UNIX!
Landon Cox
September 3, 2012
Dealing with complexity
• How do you reduce the complexity of large programs?
•
•
•
•
Break functionality into modules
Goal is to “decouple” unrelated functions
Narrow the set of interactions between modules
Hope to make whole system easier to reason about
• How do we specify interactions between code modules?
• Procedure calls (or objects = data + procedure calls)
• int foo(char *buf)
• Procedure calls reduce complexity by
• Limiting how modules can interact with one another
• Hiding implementation details
Dealing with complexity
int main () {
cout << “input: ”;
cin >> input;
output = sqrt (input);
output = pow (output,3);
cout << output << endl;
}
int main () {
getInput ();
computeResult ();
printOutput ();
}
void getInput () {
cout << “input: ”;
cin >> input;
}
void printOutput () {
cout << output << endl;
}
void computeResult () {
output = sqrt (input);
output = pow (output,3);
}
int P(int a){…}
void C(int x){
int y=P(x);
}
How do C and P share information?
Via a shared, in-memory stack
int P(int a){…}
void C(int x){
int y=P(x);
}
What info is stored on the stack?
C’s registers, call arguments, RA,
P's local vars
Review of the stack
• Each stack frame contains a function’s
•
•
•
•
Local variables
Parameters
Return address
Saved values of calling function’s registers
• The stack enables recursion
Code
0x8048347
void C () {
A (0);
}
0x8048354
void B () {
C ();
}
0x8048361
void A (int tmp){
if (tmp) B ();
}
0x804838c
Memory
Stack
0xfffffff
…
int main () {
A (1);
return 0;
}
A
tmp=0
RA=0x8048347
C
const=0
RA=0x8048354
B
RA=0x8048361
A
tmp=1
RA=0x804838c
main
0x0
const1=1
const2=0
Code
Memory
Stack
0xfffffff
0x8048361
0x804838c
void A (int bnd){
if (bnd)
A (bnd-1);
}
int main () {
A (3);
return 0;
}
How can recursion go wrong?
Can overflow the stack …
Keep adding frame after frame
…
A
bnd=0
RA=0x8048361
A
bnd=1
RA=0x8048361
A
bnd=2
RA=0x8048361
A
bnd=3
RA=0x804838c
main
0x0
const1=3
const2=0
Code
void cap (char* b){
for (int i=0;
b[i]!=‘\0’;
i++)
0x8048361 } b[i]+=32;
int main(char*arg) {
char wrd[4];
strcpy(arg, wrd);
cap (wrd);
return 0;
0x804838c }
What can go wrong?
Can overflow wrd variable …
Overwrite cap’s RA
Memory
Stack
0xfffffff
…
0x0
cap
b= 0x00234
RA=0x804838c
wrd[3]
wrd[2]
wrd[1]
main
wrd[0]
0x00234
const2=0
int P(int a){…}
void C(int x){
int y=P(x);
}
Can think of this as a contract
P agrees to return
P agrees to resume where C left off
P agrees to restore the stack pointer
P agrees to leave rest of stack alone
int P(int a){…}
void C(int x){
int y=P(x);
}
Is the call contract enforced?
At a low level, NO!
P can violate all terms of the contract
Sources of violations: attacks + bugs
int P(int a){…}
void C(int x){
int y=P(x);
}
Enforcing the contract is feasible
Interaction is purely mechanical
Programmers intention is clear
No semantic gap to cross
int P(int a){…}
void C(int x){
int y=P(x);
}
How does Java enforce the
call contract?
Language restricts expressiveness
Programmers can’t access the stack
Special “invoke” instruction expresses intent
JVM trusted to transfer control between C, P
int P(int a){…}
void C(int x){
int y=P(x);
}
Awesome, so why not run only
Java programs?
Lower-level languages are faster
(trusted JVM interposes on every instr)
Restricts programmer’s choice
(maybe, I hate programming in Java)
int P(int a){…}
void C(int x){
int y=P(x);
}
Another approach to
enforced modularity
Put C and P in separate processes
Code is fast when processes not interacting
Trust kernel to handle control transfers
Kernel ensures transitions are correct
int P(int a){…}
void C(int x){
int y=P(x);
}
Key question:
What should the interface be?
Put C and P in separate processes
Want a general interface for inter-process
communication (IPC)
Should be simple and powerful (i.e., elegant)
UNIX philosophy
• OS by programmers for programmers
• Support high-level languages (C and scripting)
• Make interactivity a first-order concern (via shell)
• Allow rapid prototyping
• How should you program for a UNIX system?
• Write programs with limited features
• Do one thing and do it well
• Support easy composition of programs
• Make data easy to understand
• Store data in plaintext (not binary formats)
• Communicate via text streams
Thompson and Ritchie
Turing Award ‘83
UNIX philosophy
Kernel
Process
C
What is the core
abstraction?
Communication via
files
?
Process
P
UNIX philosophy
Kernel
Process
C
What is the interface?
File
Process
P
Open: get a file reference (descriptor)
Read/Write: get/put data
Close: stop communicating
UNIX philosophy
Kernel
Process
C
Why is this safer than
procedure calls?
File
Process
P
Interface is narrower
Access file in a few well-defined ways
Kernel ensures things run smoothly
UNIX philosophy
Kernel
Process
C
How do we transfer
control to kernel?
File
Process
P
Special system call instruction!
CPU pauses process, runs kernel
Kind of like Java’s invoke instruction
UNIX philosophy
Kernel
Process
C
Key insight:
File
Process
P
Interface can be used for lots of things
Persistent storage (i.e., “real” files)
Devices, temporary channels (i.e., pipes)
UNIX philosophy
Kernel
Process
C
Two questions
File
Process
P
(1) How do processes start running?
(2) How do we control access to files?
Course administration
• Heap manager project
• Due a week from Friday
• Sorry, but I can’t help you …
• Questions for Vamsi?
• Piazza
• Should have received account info
• Email Jeff if not
• Other questions?
UNIX philosophy
Kernel
Process
C
Two questions
File
Process
P
(1) How do processes start running?
(2) How do we control access to files?
UNIX philosophy
Kernel
Process
C
Two questions
File
Process
P
(1) How do processes start running?
UNIX philosophy
Kernel
Process
C
File
Process
P
Maybe P is already
running?
Could just rely on kernel to
start processes
UNIX philosophy
Kernel
Process
C
File
Process
P
What might we call such a
process?
Basically what a server is
A process C wants to talk to that
someone else launched
UNIX philosophy
Kernel
Process
C
File
Process
P
All processes shouldn’t be
servers
Want to launch processes on demand
C needs primitives to create P
UNIX Shell
Kernel
Shell
Program that runs other
programs
Interactive (accepts user commands)
Essentially just a line interpreter
Allows easy composition of programs
UNIX shell
• How does a UNIX process interact with a user?
• Via standard in (fd 0) and standard out (fd 1)
• These are the default input and output for a program
• Establishes well-known data entry and exit points for a program
• How do UNIX processes communicate with each other?
• Mostly communicate with each other via pipes
• Pipes allow programs to be chained together
• Shell and OS can connect one process’s stdout to another’s stdin
• Why do we need pipes when we have files?
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•
•
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Pipes create unnamed temporary buffers between processes
Communication between programs is often ephemeral
OS knows to garbage collect resources associated with pipe on exit
Consistent with UNIX philosophy of simplifying programmers’ lives
UNIX shell
• Pipes simplify naming
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•
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Program always receives input on fd 0
Program always emits output on fd 1
Program doesn’t care what is on the other end of fd
Shell/OS handle input/output connections
• How do pipes simplify synchronization?
• Pipe accessed via read system call
• Read can block in kernel until data is ready
• Or can poll, checking to see if read returns enough data
How kernel starts a process
1.
2.
3.
4.
5.
Allocates process control block (bookkeeping data structure)
Reads program code from disk
Stores program code in memory (could be demand-loaded too)
Initializes machine registers for new process
Initializes translator data for new address space
• E.g., page table and PTBR
• Virtual addresses of code segment point to correct physical locations
6.
7.
Sets processor mode bit to “user”
Jumps to start of program
Need
hardware
support
Creating processes
• Through what commands does UNIX create processes?
• Fork: create copy child process
• Exec: initialize address space with new program
• What’s the problem of creating an exact copy process?
• Child needs to do something different than parent
• i.e., child needs to know that it is the child
• How does child know it is child?
• Pass in return point
• Parent returns from fork call, child jumps into other region of code
• Fork works slightly differently now
Fork
• Child can’t be an exact copy
• Is distinguished by one variable (the return value of fork)
if (fork () == 0) {
/* child */
execute new program
} else {
/* parent */
carry on
}
Creating processes
• Why make a complete copy of parent?
•
•
•
•
•
Sometimes you want a copy of the parent
Separating fork/exec provides flexibility
Allows child to inherit some kernel state
E.g., open files, stdin, stdout
Very useful for shell
• How do we efficiently copy an address space?
• Use “copy on write”
• Make copy of page table, set pages to read-only
• Only make physical copies of pages on write fault
Copy on write
Physical
memory
Parent
memory
Child
memory
What happens if parent writes to a page?
Copy on write
Physical
memory
Parent
memory
Child
memory
Have to create a copy of pre-write page for
the child.
Alternative approach
• Windows CreateProcess
• Combines the work of fork and exec
• UNIX’s approach
• Supports arbitrary sharing between parent and child
• Window’s approach
• Supports sharing of most common data via params
Shells (bash, explorer, finder)
• Shells are normal programs
• Though they look like part of the OS
• How would you write one?
while (1) {
print prompt (“crocus% “)
ask for input (cin)
// e.g., “ls /tmp”
first word of input is command
// e.g., ls
fork a copy of the current process (shell)
if (child) {
redirect output to a file if requested (or a pipe)
exec new program (e.g., with argument “/tmp”)
} else {
wait for child to finish
or can run child in background and ask for another command
}
}
Shell demo
UNIX philosophy
Kernel
Process
C
Two questions
File
Process
P
(1) How do processes start running?
(2) How do we control access to files?
UNIX philosophy
Kernel
Process
C
Two questions
File
Process
P
(1) How do processes start running?
(2) How do we control access to files?
Access control
• Where is most trusted code located?
• In the operating system kernel
• What are the primary responsibilities of a UNIX kernel?
• Managing the file system
• Launching/scheduling processes
• Managing memory
• How do processes invoke the kernel?
•
•
•
•
Via system calls
Hardware shepherds transition from user process to kernel
Processor knows when it is running kernel code
Represents this through protection rings or mode bit
Access control
• How does kernel know if system call is allowed?
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•
•
•
Looks at user id (uid) of process making the call
Looks at resources accessed by call (e.g., file or pipe)
Checks access-control policy associated with resource
Decides if policy allows uid to access resources
• How is a uid normally assigned to a process?
• On fork, child inherits parent’s uid
MOO accounting problem
• Multi-player game called Moo
• Want to maintain high score in a file
• Should players be able to update score?
• Yes
• Do we trust users to write file directly?
• No, they could lie about their score
Game
client
(uid x)
“x’s score = 10”
High
score
“y’s score = 11”
Game
client
(uid y)
MOO accounting problem
• Multi-player game called Moo
Game
client
(uid x)
• Want to maintain high score in a file
• Could have a trusted process update scores
Game
server
• Is this good enough?
“x’s score = 10”
“x:10
y:11”
High
score
“y’s score = 11”
Game
client
(uid y)
MOO accounting problem
• Multi-player game called Moo
Game
client
(uid x)
• Want to maintain high score in a file
• Could have a trusted process update scores
Game
server
• Is this good enough?
• Can’t be sure that reported score is genuine
• Need to ensure score was computed correctly
“x’s score = 100”
“x:100
y:11”
High
score
“y’s score = 11”
Game
client
(uid y)
Access control
• Insight: sometimes simple inheritance of uids is insufficient
• Tasks involving management of “user id” state
• Logging in (login)
• Changing passwords (passwd)
• Why isn’t this code just inside the kernel?
• This functionality doesn’t really require interaction w/ hardware
• Would like to keep kernel as small as possible
• How are “trusted” user-space processes identified?
• Run as super user or root (uid 0)
• Like a software kernel mode
• If a process runs under uid 0, then it has more privileges
Access control
• Why does login need to run as root?
• Needs to check username/password correctness
• Needs to fork/exec process under another uid
• Why does passwd need to run as root?
• Needs to modify password database (file)
• Database is shared by all users
• What makes passwd particularly tricky?
• Easy to allow process to shed privileges (e.g., login)
• passwd requires an escalation of privileges
• How does UNIX handle this?
• Executable files can have their setuid bit set
• If setuid bit is set, process inherits uid of image file’s owner on exec
MOO accounting problem
• Multi-player game called Moo
• Want to maintain high score in a file
Shell
(uid x)
• How does setuid solve our problem?
•
•
•
•
Game executable is owned by trusted entity
Game cannot be modified by normal users
Users can run executable though
High-score is also owned by trusted entity
• This is a form of trustworthy computing
• Only trusted code can update score
• Root ownership ensures code integrity
• Untrusted users can invoke trusted code
Shell
(uid y)
“fork/exec
game”
Game
client
(root)
“x’s score = 10”
High
score
“y’s score = 11”
“fork/exec
game”
Game
client
(root)