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

const1=1 const2=0 const1=1 const2=0 main tmp=1 RA=0x804838c tmp=1 RA=0x804838c A RA=0x B const=0 RA=0x const=0 RA=0x C tmp=0 RA=0x tmp=0 RA=0x A 0xfffffff 0x0 Memory void C () { A (0); } void B () { C (); } void A (int tmp){ if (tmp) B (); } int main () { A (1); return 0; } 0x x x x804838c Code Stack … SP

const1=3 const2=0 const1=3 const2=0 main bnd=3 RA=0x804838c bnd=3 RA=0x804838c A bnd=2 RA=0x bnd=2 RA=0x A bnd=1 RA=0x bnd=1 RA=0x A bnd=0 RA=0x bnd=0 RA=0x A 0xfffffff 0x0 Memory void A (int bnd){ if (bnd) A (bnd-1); } int main () { A (3); return 0; } 0x x804838c Code Stack … SP How can recursion go wrong? Can overflow the stack … Keep adding frame after frame …

wrd[3] wrd[2] wrd[1] wrd[0] const2=0 wrd[3] wrd[2] wrd[1] wrd[0] const2=0 main b= 0x00234 RA=0x804838c b= 0x00234 RA=0x804838c cap 0xfffffff 0x0 Memory void cap (char* b){ for (int i=0; b[i]!=‘\0’; i++) b[i]+=32; } int main(char*arg) { char wrd[4]; strcpy(arg, wrd); cap (wrd); return 0; } 0x x804838c Code Stack … SP 0x00234 What can go wrong? Can overflow wrd variable … Overwrite cap’s RA …

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 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 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) 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 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? 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 Process C Process P Kernel ? What is the core abstraction? Communication via files

UNIX philosophy Process C Process P Kernel What is the interface? Open: get a file reference (descriptor) Read/Write: get/put data Close: stop communicating Open: get a file reference (descriptor) Read/Write: get/put data Close: stop communicating File

UNIX philosophy Process C Process P Kernel Why is this safer than procedure calls? Interface is narrower Access file in a few well-defined ways Kernel ensures things run smoothly Interface is narrower Access file in a few well-defined ways Kernel ensures things run smoothly File

UNIX philosophy Process C Process P Kernel How do we transfer control to kernel? Special system call instruction! CPU pauses process, runs kernel Kind of like Java’s invoke instruction Special system call instruction! CPU pauses process, runs kernel Kind of like Java’s invoke instruction File

UNIX philosophy Process C Process P Kernel Key insight: Interface can be used for lots of things Persistent storage (i.e., “real” files) Devices, temporary channels (i.e., pipes) File

UNIX philosophy Process C Process P Kernel Two questions (1)How do processes start running? (2)How do we control access to files? File

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 Jeff if not Other questions?

UNIX philosophy Process C Process P Kernel Two questions (1)How do processes start running? (2)How do we control access to files? File

UNIX philosophy Process C Process P Kernel Two questions (1)How do processes start running? File

UNIX philosophy Process C Process P Kernel Maybe P is already running? Could just rely on kernel to start processes File

UNIX philosophy Process C Process P Kernel File What might we call such a process? Basically what a server is A process C wants to talk to that someone else launched Basically what a server is A process C wants to talk to that someone else launched

UNIX philosophy Process C Process P Kernel All processes shouldn’t be servers Want to launch processes on demand C needs primitives to create P File

UNIX Shell Shell Kernel 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? 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 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.Allocates process control block (bookkeeping data structure) 2.Reads program code from disk 3.Stores program code in memory (could be demand-loaded too) 4.Initializes machine registers for new process 5.Initializes translator data for new address space E.g., page table and PTBR Virtual addresses of code segment point to correct physical locations 6.Sets processor mode bit to “user” 7.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 Child memory Have to create a copy of pre-write page for the child. Physical memory Parent memory

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 Process C Process P Kernel Two questions (1)How do processes start running? (2)How do we control access to files? File

UNIX philosophy Process C Process P Kernel Two questions (1)How do processes start running? (2)How do we control access to files? File

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? 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 High score Game client (uid y) Game client (uid y) Game client (uid x) “x’s score = 10” “y’s score = 11”

MOO accounting problem Multi-player game called Moo Want to maintain high score in a file Could have a trusted process update scores Is this good enough? High score Game client (uid y) Game client (uid y) Game client (uid x) Game server “x’s score = 10” “y’s score = 11” “x:10 y:11”

MOO accounting problem Multi-player game called Moo Want to maintain high score in a file Could have a trusted process update scores Is this good enough? Can’t be sure that reported score is genuine Need to ensure score was computed correctly High score Game client (uid y) Game client (uid y) Game client (uid x) Game server “x’s score = 100” “y’s score = 11” “x:100 y:11”

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 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 High score Game client (root) Game client (root) Game client (root) Game client (root) Shell (uid y) Shell (uid y) Shell (uid x) Shell (uid x) “fork/exec game” “fork/exec game” “x’s score = 10” “y’s score = 11”