of a running program. One of the most profound ideas in computer science Not the same as “program” or “processor”

Process provides each program with two key abstractions: Logical control flow

Each program seems to have exclusive use of the CPU

Provided by kernel mechanism called context switching

Private address space Each program seems to have exclusive use of

main memory. Provided by kernel mechanism called virtual

memory

Processes

CPURegisters

Memory

StackHeap

CodeData

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Multiprocessing: The Illusion

Computer runs many processes simultaneously Applications for one or more users

Web browsers, email clients, editors, … Background tasks

Monitoring network & I/O devices

CPURegisters

Memory

StackHeap

CodeData

CPURegisters

Memory

StackHeap

CodeData …

CPURegisters

Memory

StackHeap

CodeData

Carnegie Mellon


Multiprocessing Example

Running program “top” on Mac System has 123 processes, 5 of which are active Identified by Process ID (PID)

Carnegie Mellon


Multiprocessing: The (Traditional) Reality

Single processor executes multiple processes concurrently Process executions interleaved (multitasking) Address spaces managed by virtual memory system (later in course) Register values for nonexecuting processes saved in memory

CPURegisters

Memory

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

…

Carnegie Mellon



Save current registers in memory

CPURegisters

Memory

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

…

Carnegie Mellon



Load saved registers and switch address space (context switch)

CPURegisters

Memory

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

…

Carnegie Mellon


Multiprocessing: The (Modern) Reality

Multicore processors Multiple CPUs on single chip Share main memory (and

some of the caches) Each can execute a separate

process Scheduling of processors

onto cores done by kernel

CPURegisters

Memory

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

StackHeap

CodeData

Saved registers

…

CPURegisters

Carnegie Mellon


Concurrent Processes Each process is a logical control flow. Two processes run concurrently (are

concurrent) if their flows overlap in time Otherwise, they are sequential Examples (running on single core):

Concurrent: A & B, A & C Sequential: B & C

Process A Process B Process C

Time

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User View of Concurrent Processes

Control flows for concurrent processes are physically disjoint in time

However, we can think of concurrent processes as running in parallel with each other

Time

Process A Process B Process C

Carnegie Mellon


Context Switching Processes are managed by a shared chunk

of memory-resident OS code called the kernel Important: the kernel is not a separate process, but

rather runs as part of some existing process. Control flow passes from one process to

another via a context switchProcess A Process B

user code

kernel code

user code

kernel code

user code

context switch

context switch

Time

Process Control

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System programming

Process Control

Process control – process creation, program execution, and process termination Process properties

real, effective, and saved; user and group IDs Interpreter files and the system function Process accounting

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System programming

Process Identifiers

#include <unistd.h>pid_t getpid(void);pid_t getppid(void); // get parent's piduid_t getuid(void);uid_t geteuid(void); // get effective useridgid_t getgid(void);gid_t getegid(void);

Process ID: a unique, non-negative integer

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System programming

Process Identifiers Process ID 0

The scheduler process

Process ID 1 The init process invoked by the kernel at the end of the

bootstrap procedure /sbin/init It reads the system-dependent initialization files (i.e., /etc/rc*) and brings a Unix system to a certain state.

((/etc/inittab and /etc/init.d/) or /etc/rc*)

Process ID 2 pagedaemon responsible for supporting the paging of the

virtual memory system.

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System programming

fork Function

#include <unistd.h>pid_t fork(void);

This function is called once, but returns twice. returns 0 in the child, returns the process ID of the new child in the parent.

The child is a copy of the parent (data space, heap, and stack). Often, text segment is shared.

Copy-on-write (COW)

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System programming

#include "apue.h"int glob = 6; /* external variable in initialized data */char buf[] = "a write to stdout\n";int main(void){ int var; /* automatic variable on the stack */ pid_t pid; var = 88; if (write(STDOUT_FILENO, buf, sizeof(buf)-1) != sizeof(buf)-1) err_sys("write error"); printf("before fork\n"); /* we don't flush stdout */ if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { glob++; /* modify variables */ var++; } else { sleep(2); } printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var); exit(0);}

Figure 8.1

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System programming

fork Function Figure 8.1

$ ./a.outa write to stdoutbefore forkpid = 430, glob = 7, var = 89pid = 429, glob = 6, var = 88

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System programming

fork Function

All descriptors that are open in the parent are duplicated in the child.

file status flags

current file offset

v-node ptr

file status flags

current file offset

v-node ptr

v-nodeinformation

i-nodeinformation

current file size

v-nodeinformation

i-nodeinformation

current file size

…

fd flags ptrfd 0:fd 1:fd 2:

parent process table entryfile table

v-node table

file status flags

current file offset

v-node ptrv-node

information

i-nodeinformation

current file size

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System programming

fork Function

All descriptors that are open in the parent are

duplicated in the child.

file status flags

current file offset

v-node ptr

file status flags

current file offset

v-node ptr

v-nodeinformation

i-nodeinformation

current file size

v-nodeinformation

i-nodeinformation

current file size

…


parent process table entryfile table

v-node table

file status flags

current file offset

v-node ptrv-node

information

i-nodeinformation

current file size

…


child process table entry

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System programming

fork Function

Two normal cases for handling descriptors after a fork The parent waits for the child to complete. When the

child terminates, any of shared descriptors read/written by the child will have their file offsets updated accordingly.

The parent and child each go their own way. After fork, they close the descriptors that they don’t need.

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System programming

user’s perspective(virtual memory)

System programming

System programming

System programming

System programming

System programming

fork Function Properties inherited by the child

real UID/GID, effective UID/GID, supplementary GIDs process group ID session ID controlling terminal set-user-ID flag and set-group-ID flag current working directory root directory file mode creation mask signal mask and dispositions the close-on-exec flag for any open file descriptors environment attached shared memory segments memory mapping resource limits

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System programming

fork Function

Differences between the parent and child the return value from fork the process IDs, parent process IDs the child’s values for tms_utime, tms_stime, tms_cutime,

and tms_cstime are set to 0 file locks set by the parent are not inherited by the chil

d pending alarms are cleared for the child the set of pending signals for the child is set to the em

pty set

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System programming

fork Function

Two reasons for fork to fail Too many processes in the system CHILD_MAX: the total number of processes per

real user ID Two uses for fork

The parent and child execute different sections of code at the same time, e.g. network servers.

The parent and child execute a different program, e.g. shells (the child does an exec right after returning from the fork.)

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System programming

vfork Function vfork does not fully copy the address space of th

e parent into the child (since the child won’t reference that address space – the child just calls exec or exit.) The child runs in the parent address space.

vfork guarantees that the child runs first, until the child calls exec or exit.

Figure 8.3

_exit vs. exit (flushing and closing stdout)$ a.outbefore vforkpid = 29039, glob = 7, var = 89

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System programming

#include "apue.h"int glob = 6; /* external variable in initialized data */int main(void){ int var; /* automatic variable on the stack */ pid_t pid; var = 88; printf("before vfork\n"); /* we don't flush stdio */ if ((pid = vfork()) < 0) { err_sys("vfork error"); } else if (pid == 0) { /* child */ glob++; /* modify parent's variables */ var++; _exit(0); /* child terminates */ }/* Parent continues here.*/ printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var); exit(0);}

Figure 8.3

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System programming

wait and waitpid Function

When a process terminates, the parent is notified by the kernel via the SIGCHLD signal.

#include <sys/wait.h>pid_t wait(int *statloc);pid_t waitpid(pid_t pid, int *statloc, int options);

wait can block the caller until a child terminates, while waitpid has an option that prevents it from blocking.

If a child is a zombie, wait immediately returns that child’s process ID with its termination status. Otherwise, it blocks the caller until a child terminates.

waitpid can wait for a specific process.

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System programming


Macros to examine termination status WIFEXITED(status): normal termination WEXITSTATUS(status)

WIFSIGNALED(status) : terminated by a signal WTERMSIG(status)

WCOREDUMP(status)

WIFSTOPPED(status): currently stopped WSTOPSIG(status)

WIFCONTINUED(status): continued after a job control stop

Figure 8.5 and Figure 8.6

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System programming

#include "apue.h"#include <sys/wait.h>void pr_exit(int status){ if (WIFEXITED(status)) printf("normal termination, exit status = %d\n", WEXITSTATUS(status)); else if (WIFSIGNALED(status)) printf("abnormal termination, signal number=%d%s\n", WTERMSIG(status),#ifdef WCOREDUMP WCOREDUMP(status) ? " (core file generated)" : "");#else"");#endif else if (WIFSTOPPED(status)) printf("child stopped, signal number = %d\n", WSTOPSIG(status));}

Figure 8.5

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System programming


#include "apue.h"#include <sys/wait.h>int main(void){ pid_t pid; int status; if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) /* child */ exit(7); if (wait(&status) != pid) /* wait for child */ err_sys("wait error"); pr_exit(status); /* and print its status */

if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) /* child */ abort(); /* generates SIGABRT */ if (wait(&status) != pid) /* wait for child */ err_sys("wait error"); pr_exit(status); /* and print its status */

if ((pid = fork()) < 0) err_sys("fork error"); else if (pid == 0) /* child */ status /= 0; /* divide by 0 generates SIGFPE */ if (wait(&status) != pid) /* wait for child */ err_sys("wait error"); pr_exit(status); /* and print its status */ exit(0);}

Figure 8.6

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System programming

wait and waitpid Function pid_t waitpid(pid_t pid, int *statloc, int options);

pid pid == -1 waits for any child process.

pid > 0 waits for the child whose process ID equals pid.

pid == 0 waits for any child whose process group ID equals that of the calling process.

pid < -1 waits for any child whose process group ID equals the absolute value of pid.

options WCONTINUED – the status of any child continued is returned.

WNOHANG – waitpid will not block (returns 0).

WUNTRACED – the status of any child stopped is returned.

Figure 8.8

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System programming


#include "apue.h"#include <sys/wait.h>int main(void){ pid_t pid; if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { /* first child */ if ((pid = fork()) < 0) err_sys("fork error"); else if (pid > 0) exit(0); /* parent from second fork == first child *//** We're the second child; our parent becomes init as soon* as our real parent calls exit() in the statement above.* Here's where we'd continue executing, knowing that when* we're done, init will reap our status.*/ sleep(2); printf("second child, parent pid = %d\n", getppid()); exit(0); }

if (waitpid(pid, NULL, 0) != pid) /* wait for first child */ err_sys("waitpid error");

/** We're the parent (the original process); * we continue executing,* knowing that we're not the parent of * the second child.*/ exit(0);}

Figure 8.8

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System programming

wait3 and wait4 Functions

#include <sys/types.h>#include <sys/wait.h>#include <sys/time.h>#include <sys/resource.h>pid_t wait3(int *statloc, int options, struct rusage *rusage);pid_t wait4(pid_t pid, int *statloc, int options, struct rusage *ruage);

It returns a summary of the resources used by the terminated process and all its child processes.

User/system CPU time, number of page faults, number of signals received, and the like

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System programming

exec Functions

#include <unistd.h>int execl(const char *pathname, const char *arg0, … /* (char *) 0 */);int execv(const char *pathname, char *const argv[]);int execle(const char *pathname, const char *arg0, … /* (char *) 0, char *const envp[] */);int execve(const char *pathname, char *const argv[], char *const envp[]);int execlp(const char *filename, const char *arg0, … /* (char *) 0 */)int execvp(const char *filename, char *const argv[]);

exec merely replaces the current process (its text, data, heap, and stack segments) with a brand new program from disk.

l – list of arguments, v – argv[] vector, e – an envp[] array, and p – a filename argument.

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System programming

exec Functions

Filename argument (execlp/execvp) If filename contains a slash, it is taken as a pathname. Otherwise, the executable is searched for in PATH environ

ment variable directories. If not a machine executable, it invokes /bin/sh with the

filename as input to the shell. Argument passing

execl/execlp/execle require separate command-line arguments with the end of the arguments marked with a null pointer.

Environment list passing execle/execve passing const *char envp[] instead

of using extern char **environ

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System programming

exec Functions Properties inherited from the calling process

pid, ppid, real UID/GID, supplementary GIDs process group ID, session ID controlling terminal time left until alarm clock current working directory, root directory file mode creation mask, file locks process signal mask, pending signals resource limits tms_utime, tms_stime, tms_cutime, and tms_cstime

Handling of open files the close-on-exec flag of every open descriptor: if set, the

descriptor is closed across an exec. FD_CLOEXEC flag

Effective UID/GID can change, depending on the status of the set-user-ID and the set-group-ID bits for the program file.

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System programming

exec Functions#include "apue.h"#include <sys/wait.h>char *env_init[] = { "USER=unknown", "PATH=/tmp", NULL };int main(void){ pid_t pid; if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { /* specify pathname, specify environment */ if (execle("/home/sar/bin/echoall", "echoall", "myarg1", "MY ARG2", (char *)0, env_init) < 0) err_sys("execle error"); }

Figure 8.16

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System programming

if (waitpid(pid, NULL, 0) < 0) err_sys("wait error"); if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { /* specify filename, inherit environment */ if (execlp("echoall", "echoall", "only 1 arg", (char *)0) < 0) err_sys("execlp error"); } exit(0);}

System programming

Changing User IDs and Group IDs

#include <unistd.h>int setuid(uid_t uid);int setgid(gid_t gid);

A superuser process can set the real UID, effective UID, and saved set-user-ID to uid.

If uid equals either the real UID or the saved set-user-ID, setuid sets only the effective UID to uid.

Otherwise, errno is set to EPERM.

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System programming


Only a superuser process can change the real user ID. The effective UID is set by the exec function, only if the set-user-ID bit is set f

or the program file. We can call setuid at any time to set the effective UID to either the real UID or the saved set-user-ID.

The saved set-user-ID is copied from the effective UID by exec.

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ID

exec setuid(uid)

set-user-ID bit of set-user-ID bit on superuser unprivi-

leged user

real user IDefective user ID

saved set-user ID

unchangedunchanged

copied from ef -fective user ID

unchangedset from user ID of program filecopied from efective user ID

set to uidset to uid

set to uid

unchangedset to uid

unchanged

System programming


saved set-user-ID feature

Assuming that the man program file is owned by the user name man and has its set-user-ID bit set1. When we exec it,2. real user ID = our user ID

3. effective user ID = man

4. saved set-user-ID = man

5. The man program accesses the required configuration files and manual pages (owned by the user name man.)

6. Before man runs any command on our behalf, it calls setuid(getuid()) to safely execute filter programs.

7. real user ID = our user ID (unchanged)

8. effective user ID = our user ID

9. saved set-user-ID = man (unchanged)

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System programming


4. When the filter is done, man calls setuid(maneuid). This call is allowed because maneuid equals the saved set-user-ID.

5. real user ID = our user ID (unchanged)

6. effective user ID = man

7. saved set-user-ID = man (unchanged)

5. The man program can now operate on its files, as its effective UID is man.

Extra privileges at the beginning and end, but our normal privilege most of the time.

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System programming


#include <unistd.h>int setreuid(uid_t ruid, uid_t euid);int setregid(gid_t rgid, gid_t egid); Swapping of the real UID and the effective UID

Either the real UID can be set to the effective UID, or the effective UID can either be set to the saved set-user ID or the real UID.

#include <unistd.h>int seteuid(uid_t uid);int setegid(gid_t gid); The effective UID can be set to either the real UID

or the saved set-user-ID. For a privileged user, only the effective UID is set to uid.

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System programming


49


efective user ID

real user ID

saved set-user-ID

superusersetuid(uid)

superusersetreuid(ruid, euid)

superuserseteuid(uid)

unprivilegedsetuid or seteuid

unprivilegedsetuid or seteuid

unprivilegedsetreuid

unprivilegedsetreuid

exec ofset-user-ID

uid

uiduid uideuid

ruid

System programming

#include "apue.h"#include <sys/wait.h>int main(void){ pid_t pid; if ((pid = fork()) < 0) { err_sys("fork error"); } else if (pid == 0) { /* child */ if (execl("/home/sar/bin/testinterp", "testinterp", "myarg1", "MY ARG2", (char *)0) < 0) err_sys("execl error"); } if (waitpid(pid, NULL, 0) < 0) /* parent */ err_sys("waitpid error"); exit(0);}

Figure 8.20

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System programming

Interpreter Files

Interpreter files #! pathname [optional-argument] The actual file got execed by the kernel is the file specifie

d by the pathname on the first line. Interpreter file vs. interpreter

Figure 8.20$ cat /home/sar/bin/testinterp#!/home/sar/bin/echarg foo$ ./a.outargv[0]: /home/sar/bin/echoargargv[1]: fooargv[2]: /home/sar/bin/testinterpargv[3]: myarg1argv[4]: MY ARGS2

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System programming

system Function

#include <stdlib.h>int system(const char *cmdstring); An interface to a shell (not to OS) Implemented by calling fork, exec, and waitpid Three different types of return values

If either the fork fails or waitpid returns an error other than EINTR, -1 with errno set to indicate the error.

If the exec fails, the return value is as if the shell had executed exit(127).

Otherwise, the return value from system is the termination status of the shell.

Figure 8.22 & Figure 8.23

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System programming

system Function#include <sys/wait.h>#include <errno.h>#include <unistd.h>int system(const char *cmdstring) /* version without signal handling */{ pid_t pid; int status; if (cmdstring == NULL) return(1); /* always a command processor with UNIX */ if ((pid = fork()) < 0) { status = -1; /* probably out of processes */ } else if (pid == 0) { /* child */ execl("/bin/sh", "sh", "-c", cmdstring, (char *)0); _exit(127); /* execl error */ } else { /* parent */ while (waitpid(pid, &status, 0) < 0) { if (errno != EINTR) { status = -1; /* error other than EINTR from waitpid() */ break; } } } return(status);}

Figure 8.22

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System programming

system Function#include "apue.h"#include <sys/wait.h>int main(void){ int status; if ((status = system("date")) < 0) err_sys("system() error");

pr_exit(status); if ((status = system("nosuchcommand")) < 0) err_sys("system() error");

pr_exit(status);

if ((status = system("who; exit 44")) < 0) err_sys("system() error");

pr_exit(status); exit(0);}

Figure 8.23

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System programming

system Function An advantage over fork and exec, is that system does all the required error/signal handling.

A security hole if we call system from a set-user-ID program Figure 8.24 & 8.25 on page 249 A program with set-user-ID or set-group-ID shou

ld use fork and exec directly, being certain to change back to normal permission after the fork, before calling exec. (no system with setuid/setgid programs)

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System programming

system Function#include "apue.h"int main(int argc, char *argv[]){ int status; if (argc < 2) err_quit("command-line argument required"); if ((status = system(argv[1])) < 0) err_sys("system() error"); pr_exit(status);

exit(0);}

#include "apue.h"int main(void){ printf("real uid = %d, effective uid = %d\n", getuid(), geteuid()); exit(0);}

Figure 8.24

Figure 8.25

$ tsys printuids normal execution, no special privileges

real uid = 205, effective uid = 205

normal termination, exit status = 0

$ su become superuser

Password: enter superuser password

# chown root tsys change owner

# chmod u+s tsys make set-user-ID

# ls -l tsys verify file's permissions and owner

-rwsrwxr-x 1 root 16361 Mar 16 16:59 tsys

# exit leave superuser shell

$ tsys printuids

real uid = 205, effective uid = 0 oops, this is a security hole

normal termination, exit status = 0

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System programming

Process Accounting Accounting records

Typically a small amount of binary data with command

name, system/user CPU time, UID, GID, starting/elapsed

time, memory usage, termination status, read/write bytes, etc. (struct acct in <sys/acct.h>)

Accounting data are all kept by the kernel in the

process table (initialized whenever a new process is created.)

Each accounting record is written into the account file (/var/adm/pacct on Solaris) when the

process terminates.

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System programming

User Identification

Does getpwuid(getuid()) enable us to find out the login name of the user running the program?

What if a single user has multiple login names? (i.e. multiple entries in the password file with the same user ID)

Environment variable LOGNAME What if a user modifies the environment variable?

#include <unistd.h>char *getlogin(void); It returns a pointer to a login name string. It can fail if the process is not attached to a termina

l that a user logged into, i.e. daemon processes.

58

System programming

Process Times

#include <sys/times.h>clock_t times(struct tms *buf);struct tms {

clock_t tms_utime; /* user CPU time */

clock_t tms_stime; /* system CPU time */

clock_t tms_cutime; /* user CPU time, terminated children */

clock_t tms_cstime; /* system CPU time, terminated children */

}

It returns the elapsed wall clock time from some arbitrary point in the past.

The clock_t values / sysconf(_SC_CLK_TCK) Figure 8.30

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System programming

Process Times#include "apue.h"#include <sys/times.h>static void pr_times(clock_t, struct tms *, struct tms *);static void do_cmd(char *);int main(int argc, char *argv[]){ int i; setbuf(stdout, NULL); for (i = 1; i < argc; i++) do_cmd(argv[i]); /* once for each command-line arg */ exit(0);}static void do_cmd(char *cmd) /* execute and time the "cmd" */{ struct tms tmsstart, tmsend; clock_t start, end; int status;

printf("\ncommand: %s\n", cmd); /* starting values */ if ((start = times(&tmsstart)) == -1) err_sys("times error");

if ((status = system(cmd)) < 0) /* execute command */ err_sys("system() error");

if ((end = times(&tmsend)) == -1) /* ending values */ err_sys("times error");

pr_times(end-start, &tmsstart, &tmsend); pr_exit(status);}

Figure 8.30

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System programming

Process Timesstatic void pr_times(clock_t real, struct tms *tmsstart, struct tms *tmsend){ static long clktck = 0;

if (clktck == 0) /* fetch clock ticks per second first time */ if ((clktck = sysconf(_SC_CLK_TCK)) < 0) err_sys("sysconf error");

printf(" real: %7.2f\n", real / (double) clktck); printf(" user: %7.2f\n", (tmsend->tms_utime - tmsstart->tms_utime) / (double) clktck);

printf(" sys: %7.2f\n", (tmsend->tms_stime - tmsstart->tms_stime) / (double) clktck); printf(" child user: %7.2f\n", (tmsend->tms_cutime - tmsstart->tms_cutime) / (double) clktck);

printf(" child sys: %7.2f\n", (tmsend->tms_cstime - tmsstart->tms_cstime) / (double) clktck);}

Figure 8.30

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Documents

Process Control - calab.hanyang.ac.krcalab.hanyang.ac.kr/courses/SP_taesoo/08_process.pdf · Carnegie Mellon Bryant and O’Hallaron, Computer Systems: A Programmer’s Perspective,